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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
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> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
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> > > Add a new option to force index recreation in vacuum
> > > and if index recreation is specified.
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> > I don't know how people estimate this disadvantage.
>
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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
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> > > In fact, for REINDEX cases where you don't have a lock on the entire
> > > table as you do in vacuum, you could reindex the table with a simple
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> I heard from someone that old vacuum had been like so.
> Probably 2x disk space for big tables was a big disadvantage.
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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
References: <000f01bf622a$bf423940$2801007e@tpf.co.jp>
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Hiroshi Inoue wrote:
> > > Yes,I believe so. It's necessary to keep consistency between heap
References: <200001192132.QAA26048@candle.pha.pa.us>
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Bruce Momjian wrote:
>
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I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is
400MB and index is 160MB.
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Bruce Momjian wrote:
>
References: <200001210543.AAA13592@candle.pha.pa.us>
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Bruce Momjian wrote:
>
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> -----Original Message-----
> From: owner-pgsql-hackers@postgreSQL.org
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> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is
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On Fri, 21 Jan 2000, Bruce Momjian wrote:
Date: Fri, 21 Jan 2000 14:06:31 -0500
Message-ID: <16498.948481591@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: OR
+Status: RO
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Conclusions:
References: <200001211751.MAA12106@candle.pha.pa.us> <16498.948481591@sss.pgh.pa.us>
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Tom Lane wrote:
>
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> Tom Lane wrote:
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On Fri, 21 Jan 2000, Bruce Momjian wrote:
Date: Fri, 21 Jan 2000 17:02:06 -0500
Message-ID: <9694.948492126@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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+Status: RO
The Hermit Hacker <scrappy@hub.org> writes:
>> lock table for less duration, or read lock
Date: Fri, 21 Jan 2000 23:50:13 -0500
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Conclusions:
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> -----Original Message-----
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From: Tom Lane <tgl@sss.pgh.pa.us>
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"Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
> Vacuum after deleting half of rows may be one of the worst case.
Date: Fri, 21 Jan 2000 00:51:51 -0500
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is
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Here's two ideas I had for optimizing vacuum, I apologize in advance
if the ideas presented here are niave and don't take into account
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Alfred Perlstein wrote:
> #1
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Alfred Perlstein wrote:
PGP key available upon request, | /
and from pgp5.ai.mit.edu:11371 |/
-From pgsql-hackers-owner+M8931@postgresql.org Thu May 17 19:14:23 2001
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-To: pgsql-hackers@postgresql.org
-Subject: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 17 May 2001 19:05:24 -0400
-Message-ID: <12833.990140724@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
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-
-I have been thinking about the problem of VACUUM and how we might fix it
-for 7.2. Vadim has suggested that we should attack this by implementing
-an overwriting storage manager and transaction UNDO, but I'm not totally
-comfortable with that approach: it seems to me that it's an awfully large
-change in the way Postgres works. Instead, here is a sketch of an attack
-that I think fits better into the existing system structure.
-
-First point: I don't think we need to get rid of VACUUM, exactly. What
-we want for 24x7 operation is to be able to do whatever housekeeping we
-need without locking out normal transaction processing for long intervals.
-We could live with routine VACUUMs if they could run in parallel with
-reads and writes of the table being vacuumed. They don't even have to run
-in parallel with schema updates of the target table (CREATE/DROP INDEX,
-ALTER TABLE, etc). Schema updates aren't things you do lightly for big
-tables anyhow. So what we want is more of a "background VACUUM" than a
-"no VACUUM" solution.
-
-Second: if VACUUM can run in the background, then there's no reason not
-to run it fairly frequently. In fact, it could become an automatically
-scheduled activity like CHECKPOINT is now, or perhaps even a continuously
-running daemon (which was the original conception of it at Berkeley, BTW).
-This is important because it means that VACUUM doesn't have to be perfect.
-The existing VACUUM code goes to huge lengths to ensure that it compacts
-the table as much as possible. We don't need that; if we miss some free
-space this time around, but we can expect to get it the next time (or
-eventually), we can be happy. This leads to thinking of space management
-in terms of steady-state behavior, rather than the periodic "big bang"
-approach that VACUUM represents now.
-
-But having said that, there's no reason to remove the existing VACUUM
-code: we can keep it around for situations where you need to crunch a
-table as much as possible and you can afford to lock the table while
-you do it. The new code would be a new command, maybe "VACUUM LAZY"
-(or some other name entirely).
-
-Enough handwaving, what about specifics?
-
-1. Forget moving tuples from one page to another. Doing that in a
-transaction-safe way is hugely expensive and complicated. Lazy VACUUM
-will only delete dead tuples and coalesce the free space thus made
-available within each page of a relation.
-
-2. This does no good unless there's a provision to re-use that free space.
-To do that, I propose a free space map (FSM) kept in shared memory, which
-will tell backends which pages of a relation have free space. Only if the
-FSM shows no free space available will the relation be extended to insert
-a new or updated tuple.
-
-3. Lazy VACUUM processes a table in five stages:
- A. Scan relation looking for dead tuples; accumulate a list of their
- TIDs, as well as info about existing free space. (This pass is
- completely read-only and so incurs no WAL traffic.)
- B. Remove index entries for the dead tuples. (See below for details.)
- C. Physically delete dead tuples and compact free space on their pages.
- D. Truncate any completely-empty pages at relation's end. (Optional,
- see below.)
- E. Create/update FSM entry for the table.
-Note that this is crash-safe as long as the individual update operations
-are atomic (which can be guaranteed by WAL entries for them). If a tuple
-is dead, we care not whether its index entries are still around or not;
-so there's no risk to logical consistency.
-
-4. Observe that lazy VACUUM need not really be a transaction at all, since
-there's nothing it does that needs to be cancelled or undone if it is
-aborted. This means that its WAL entries do not have to hang around past
-the next checkpoint, which solves the huge-WAL-space-usage problem that
-people have noticed while VACUUMing large tables under 7.1.
-
-5. Also note that there's nothing saying that lazy VACUUM must do the
-entire table in one go; once it's accumulated a big enough batch of dead
-tuples, it can proceed through steps B,C,D,E even though it's not scanned
-the whole table. This avoids a rather nasty problem that VACUUM has
-always had with running out of memory on huge tables.
-
-
-Free space map details
-----------------------
-
-I envision the FSM as a shared hash table keyed by table ID, with each
-entry containing a list of page numbers and free space in each such page.
-
-The FSM is empty at system startup and is filled by lazy VACUUM as it
-processes each table. Backends then decrement/remove page entries as they
-use free space.
-
-Critical point: the FSM is only a hint and does not have to be perfectly
-accurate. It can omit space that's actually available without harm, and
-if it claims there's more space available on a page than there actually
-is, we haven't lost much except a wasted ReadBuffer cycle. This allows
-us to take shortcuts in maintaining it. In particular, we can constrain
-the FSM to a prespecified size, which is critical for keeping it in shared
-memory. We just discard entries (pages or whole relations) as necessary
-to keep it under budget. Obviously, we'd not bother to make entries in
-the first place for pages with only a little free space. Relation entries
-might be discarded on a least-recently-used basis.
-
-Accesses to the FSM could create contention problems if we're not careful.
-I think this can be dealt with by having each backend remember (in its
-relcache entry for a table) the page number of the last page it chose from
-the FSM to insert into. That backend will keep inserting new tuples into
-that same page, without touching the FSM, as long as there's room there.
-Only then does it go back to the FSM, update or remove that page entry,
-and choose another page to start inserting on. This reduces the access
-load on the FSM from once per tuple to once per page. (Moreover, we can
-arrange that successive backends consulting the FSM pick different pages
-if possible. Then, concurrent inserts will tend to go to different pages,
-reducing contention for shared buffers; yet any single backend does
-sequential inserts in one page, so that a bulk load doesn't cause
-disk traffic scattered all over the table.)
-
-The FSM can also cache the overall relation size, saving an lseek kernel
-call whenever we do have to extend the relation for lack of internal free
-space. This will help pay for the locking cost of accessing the FSM.
-
-
-Locking issues
---------------
-
-We will need two extensions to the lock manager:
-
-1. A new lock type that allows concurrent reads and writes
-(AccessShareLock, RowShareLock, RowExclusiveLock) but not anything else.
-Lazy VACUUM will grab this type of table lock to ensure the table schema
-doesn't change under it. Call it a VacuumLock until we think of a better
-name.
-
-2. A "conditional lock" operation that acquires a lock if available, but
-doesn't block if not.
-
-The conditional lock will be used by lazy VACUUM to try to upgrade its
-VacuumLock to an AccessExclusiveLock at step D (truncate table). If it's
-able to get exclusive lock, it's safe to truncate any unused end pages.
-Without exclusive lock, it's not, since there might be concurrent
-transactions scanning or inserting into the empty pages. We do not want
-lazy VACUUM to block waiting to do this, since if it does that it will
-create a lockout situation (reader/writer transactions will stack up
-behind it in the lock queue while everyone waits for the existing
-reader/writer transactions to finish). Better to not do the truncation.
-
-Another place where lazy VACUUM may be unable to do its job completely
-is in compaction of space on individual disk pages. It can physically
-move tuples to perform compaction only if there are not currently any
-other backends with pointers into that page (which can be tested by
-looking to see if the buffer reference count is one). Again, we punt
-and leave the space to be compacted next time if we can't do it right
-away.
-
-The fact that inserted/updated tuples might wind up anywhere in the table,
-not only at the end, creates no headaches except for heap_update. That
-routine needs buffer locks on both the page containing the old tuple and
-the page that will contain the new. To avoid possible deadlocks between
-different backends locking the same two pages in opposite orders, we need
-to constrain the lock ordering used by heap_update. This is doable but
-will require slightly more code than is there now.
-
-
-Index access method improvements
---------------------------------
-
-Presently, VACUUM deletes index tuples by doing a standard index scan
-and checking each returned index tuple to see if it points at any of
-the tuples to be deleted. If so, the index AM is called back to delete
-the tested index tuple. This is horribly inefficient: it means one trip
-into the index AM (with associated buffer lock/unlock and search overhead)
-for each tuple in the index, plus another such trip for each tuple actually
-deleted.
-
-This is mainly a problem of a poorly chosen API. The index AMs should
-offer a "bulk delete" call, which is passed a sorted array of main-table
-TIDs. The loop over the index tuples should happen internally to the
-index AM. At least in the case of btree, this could be done by a
-sequential scan over the index pages, which avoids the random I/O of an
-index-order scan and so should offer additional speedup.
-
-Further out (possibly not for 7.2), we should also look at making the
-index AMs responsible for shrinking indexes during deletion, or perhaps
-via a separate "vacuum index" API. This can be done without exclusive
-locks on the index --- the original Lehman & Yao concurrent-btrees paper
-didn't describe how, but more recent papers show how to do it. As with
-the main tables, I think it's sufficient to recycle freed space within
-the index, and not necessarily try to give it back to the OS.
-
-We will also want to look at upgrading the non-btree index types to allow
-concurrent operations. This may be a research problem; I don't expect to
-touch that issue for 7.2. (Hence, lazy VACUUM on tables with non-btree
-indexes will still create lockouts until this is addressed. But note that
-the lockout only lasts through step B of the VACUUM, not the whole thing.)
-
-
-There you have it. If people like this, I'm prepared to commit to
-making it happen for 7.2. Comments, objections, better ideas?
-
- regards, tom lane
-
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-From tgl@sss.pgh.pa.us Fri May 18 01:41:34 2001
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-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105180227.f4I2Rpa13258@candle.pha.pa.us>
-References: <200105180227.f4I2Rpa13258@candle.pha.pa.us>
-Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
- message dated "Thu, 17 May 2001 22:27:51 -0400"
-Date: Fri, 18 May 2001 01:41:33 -0400
-Message-ID: <14010.990164493@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: ORr
-
-Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> The only question I have is about the Free Space Map. It would seem
-> better to me if we could get this map closer to the table itself, rather
-> than having every table of every database mixed into the same shared
-> memory area. I can just see random table access clearing out most of
-> the map cache and perhaps making it less useless.
-
-What random access? Read transactions will never touch the FSM at all.
-As for writes, seems to me the places you are writing are exactly the
-places you need info for.
-
-You make a good point, which is that we don't want a schedule-driven
-VACUUM to load FSM entries for unused tables into the map at the cost
-of throwing out entries that *are* being used. But it seems to me that
-that's easily dealt with if we recognize the risk.
-
-> It would be nice if we could store the map on the first page of the disk
-> table, or store it in a flat file per table. I know both of these ideas
-> will not work,
-
-You said it. What's wrong with shared memory? You can't get any closer
-than shared memory: keeping maps in the files would mean you'd need to
-chew up shared-buffer space to get at them. (And what was that about
-random accesses causing your maps to get dropped? That would happen
-for sure if they live in shared buffers.)
-
-Another problem with keeping stuff in the first page: what happens when
-the table gets big enough that 8k of map data isn't really enough?
-With a shared-memory area, we can fairly easily allocate a variable
-amount of space based on total size of a relation vs. total size of
-relations under management.
-
-It is true that a shared-memory map would be useless at system startup,
-until VACUUM has run and filled in some info. But I don't see that as
-a big drawback. People who aren't developers like us don't restart
-their postmasters every five minutes.
-
-> Another advantage of centralization is that we can record update/delete
-> counters per table, helping tell vacuum where to vacuum next. Vacuum
-> roaming around looking for old tuples seems wasteful.
-
-Indeed. But I thought you were arguing against centralization?
-
- regards, tom lane
-
-From pgsql-hackers-owner+M8982@postgresql.org Fri May 18 14:13:26 2001
-Return-path: <pgsql-hackers-owner+M8982@postgresql.org>
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-Date: Fri, 18 May 2001 18:45:46 +0300 (GMT)
-From: Oleg Bartunov <oleg@sai.msu.su>
-X-X-Sender: <megera@ra.sai.msu.su>
-To: Tom Lane <tgl@sss.pgh.pa.us>
-cc: <pgsql-hackers@postgresql.org>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <12833.990140724@sss.pgh.pa.us>
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-On Thu, 17 May 2001, Tom Lane wrote:
-
->
-> We will also want to look at upgrading the non-btree index types to allow
-> concurrent operations. This may be a research problem; I don't expect to
-> touch that issue for 7.2. (Hence, lazy VACUUM on tables with non-btree
-> indexes will still create lockouts until this is addressed. But note that
-> the lockout only lasts through step B of the VACUUM, not the whole thing.)
-
-am I right you plan to work with GiST indexes as well ?
-We read a paper "Concurrency and Recovery in Generalized Search Trees"
-by Marcel Kornacker, C. Mohan, Joseph Hellerstein
-(http://citeseer.nj.nec.com/kornacker97concurrency.html)
-and probably we could go in this direction. Right now we're working
-on adding of multi-key support to GiST.
-
-btw, I have a question about function gistPageAddItem in gist.c
-it just decompress - compress key and calls PageAddItem to
-write tuple. We don't understand why do we need this function -
-why not use PageAddItem function. Adding multi-key support requires
-a lot of work and we don't want to waste our efforts and time.
-We already done some tests (gistPageAddItem -> PageAddItem) and
-everything is ok. Bruce, you're enthuasistic in removing unused code :-)
-
-
-
->
-> regards, tom lane
->
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->
-
- Regards,
- Oleg
-_____________________________________________________________
-Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
-Sternberg Astronomical Institute, Moscow University (Russia)
-Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
-phone: +007(095)939-16-83, +007(095)939-23-83
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-From pgsql-hackers-owner+M8987@postgresql.org Fri May 18 14:54:09 2001
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-Date: Fri, 18 May 2001 20:10:10 +0300 (GMT)
-From: Oleg Bartunov <oleg@sai.msu.su>
-X-X-Sender: <megera@ra.sai.msu.su>
-To: Tom Lane <tgl@sss.pgh.pa.us>
-cc: <pgsql-hackers@postgresql.org>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <20032.990203902@sss.pgh.pa.us>
-Message-ID: <Pine.GSO.4.33.0105181947520.12431-100000@ra.sai.msu.su>
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-
-On Fri, 18 May 2001, Tom Lane wrote:
-
-> Oleg Bartunov <oleg@sai.msu.su> writes:
-> > On Thu, 17 May 2001, Tom Lane wrote:
-> >> We will also want to look at upgrading the non-btree index types to allow
-> >> concurrent operations.
->
-> > am I right you plan to work with GiST indexes as well ?
-> > We read a paper "Concurrency and Recovery in Generalized Search Trees"
-> > by Marcel Kornacker, C. Mohan, Joseph Hellerstein
-> > (http://citeseer.nj.nec.com/kornacker97concurrency.html)
-> > and probably we could go in this direction. Right now we're working
-> > on adding of multi-key support to GiST.
-
-Another paper to read:
-"Efficient Concurrency Control in Multidimensional Access Methods"
-by Kaushik Chakrabarti
-http://www.ics.uci.edu/~kaushik/research/pubs.html
-
->
-> Yes, GIST should be upgraded to do concurrency. But I have no objection
-> if you want to work on multi-key support first.
->
-> My feeling is that a few releases from now we will have btree and GIST
-> as the preferred/well-supported index types. Hash and rtree might go
-> away altogether --- AFAICS they don't do anything that's not done as
-> well or better by btree or GIST, so what's the point of maintaining
-> them?
-
-Cool ! We could write rtree (and btree) ops using GiST. We have already
-realization of rtree for box ops and there are no problem to write
-additional ops for points, polygons etc.
-
->
-> > btw, I have a question about function gistPageAddItem in gist.c
-> > it just decompress - compress key and calls PageAddItem to
-> > write tuple. We don't understand why do we need this function -
->
-> The comment says
->
-> ** Take a compressed entry, and install it on a page. Since we now know
-> ** where the entry will live, we decompress it and recompress it using
-> ** that knowledge (some compression routines may want to fish around
-> ** on the page, for example, or do something special for leaf nodes.)
->
-> Are you prepared to say that you will no longer support the ability for
-> GIST compression routines to do those things? That seems shortsighted.
->
-
-No-no !!! we don't intend to lose that (compression) functionality.
-
-there are several reason we want to eliminate gistPageAddItem:
-1. It seems there are no examples where compress uses information about
- the page.
-2. There is some discrepancy between calculation of free space on page and
- the size of tuple saved on page - calculation of free space on page
- by gistNoSpace uses compressed tuple but tuple itself saved after
- recompression. It's possible that size of tupple could changed
- after recompression.
-3. decompress/compress could slowdown insert because it happens
- for every tuple.
-4. Currently gistPageAddItem is broken because it's not toast safe
- (see call gist_tuple_replacekey in gistPageAddItem)
-
-Right now we use #define GIST_PAGEADDITEM in gist.c and
-working with original PageAddItem. If people insist on gistPageAddItem
-we'll totally rewrite it. But for now we have enough job to do.
-
-
-> regards, tom lane
->
-
- Regards,
- Oleg
-_____________________________________________________________
-Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
-Sternberg Astronomical Institute, Moscow University (Russia)
-Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
-phone: +007(095)939-16-83, +007(095)939-23-83
-
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-From pgsql-hackers-owner+M9001@postgresql.org Fri May 18 20:22:26 2001
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-Message-ID: <3705826352029646A3E91C53F7189E3201662C@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Tom Lane'" <tgl@sss.pgh.pa.us>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 18 May 2001 17:08:07 -0700
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-
-> I have been thinking about the problem of VACUUM and how we
-> might fix it for 7.2. Vadim has suggested that we should
-> attack this by implementing an overwriting storage manager
-> and transaction UNDO, but I'm not totally comfortable with
-> that approach: it seems to me that it's an awfully large
-> change in the way Postgres works.
-
-I'm not sure if we should implement overwriting smgr at all.
-I was/is going to solve space reusing problem with non-overwriting
-one, though I'm sure that we have to reimplement it (> 1 table
-per data file, stored on disk FSM etc).
-
-> Second: if VACUUM can run in the background, then there's no
-> reason not to run it fairly frequently. In fact, it could become
-> an automatically scheduled activity like CHECKPOINT is now,
-> or perhaps even a continuously running daemon (which was the
-> original conception of it at Berkeley, BTW).
-
-And original authors concluded that daemon was very slow in
-reclaiming dead space, BTW.
-
-> 3. Lazy VACUUM processes a table in five stages:
-> A. Scan relation looking for dead tuples;...
-> B. Remove index entries for the dead tuples...
-> C. Physically delete dead tuples and compact free space...
-> D. Truncate any completely-empty pages at relation's end.
-> E. Create/update FSM entry for the table.
-...
-> If a tuple is dead, we care not whether its index entries are still
-> around or not; so there's no risk to logical consistency.
-
-What does this sentence mean? We canNOT remove dead heap tuple untill
-we know that there are no index tuples referencing it and your A,B,C
-reflect this, so ..?
-
-> Another place where lazy VACUUM may be unable to do its job completely
-> is in compaction of space on individual disk pages. It can physically
-> move tuples to perform compaction only if there are not currently any
-> other backends with pointers into that page (which can be tested by
-> looking to see if the buffer reference count is one). Again, we punt
-> and leave the space to be compacted next time if we can't do it right
-> away.
-
-We could keep share buffer lock (or add some other kind of lock)
-untill tuple projected - after projection we need not to read data
-for fetched tuple from shared buffer and time between fetching
-tuple and projection is very short, so keeping lock on buffer will
-not impact concurrency significantly.
-
-Or we could register callback cleanup function with buffer so bufmgr
-would call it when refcnt drops to 0.
-
-> Presently, VACUUM deletes index tuples by doing a standard index
-> scan and checking each returned index tuple to see if it points
-> at any of the tuples to be deleted. If so, the index AM is called
-> back to delete the tested index tuple. This is horribly inefficient:
-...
-> This is mainly a problem of a poorly chosen API. The index AMs
-> should offer a "bulk delete" call, which is passed a sorted array
-> of main-table TIDs. The loop over the index tuples should happen
-> internally to the index AM.
-
-I agreed with others who think that the main problem of index cleanup
-is reading all index data pages to remove some index tuples. You told
-youself about partial heap scanning - so for each scanned part of table
-you'll have to read all index pages again and again - very good way to
-trash buffer pool with big indices.
-
-Well, probably it's ok for first implementation and you'll win some CPU
-with "bulk delete" - I'm not sure how much, though, and there is more
-significant issue with index cleanup if table is not locked exclusively:
-concurrent index scan returns tuple (and unlock index page), heap_fetch
-reads table row and find that it's dead, now index scan *must* find
-current index tuple to continue, but background vacuum could already
-remove that index tuple => elog(FATAL, "_bt_restscan: my bits moved...");
-
-Two ways: hold index page lock untill heap tuple is checked or (rough
-schema)
-store info in shmem (just IndexTupleData.t_tid and flag) that an index tuple
-is used by some scan so cleaner could change stored TID (get one from prev
-index tuple) and set flag to help scan restore its current position on
-return.
-
-I'm particularly interested in discussing this issue because of it must be
-resolved for UNDO and chosen way will affect in what volume we'll be able
-to implement dirty reads (first way doesn't allow to implement them in full
-- ie selects with joins, - but good enough to resolve RI constraints
-concurrency issue).
-
-> There you have it. If people like this, I'm prepared to commit to
-> making it happen for 7.2. Comments, objections, better ideas?
-
-Well, my current TODO looks as (ORDER BY PRIORITY DESC):
-
-1. UNDO;
-2. New SMGR;
-3. Space reusing.
-
-and I cannot commit at this point anything about 3. So, why not to refine
-vacuum if you want it. I, personally, was never be able to convince myself
-to spend time for this.
-
-Vadim
-
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-From pgsql-hackers-owner+M9006@postgresql.org Fri May 18 21:04:21 2001
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-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3705826352029646A3E91C53F7189E3201662C@sectorbase2.sectorbase.com>
-References: <3705826352029646A3E91C53F7189E3201662C@sectorbase2.sectorbase.com>
-Comments: In-reply-to "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
- message dated "Fri, 18 May 2001 17:08:07 -0700"
-Date: Fri, 18 May 2001 20:27:15 -0400
-Message-ID: <27248.990232035@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
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-
-"Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
->> If a tuple is dead, we care not whether its index entries are still
->> around or not; so there's no risk to logical consistency.
-
-> What does this sentence mean? We canNOT remove dead heap tuple untill
-> we know that there are no index tuples referencing it and your A,B,C
-> reflect this, so ..?
-
-Sorry if it wasn't clear. I meant that if the vacuum process fails
-after removing an index tuple but before removing the (dead) heap tuple
-it points to, there's no need to try to undo. That state is OK, and
-when we next get a chance to vacuum we'll still be able to finish
-removing the heap tuple.
-
->> Another place where lazy VACUUM may be unable to do its job completely
->> is in compaction of space on individual disk pages. It can physically
->> move tuples to perform compaction only if there are not currently any
->> other backends with pointers into that page (which can be tested by
->> looking to see if the buffer reference count is one). Again, we punt
->> and leave the space to be compacted next time if we can't do it right
->> away.
-
-> We could keep share buffer lock (or add some other kind of lock)
-> untill tuple projected - after projection we need not to read data
-> for fetched tuple from shared buffer and time between fetching
-> tuple and projection is very short, so keeping lock on buffer will
-> not impact concurrency significantly.
-
-Or drop the pin on the buffer to show we no longer have a pointer to it.
-I'm not sure that the time to do projection is short though --- what
-if there are arbitrary user-defined functions in the quals or the
-projection targetlist?
-
-> Or we could register callback cleanup function with buffer so bufmgr
-> would call it when refcnt drops to 0.
-
-Hmm ... might work. There's no guarantee that the refcnt would drop to
-zero before the current backend exits, however. Perhaps set a flag in
-the shared buffer header, and the last guy to drop his pin is supposed
-to do the cleanup? But then you'd be pushing VACUUM's work into
-productive transactions, which is probably not the way to go.
-
->> This is mainly a problem of a poorly chosen API. The index AMs
->> should offer a "bulk delete" call, which is passed a sorted array
->> of main-table TIDs. The loop over the index tuples should happen
->> internally to the index AM.
-
-> I agreed with others who think that the main problem of index cleanup
-> is reading all index data pages to remove some index tuples.
-
-For very small numbers of tuples that might be true. But I'm not
-convinced it's worth worrying about. If there aren't many tuples to
-be freed, perhaps VACUUM shouldn't do anything at all.
-
-> Well, probably it's ok for first implementation and you'll win some CPU
-> with "bulk delete" - I'm not sure how much, though, and there is more
-> significant issue with index cleanup if table is not locked exclusively:
-> concurrent index scan returns tuple (and unlock index page), heap_fetch
-> reads table row and find that it's dead, now index scan *must* find
-> current index tuple to continue, but background vacuum could already
-> remove that index tuple => elog(FATAL, "_bt_restscan: my bits moved...");
-
-Hm. Good point ...
-
-> Two ways: hold index page lock untill heap tuple is checked or (rough
-> schema)
-> store info in shmem (just IndexTupleData.t_tid and flag) that an index tuple
-> is used by some scan so cleaner could change stored TID (get one from prev
-> index tuple) and set flag to help scan restore its current position on
-> return.
-
-Another way is to mark the index tuple "gone but not forgotten", so to
-speak --- mark it dead without removing it. (We could know that we need
-to do that if we see someone else has a buffer pin on the index page.)
-In this state, the index scan coming back to work would still be allowed
-to find the index tuple, but no other index scan would stop on the
-tuple. Later passes of vacuum would eventually remove the index tuple,
-whenever vacuum happened to pass through at an instant where no one has
-a pin on that index page.
-
-None of these seem real clean though. Needs more thought.
-
-> Well, my current TODO looks as (ORDER BY PRIORITY DESC):
-
-> 1. UNDO;
-> 2. New SMGR;
-> 3. Space reusing.
-
-> and I cannot commit at this point anything about 3. So, why not to refine
-> vacuum if you want it. I, personally, was never be able to convince myself
-> to spend time for this.
-
-Okay, good. I was worried that this idea would conflict with what you
-were doing, but it seems it won't.
-
- regards, tom lane
-
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-From vmikheev@SECTORBASE.COM Fri May 18 21:11:10 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-cc: "'Tom Lane'" <tgl@sss.pgh.pa.us>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 18 May 2001 18:10:10 -0700
-MIME-Version: 1.0
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-
-> Vadim, can you remind me what UNDO is used for?
-
-Ok, last reminder -:))
-
-On transaction abort, read WAL records and undo (rollback)
-changes made in storage. Would allow:
-
-1. Reclaim space allocated by aborted transactions.
-2. Implement SAVEPOINTs.
- Just to remind -:) - in the event of error discovered by server
- - duplicate key, deadlock, command mistyping, etc, - transaction
- will be rolled back to the nearest implicit savepoint setted
- just before query execution; - or transaction can be aborted by
- ROLLBACK TO <savepoint_name> command to some explicit savepoint
- setted by user. Transaction rolled back to savepoint may be continued.
-3. Reuse transaction IDs on postmaster restart.
-4. Split pg_log into small files with ability to remove old ones (which
- do not hold statuses for any running transactions).
-
-Vadim
-
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-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com>
-Comments: In-reply-to "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
- message dated "Fri, 18 May 2001 18:10:10 -0700"
-Date: Fri, 18 May 2001 21:37:37 -0400
-Message-ID: <27745.990236257@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-"Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
->> Vadim, can you remind me what UNDO is used for?
-> Ok, last reminder -:))
-
-> On transaction abort, read WAL records and undo (rollback)
-> changes made in storage. Would allow:
-
-> 1. Reclaim space allocated by aborted transactions.
-> 2. Implement SAVEPOINTs.
-> Just to remind -:) - in the event of error discovered by server
-> - duplicate key, deadlock, command mistyping, etc, - transaction
-> will be rolled back to the nearest implicit savepoint setted
-> just before query execution; - or transaction can be aborted by
-> ROLLBACK TO <savepoint_name> command to some explicit savepoint
-> setted by user. Transaction rolled back to savepoint may be continued.
-> 3. Reuse transaction IDs on postmaster restart.
-> 4. Split pg_log into small files with ability to remove old ones (which
-> do not hold statuses for any running transactions).
-
-Hm. On the other hand, relying on WAL for undo means you cannot drop
-old WAL segments that contain records for any open transaction. We've
-already seen several complaints that the WAL logs grow unmanageably huge
-when there is a long-running transaction, and I think we'll see a lot
-more.
-
-It would be nicer if we could drop WAL records after a checkpoint or two,
-even in the presence of long-running transactions. We could do that if
-we were only relying on them for crash recovery and not for UNDO.
-
-Looking at the advantages:
-
-1. Space reclamation via UNDO doesn't excite me a whole lot, if we can
-make lightweight VACUUM work well. (I definitely don't like the idea
-that after a very long transaction fails and aborts, I'd have to wait
-another very long time for UNDO to do its thing before I could get on
-with my work. Would much rather have the space reclamation happen in
-background.)
-
-2. SAVEPOINTs would be awfully nice to have, I agree.
-
-3. Reusing xact IDs would be nice, but there's an answer with a lot less
-impact on the system: go to 8-byte xact IDs. Having to shut down the
-postmaster when you approach the 4Gb transaction mark isn't going to
-impress people who want a 24x7 commitment, anyway.
-
-4. Recycling pg_log would be nice too, but we've already discussed other
-hacks that might allow pg_log to be kept finite without depending on
-UNDO (or requiring postmaster restarts, IIRC).
-
-I'm sort of thinking that undoing back to a savepoint is the only real
-usefulness of WAL-based UNDO. Is it practical to preserve the WAL log
-just back to the last savepoint in each xact, not the whole xact?
-
-Another thought: do we need WAL UNDO at all to implement savepoints?
-Is there some way we could do them like nested transactions, wherein
-each savepoint-to-savepoint segment is given its own transaction number?
-Committing multiple xact IDs at once might be a little tricky, but it
-seems like a narrow, soluble problem. Implementing UNDO without
-creating lots of performance issues looks a lot harder.
-
- regards, tom lane
-
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-From tgl@sss.pgh.pa.us Fri May 18 21:37:41 2001
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- Fri, 18 May 2001 21:37:37 -0400 (EDT)
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com>
-Comments: In-reply-to "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
- message dated "Fri, 18 May 2001 18:10:10 -0700"
-Date: Fri, 18 May 2001 21:37:37 -0400
-Message-ID: <27745.990236257@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: ORr
-
-"Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
->> Vadim, can you remind me what UNDO is used for?
-> Ok, last reminder -:))
-
-> On transaction abort, read WAL records and undo (rollback)
-> changes made in storage. Would allow:
-
-> 1. Reclaim space allocated by aborted transactions.
-> 2. Implement SAVEPOINTs.
-> Just to remind -:) - in the event of error discovered by server
-> - duplicate key, deadlock, command mistyping, etc, - transaction
-> will be rolled back to the nearest implicit savepoint setted
-> just before query execution; - or transaction can be aborted by
-> ROLLBACK TO <savepoint_name> command to some explicit savepoint
-> setted by user. Transaction rolled back to savepoint may be continued.
-> 3. Reuse transaction IDs on postmaster restart.
-> 4. Split pg_log into small files with ability to remove old ones (which
-> do not hold statuses for any running transactions).
-
-Hm. On the other hand, relying on WAL for undo means you cannot drop
-old WAL segments that contain records for any open transaction. We've
-already seen several complaints that the WAL logs grow unmanageably huge
-when there is a long-running transaction, and I think we'll see a lot
-more.
-
-It would be nicer if we could drop WAL records after a checkpoint or two,
-even in the presence of long-running transactions. We could do that if
-we were only relying on them for crash recovery and not for UNDO.
-
-Looking at the advantages:
-
-1. Space reclamation via UNDO doesn't excite me a whole lot, if we can
-make lightweight VACUUM work well. (I definitely don't like the idea
-that after a very long transaction fails and aborts, I'd have to wait
-another very long time for UNDO to do its thing before I could get on
-with my work. Would much rather have the space reclamation happen in
-background.)
-
-2. SAVEPOINTs would be awfully nice to have, I agree.
-
-3. Reusing xact IDs would be nice, but there's an answer with a lot less
-impact on the system: go to 8-byte xact IDs. Having to shut down the
-postmaster when you approach the 4Gb transaction mark isn't going to
-impress people who want a 24x7 commitment, anyway.
-
-4. Recycling pg_log would be nice too, but we've already discussed other
-hacks that might allow pg_log to be kept finite without depending on
-UNDO (or requiring postmaster restarts, IIRC).
-
-I'm sort of thinking that undoing back to a savepoint is the only real
-usefulness of WAL-based UNDO. Is it practical to preserve the WAL log
-just back to the last savepoint in each xact, not the whole xact?
-
-Another thought: do we need WAL UNDO at all to implement savepoints?
-Is there some way we could do them like nested transactions, wherein
-each savepoint-to-savepoint segment is given its own transaction number?
-Committing multiple xact IDs at once might be a little tricky, but it
-seems like a narrow, soluble problem. Implementing UNDO without
-creating lots of performance issues looks a lot harder.
-
- regards, tom lane
-
-From pgsql-hackers-owner+M9012@postgresql.org Fri May 18 22:02:39 2001
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-Date: Fri, 18 May 2001 18:56:25 -0700
-To: pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Message-ID: <20010518185625.F18121@store.zembu.com>
-Reply-To: pgsql-hackers@postgresql.org
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-In-Reply-To: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com>; from vmikheev@SECTORBASE.COM on Fri, May 18, 2001 at 06:10:10PM -0700
-From: ncm@zembu.com (Nathan Myers)
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-On Fri, May 18, 2001 at 06:10:10PM -0700, Mikheev, Vadim wrote:
-> > Vadim, can you remind me what UNDO is used for?
->
-> Ok, last reminder -:))
->
-> On transaction abort, read WAL records and undo (rollback)
-> changes made in storage. Would allow:
->
-> 1. Reclaim space allocated by aborted transactions.
-> 2. Implement SAVEPOINTs.
-> Just to remind -:) - in the event of error discovered by server
-> - duplicate key, deadlock, command mistyping, etc, - transaction
-> will be rolled back to the nearest implicit savepoint setted
-> just before query execution; - or transaction can be aborted by
-> ROLLBACK TO <savepoint_name> command to some explicit savepoint
-> setted by user. Transaction rolled back to savepoint may be continued.
-> 3. Reuse transaction IDs on postmaster restart.
-> 4. Split pg_log into small files with ability to remove old ones (which
-> do not hold statuses for any running transactions).
-
-I missed the original discussions; apologies if this has already been
-beaten into the ground. But... mightn't sub-transactions be a
-better-structured way to expose this service?
-
-Nathan Myers
-ncm@zembu.com
-
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-From pgsql-hackers-owner+M9016@postgresql.org Fri May 18 23:17:40 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105190312.f4J3Cfs14576@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <27745.990236257@sss.pgh.pa.us> "from Tom Lane at May 18, 2001 09:37:37
- pm"
-To: Tom Lane <tgl@sss.pgh.pa.us>
-Date: Fri, 18 May 2001 23:12:41 -0400 (EDT)
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
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-
-> Another thought: do we need WAL UNDO at all to implement savepoints?
-> Is there some way we could do them like nested transactions, wherein
-> each savepoint-to-savepoint segment is given its own transaction number?
-> Committing multiple xact IDs at once might be a little tricky, but it
-> seems like a narrow, soluble problem. Implementing UNDO without
-> creating lots of performance issues looks a lot harder.
-
-I am confused why we can't implement subtransactions as part of our
-command counter? The counter is already 4 bytes long. Couldn't we
-rollback to counter number X-10?
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-From pgsql-hackers-owner+M9017@postgresql.org Fri May 18 23:20:00 2001
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- Fri, 18 May 2001 23:15:14 -0400 (EDT)
-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105190312.f4J3Cfs14576@candle.pha.pa.us>
-References: <200105190312.f4J3Cfs14576@candle.pha.pa.us>
-Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
- message dated "Fri, 18 May 2001 23:12:41 -0400"
-Date: Fri, 18 May 2001 23:15:13 -0400
-Message-ID: <28236.990242113@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> I am confused why we can't implement subtransactions as part of our
-> command counter? The counter is already 4 bytes long. Couldn't we
-> rollback to counter number X-10?
-
-That'd work within your own transaction, but not from outside it.
-After you commit, how will other backends know which command-counter
-values of your transaction to believe, and which not?
-
- regards, tom lane
-
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-From tgl@sss.pgh.pa.us Fri May 18 23:15:13 2001
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-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105190312.f4J3Cfs14576@candle.pha.pa.us>
-References: <200105190312.f4J3Cfs14576@candle.pha.pa.us>
-Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
- message dated "Fri, 18 May 2001 23:12:41 -0400"
-Date: Fri, 18 May 2001 23:15:13 -0400
-Message-ID: <28236.990242113@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: ORr
-
-Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> I am confused why we can't implement subtransactions as part of our
-> command counter? The counter is already 4 bytes long. Couldn't we
-> rollback to counter number X-10?
-
-That'd work within your own transaction, but not from outside it.
-After you commit, how will other backends know which command-counter
-values of your transaction to believe, and which not?
-
- regards, tom lane
-
-From pgsql-hackers-owner+M9020@postgresql.org Fri May 18 23:44:09 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105190329.f4J3TtI15796@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <28236.990242113@sss.pgh.pa.us> "from Tom Lane at May 18, 2001 11:15:13
- pm"
-To: Tom Lane <tgl@sss.pgh.pa.us>
-Date: Fri, 18 May 2001 23:29:55 -0400 (EDT)
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
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-> Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> > I am confused why we can't implement subtransactions as part of our
-> > command counter? The counter is already 4 bytes long. Couldn't we
-> > rollback to counter number X-10?
->
-> That'd work within your own transaction, but not from outside it.
-> After you commit, how will other backends know which command-counter
-> values of your transaction to believe, and which not?
-
-Seems we would have to store the command counters for the parts of the
-transaction that committed, or the ones that were rolled back. Yuck.
-
-I hate to add UNDO complexity just for subtransactions.
-
-Hey, I have an idea. Can we do subtransactions as separate transactions
-(as Tom mentioned), and put the subtransaction id's in the WAL, so they
-an be safely committed/rolledback as a group?
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105190329.f4J3TtI15796@candle.pha.pa.us>
-References: <200105190329.f4J3TtI15796@candle.pha.pa.us>
-Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
- message dated "Fri, 18 May 2001 23:29:55 -0400"
-Date: Fri, 18 May 2001 23:43:42 -0400
-Message-ID: <28481.990243822@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
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-Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> Hey, I have an idea. Can we do subtransactions as separate transactions
-> (as Tom mentioned), and put the subtransaction id's in the WAL, so they
-> an be safely committed/rolledback as a group?
-
-It's not quite that easy: all the subtransactions have to commit at
-*the same time* from the point of view of other xacts, or you have
-consistency problems. So there'd need to be more xact-commit mechanism
-than there is now. Snapshots are also interesting; we couldn't use a
-single xact ID per backend to show the open-transaction state.
-
-WAL doesn't really enter into it AFAICS...
-
- regards, tom lane
-
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-From tgl@sss.pgh.pa.us Fri May 18 23:43:41 2001
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-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105190329.f4J3TtI15796@candle.pha.pa.us>
-References: <200105190329.f4J3TtI15796@candle.pha.pa.us>
-Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
- message dated "Fri, 18 May 2001 23:29:55 -0400"
-Date: Fri, 18 May 2001 23:43:42 -0400
-Message-ID: <28481.990243822@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: ORr
-
-Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> Hey, I have an idea. Can we do subtransactions as separate transactions
-> (as Tom mentioned), and put the subtransaction id's in the WAL, so they
-> an be safely committed/rolledback as a group?
-
-It's not quite that easy: all the subtransactions have to commit at
-*the same time* from the point of view of other xacts, or you have
-consistency problems. So there'd need to be more xact-commit mechanism
-than there is now. Snapshots are also interesting; we couldn't use a
-single xact ID per backend to show the open-transaction state.
-
-WAL doesn't really enter into it AFAICS...
-
- regards, tom lane
-
-From pgsql-hackers-owner+M9024@postgresql.org Sat May 19 00:05:43 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105190357.f4J3v1h17419@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <28481.990243822@sss.pgh.pa.us> "from Tom Lane at May 18, 2001 11:43:42
- pm"
-To: Tom Lane <tgl@sss.pgh.pa.us>
-Date: Fri, 18 May 2001 23:57:01 -0400 (EDT)
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
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-> Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> > Hey, I have an idea. Can we do subtransactions as separate transactions
-> > (as Tom mentioned), and put the subtransaction id's in the WAL, so they
-> > an be safely committed/rolledback as a group?
->
-> It's not quite that easy: all the subtransactions have to commit at
-> *the same time* from the point of view of other xacts, or you have
-> consistency problems. So there'd need to be more xact-commit mechanism
-> than there is now. Snapshots are also interesting; we couldn't use a
-> single xact ID per backend to show the open-transaction state.
-
-Yes, I knew that was going to come up that you have to add a lock to the
-pg_log that is only in affect when someone is commiting a transaction
-with subtransactions. Normal transactions get read/sharedlock, while
-subtransaction needs exclusive/writelock.
-
-Seems a lot easier than UNDO. Vadim you mentioned UNDO would allow
-space reuse for rolledback transactions, but in most cases the space
-reuse is going to be for old copies of committed transactions, right?
-Were you going to use WAL to get free space from old copies too?
-
-Vadim, I think I am missing something. You mentioned UNDO would be used
-for these cases and I don't understand the purpose of adding what would
-seem to be a pretty complex capability:
-
-> 1. Reclaim space allocated by aborted transactions.
-
-Is there really a lot to be saved here vs. old tuples of committed
-transactions?
-
-> 2. Implement SAVEPOINTs.
-> Just to remind -:) - in the event of error discovered by server
-> - duplicate key, deadlock, command mistyping, etc, - transaction
-> will be rolled back to the nearest implicit savepoint setted
-> just before query execution; - or transaction can be aborted by
-> ROLLBACK TO <savepoint_name> command to some explicit savepoint
-> setted by user. Transaction rolled back to savepoint may be
-> continued.
-
-Discussing, perhaps using multiple transactions.
-
-> 3. Reuse transaction IDs on postmaster restart.
-
-Doesn't seem like a huge win.
-
-> 4. Split pg_log into small files with ability to remove old ones (which
-> do not hold statuses for any running transactions).
-
-That one is interesting. Seems the only workaround for that would be to
-allow a global scan of all databases and tables to set commit flags,
-then shrink pg_log and set XID offset as start of file.
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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-From pgsql-hackers-owner+M9028@postgresql.org Sat May 19 05:00:37 2001
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-From: Kaare Rasmussen <kar@webline.dk>
-To: pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sat, 19 May 2001 10:38:29 +0200
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-> Second: if VACUUM can run in the background, then there's no reason not
-> to run it fairly frequently. In fact, it could become an automatically
-> scheduled activity like CHECKPOINT is now, or perhaps even a continuously
-> running daemon (which was the original conception of it at Berkeley, BTW).
-
-Maybe it's obvious, but I'd like to mention that you need some way of setting
-priority. If it's a daemon, or a process, you an nice it. If not, you need to
-implement something by yourself.
-
---
-Kaare Rasmussen --Linux, spil,-- Tlf: 3816 2582
-Kaki Data tshirts, merchandize Fax: 3816 2501
-Howitzvej 75 Åben 14.00-18.00 Web: www.suse.dk
-2000 Frederiksberg Lørdag 11.00-17.00 Email: kar@webline.dk
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-From pgman Sat May 19 08:12:28 2001
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- Sat, 19 May 2001 08:12:28 -0400 (EDT)
-From: Bruce Momjian <pgman>
-Message-ID: <200105191212.f4JCCSc15349@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105190357.f4J3v1h17419@candle.pha.pa.us> "from Bruce Momjian
- at May 18, 2001 11:57:01 pm"
-To: Bruce Momjian <pgman@candle.pha.pa.us>
-Date: Sat, 19 May 2001 08:12:28 -0400 (EDT)
-cc: Tom Lane <tgl@sss.pgh.pa.us>, "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- pgsql-hackers@postgresql.org
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-> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> > > Hey, I have an idea. Can we do subtransactions as separate transactions
-> > > (as Tom mentioned), and put the subtransaction id's in the WAL, so they
-> > > an be safely committed/rolledback as a group?
-> >
-> > It's not quite that easy: all the subtransactions have to commit at
-> > *the same time* from the point of view of other xacts, or you have
-> > consistency problems. So there'd need to be more xact-commit mechanism
-> > than there is now. Snapshots are also interesting; we couldn't use a
-> > single xact ID per backend to show the open-transaction state.
->
-> Yes, I knew that was going to come up that you have to add a lock to the
-> pg_log that is only in affect when someone is commiting a transaction
-> with subtransactions. Normal transactions get read/sharedlock, while
-> subtransaction needs exclusive/writelock.
-
-I was wrong here. Multiple backends can write to pg_log at the same
-time, even subtraction ones. It is just that no backend can read from
-pg_log during a subtransaction commit. Acctually, they can if the are
-reading a transaction status that is less than the minium active
-transaction id, see GetXmaxRecent().
-
-Doesn't seem too bad.
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
-From pgsql-hackers-owner+M9034@postgresql.org Sat May 19 08:18:54 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105191212.f4JCCSc15349@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105190357.f4J3v1h17419@candle.pha.pa.us> "from Bruce Momjian
- at May 18, 2001 11:57:01 pm"
-To: Bruce Momjian <pgman@candle.pha.pa.us>
-Date: Sat, 19 May 2001 08:12:28 -0400 (EDT)
-cc: Tom Lane <tgl@sss.pgh.pa.us>, "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- pgsql-hackers@postgresql.org
-X-Mailer: ELM [version 2.4ME+ PL90 (25)]
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-
-> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> > > Hey, I have an idea. Can we do subtransactions as separate transactions
-> > > (as Tom mentioned), and put the subtransaction id's in the WAL, so they
-> > > an be safely committed/rolledback as a group?
-> >
-> > It's not quite that easy: all the subtransactions have to commit at
-> > *the same time* from the point of view of other xacts, or you have
-> > consistency problems. So there'd need to be more xact-commit mechanism
-> > than there is now. Snapshots are also interesting; we couldn't use a
-> > single xact ID per backend to show the open-transaction state.
->
-> Yes, I knew that was going to come up that you have to add a lock to the
-> pg_log that is only in affect when someone is commiting a transaction
-> with subtransactions. Normal transactions get read/sharedlock, while
-> subtransaction needs exclusive/writelock.
-
-I was wrong here. Multiple backends can write to pg_log at the same
-time, even subtraction ones. It is just that no backend can read from
-pg_log during a subtransaction commit. Acctually, they can if the are
-reading a transaction status that is less than the minium active
-transaction id, see GetXmaxRecent().
-
-Doesn't seem too bad.
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-From pgsql-hackers-owner+M9036@postgresql.org Sat May 19 08:30:41 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105191223.f4JCNb815894@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <28481.990243822@sss.pgh.pa.us> "from Tom Lane at May 18, 2001 11:43:42
- pm"
-To: Tom Lane <tgl@sss.pgh.pa.us>
-Date: Sat, 19 May 2001 08:23:37 -0400 (EDT)
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
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-> Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> > Hey, I have an idea. Can we do subtransactions as separate transactions
-> > (as Tom mentioned), and put the subtransaction id's in the WAL, so they
-> > an be safely committed/rolledback as a group?
->
-> It's not quite that easy: all the subtransactions have to commit at
-> *the same time* from the point of view of other xacts, or you have
-> consistency problems. So there'd need to be more xact-commit mechanism
-> than there is now. Snapshots are also interesting; we couldn't use a
-> single xact ID per backend to show the open-transaction state.
-
-OK, I have another idea about subtransactions as multiple transaction
-ids.
-
-I realize that the snapshot problem would be an issue, because now
-instead of looking at your own transaction id, you have to look at
-multiple transaction ids. We could do this as a List of xid's, but that
-will not scale well.
-
-My idea is for a subtransaction backend to have its own pg_log-style
-memory area that shows which transactions it owns and has
-committed/aborted. It can have the log start at its start xid, and can
-look in pg_log and in there anytime it needs to check the visibility of
-a transaction greater than its minium xid. 16k can hold 64k xids, so it
-seems it should scale pretty well. (Each xid is two bits in pg_log.)
-
-In fact, multi-query transactions are just a special case of
-subtransactions, where all previous subtransactions are
-committed/visible. We could use the same pg_log-style memory area for
-multi-query transactions, eliminating the command counter and saving 8
-bytes overhead per tuple.
-
-Currently, the XMIN/XMAX command counters are used only by the current
-transaction, and they are useless once the transaction finishes and take
-up 8 bytes on disk.
-
-So, this idea gets us subtransactions and saves 8 bytes overhead. This
-reduces our per-tuple overhead from 36 to 28 bytes, a 22% reduction!
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105191223.f4JCNb815894@candle.pha.pa.us>
-References: <200105191223.f4JCNb815894@candle.pha.pa.us>
-Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
- message dated "Sat, 19 May 2001 08:23:37 -0400"
-Date: Sat, 19 May 2001 11:13:11 -0400
-Message-ID: <132.990285191@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: OR
-
-Bruce Momjian <pgman@candle.pha.pa.us> writes:
-> In fact, multi-query transactions are just a special case of
-> subtransactions, where all previous subtransactions are
-> committed/visible. We could use the same pg_log-style memory area for
-> multi-query transactions, eliminating the command counter and saving 8
-> bytes overhead per tuple.
-
-Interesting thought, but command IDs don't act the same as transactions;
-in particular, visibility of one scan to another doesn't necessarily
-depend on whether the scan has finished.
-
-Possibly that could be taken into account by having different rules for
-"do we think it's committed" in the local pg_log than the global one.
-
-Also, this distinction would propagate out of the xact status code;
-for example, it wouldn't do for heapam to set the "known committed"
-bit on a tuple just because it's from a previous subtransaction of the
-current xact. Right now that works because heapam knows the difference
-between xacts and commands; it would still have to know the difference.
-
-A much more significant objection is that such a design would eat xact
-IDs at a tremendous rate, to no purpose. CommandCounterIncrement is a
-cheap operation now, and we do it with abandon. It would not be cheap
-if it implied allocating a new xact ID that would eventually need to be
-marked committed. I don't mind allocating a new xact ID for each
-explicitly-created savepoint, but a new ID per CommandCounterIncrement
-is a different story.
-
- regards, tom lane
-
-From pgsql-hackers-owner+M9081@postgresql.org Sun May 20 02:45:24 2001
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-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "Tom Lane" <tgl@sss.pgh.pa.us>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, <pgsql-hackers@postgresql.org>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com> <27745.990236257@sss.pgh.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sat, 19 May 2001 23:36:34 -0700
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-
-> Hm. On the other hand, relying on WAL for undo means you cannot drop
-> old WAL segments that contain records for any open transaction. We've
-> already seen several complaints that the WAL logs grow unmanageably huge
-> when there is a long-running transaction, and I think we'll see a lot
-> more.
->
-> It would be nicer if we could drop WAL records after a checkpoint or two,
-> even in the presence of long-running transactions. We could do that if
-> we were only relying on them for crash recovery and not for UNDO.
-
-As you understand this is old, well-known problem in database practice,
-described in books. Two ways - either abort too long running transactions
-or (/and) compact old log segments: fetch and save (to use for undo)
-records of long-running transactions and remove other records. Neither
-way is perfect but nothing is perfect at all -:)
-
-> 1. Space reclamation via UNDO doesn't excite me a whole lot, if we can
-> make lightweight VACUUM work well. (I definitely don't like the idea
-
-Sorry, but I'm going to consider background vacuum as temporary solution
-only. As I've already pointed, original PG authors finally became
-disillusioned with the same approach. What is good in using UNDO for 1.
-is the fact that WAL records give you *direct* physical access to changes
-which should be rolled back.
-
-> that after a very long transaction fails and aborts, I'd have to wait
-> another very long time for UNDO to do its thing before I could get on
-> with my work. Would much rather have the space reclamation happen in
-> background.)
-
-Understandable, but why other transactions should read dirty data again
-and again waiting for background vacuum? I think aborted transaction
-should take some responsibility for mess made by them -:)
-And keeping in mind 2. very long transactions could be continued -:)
-
-> 2. SAVEPOINTs would be awfully nice to have, I agree.
->
-> 3. Reusing xact IDs would be nice, but there's an answer with a lot less
-> impact on the system: go to 8-byte xact IDs. Having to shut down the
-> postmaster when you approach the 4Gb transaction mark isn't going to
-> impress people who want a 24x7 commitment, anyway.
-
-+8 bytes in tuple header is not so tiny thing.
-
-> 4. Recycling pg_log would be nice too, but we've already discussed other
-> hacks that might allow pg_log to be kept finite without depending on
-> UNDO (or requiring postmaster restarts, IIRC).
-
-We did... and didn't get agreement.
-
-> I'm sort of thinking that undoing back to a savepoint is the only real
-> usefulness of WAL-based UNDO. Is it practical to preserve the WAL log
-> just back to the last savepoint in each xact, not the whole xact?
-
-No, it's not. It's not possible in overwriting systems at all - all
-transaction records are required.
-
-> Another thought: do we need WAL UNDO at all to implement savepoints?
-> Is there some way we could do them like nested transactions, wherein
-> each savepoint-to-savepoint segment is given its own transaction number?
-> Committing multiple xact IDs at once might be a little tricky, but it
-> seems like a narrow, soluble problem.
-
-Implicit savepoints wouldn't be possible - this is very convenient
-feature I've found in Oracle.
-And additional code in tqual.c wouldn't be good addition.
-
-> Implementing UNDO without creating lots of performance issues looks
-> a lot harder.
-
-What *performance* issues?!
-The only issue is additional disk requirements.
-
-Vadim
-
-
-
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-From pgsql-hackers-owner+M9088@postgresql.org Sun May 20 13:17:50 2001
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-To: "Vadim Mikheev" <vmikheev@sectorbase.com>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <002d01c0e0f7$376b59a0$4c79583f@sectorbase.com>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com> <27745.990236257@sss.pgh.pa.us> <002d01c0e0f7$376b59a0$4c79583f@sectorbase.com>
-Comments: In-reply-to "Vadim Mikheev" <vmikheev@sectorbase.com>
- message dated "Sat, 19 May 2001 23:36:34 -0700"
-Date: Sun, 20 May 2001 13:09:45 -0400
-Message-ID: <12003.990378585@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
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-
-"Vadim Mikheev" <vmikheev@sectorbase.com> writes:
->> 1. Space reclamation via UNDO doesn't excite me a whole lot, if we can
->> make lightweight VACUUM work well.
-
-> Sorry, but I'm going to consider background vacuum as temporary solution
-> only. As I've already pointed, original PG authors finally became
-> disillusioned with the same approach.
-
-How could they become disillusioned with it, when they never tried it?
-I know of no evidence that any version of PG has had backgroundable
-(non-blocking-to-other-transactions) VACUUM, still less within-relation
-space recycling. They may have become disillusioned with the form of
-VACUUM that they actually had (ie, the same one we've inherited) --- but
-please don't call that "the same approach" I'm proposing.
-
-Certainly, doing VACUUM this way is an experiment that may fail, or may
-require further work before it really works well. But I'd appreciate it
-if you wouldn't prejudge the results of the experiment.
-
->> Would much rather have the space reclamation happen in
->> background.)
-
-> Understandable, but why other transactions should read dirty data again
-> and again waiting for background vacuum? I think aborted transaction
-> should take some responsibility for mess made by them -:)
-
-They might read it again and again before the failed xact gets around to
-removing the data, too. You cannot rely on UNDO for correctness; at
-most it can be a speed/space optimization. I see no reason to assume
-that it's a more effective optimization than a background vacuum
-process.
-
->> 3. Reusing xact IDs would be nice, but there's an answer with a lot less
->> impact on the system: go to 8-byte xact IDs.
-
-> +8 bytes in tuple header is not so tiny thing.
-
-Agreed, but the people who need 8-byte IDs are not running small
-installations. I think they'd sooner pay a little more in disk space
-than risk costs in performance or reliability.
-
->> Another thought: do we need WAL UNDO at all to implement savepoints?
->> Is there some way we could do them like nested transactions, wherein
->> each savepoint-to-savepoint segment is given its own transaction number?
-
-> Implicit savepoints wouldn't be possible - this is very convenient
-> feature I've found in Oracle.
-
-Why not? Seems to me that establishing implicit savepoints is just a
-user-interface issue; you can do it, or not do it, regardless of the
-underlying mechanism.
-
->> Implementing UNDO without creating lots of performance issues looks
->> a lot harder.
-
-> What *performance* issues?!
-> The only issue is additional disk requirements.
-
-Not so. UNDO does failed-transaction cleanup work in the interactive
-backends, where it necessarily delays clients who might otherwise be
-issuing their next command. A VACUUM-based approach does the cleanup
-work in the background. Same work, more or less, but it's not in the
-clients' critical path.
-
-BTW, UNDO for failed transactions alone will not eliminate the need for
-VACUUM. Will you also make successful transactions go back and
-physically remove the tuples they deleted?
-
- regards, tom lane
-
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-From pgsql-hackers-owner+M9099@postgresql.org Sun May 20 17:09:01 2001
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-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "Tom Lane" <tgl@sss.pgh.pa.us>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, <pgsql-hackers@postgresql.org>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com> <27745.990236257@sss.pgh.pa.us> <002d01c0e0f7$376b59a0$4c79583f@sectorbase.com> <12003.990378585@sss.pgh.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sun, 20 May 2001 14:00:48 -0700
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-
-> >> 1. Space reclamation via UNDO doesn't excite me a whole lot, if we can
-> >> make lightweight VACUUM work well.
->
-> > Sorry, but I'm going to consider background vacuum as temporary solution
-> > only. As I've already pointed, original PG authors finally became
-> > disillusioned with the same approach.
->
-> How could they become disillusioned with it, when they never tried it?
-> I know of no evidence that any version of PG has had backgroundable
-> (non-blocking-to-other-transactions) VACUUM, still less within-relation
-> space recycling. They may have become disillusioned with the form of
-> VACUUM that they actually had (ie, the same one we've inherited) --- but
-> please don't call that "the same approach" I'm proposing.
-
-Pre-Postgres'95 (original) versions had vacuum daemon running in
-background. I don't know if that vacuum shrinked relations or not
-(there was no shrinking in '95 version), I know that daemon had to
-do some extra work in moving old tuples to archival storage, but
-anyway as you can read in old papers in the case of consistent heavy
-load daemon was not able to cleanup storage fast enough. And the
-reason is obvious - no matter how optimized your daemon will be
-(in regard to blocking other transactions etc), it will have to
-perform huge amount of IO just to find space available for reclaiming.
-
-> Certainly, doing VACUUM this way is an experiment that may fail, or may
-> require further work before it really works well. But I'd appreciate it
-> if you wouldn't prejudge the results of the experiment.
-
-Why not, Tom? Why shouldn't I say my opinion?
-Last summer your comment about WAL, may experiment that time, was that
-it will save just a few fsyncs. It was your right to make prejudment,
-what's wrong with my rights? And you appealed to old papers as well, BTW.
-
-> > Understandable, but why other transactions should read dirty data again
-> > and again waiting for background vacuum? I think aborted transaction
-> > should take some responsibility for mess made by them -:)
->
-> They might read it again and again before the failed xact gets around to
-> removing the data, too. You cannot rely on UNDO for correctness; at
-> most it can be a speed/space optimization. I see no reason to assume
-> that it's a more effective optimization than a background vacuum
-> process.
-
-Really?! Once again: WAL records give you *physical* address of tuples
-(both heap and index ones!) to be removed and size of log to read
-records from is not comparable with size of data files.
-
-> >> Another thought: do we need WAL UNDO at all to implement savepoints?
-> >> Is there some way we could do them like nested transactions, wherein
-> >> each savepoint-to-savepoint segment is given its own transaction number?
->
-> > Implicit savepoints wouldn't be possible - this is very convenient
-> > feature I've found in Oracle.
->
-> Why not? Seems to me that establishing implicit savepoints is just a
-> user-interface issue; you can do it, or not do it, regardless of the
-> underlying mechanism.
-
-Implicit savepoints are setted by server automatically before each
-query execution - you wouldn't use transaction IDs for this.
-
-> >> Implementing UNDO without creating lots of performance issues looks
-> >> a lot harder.
->
-> > What *performance* issues?!
-> > The only issue is additional disk requirements.
->
-> Not so. UNDO does failed-transaction cleanup work in the interactive
-> backends, where it necessarily delays clients who might otherwise be
-> issuing their next command. A VACUUM-based approach does the cleanup
-> work in the background. Same work, more or less, but it's not in the
-> clients' critical path.
-
-Not same work but much more and in the critical pathes of all clients.
-And - is overall performance of Oracle or Informix worse then in PG?
-Seems delays in clients for rollback doesn't affect performance so much.
-But dirty storage does it.
-
-> BTW, UNDO for failed transactions alone will not eliminate the need for
-> VACUUM. Will you also make successful transactions go back and
-> physically remove the tuples they deleted?
-
-They can't do this, as you know pretty well. But using WAL to get TIDs to
-be deleted is considerable, no?
-
-Vadim
-
-
-
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-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "Bruce Momjian" <pgman@candle.pha.pa.us>, "Tom Lane" <tgl@sss.pgh.pa.us>
-cc: <pgsql-hackers@postgresql.org>
-References: <200105190357.f4J3v1h17419@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sun, 20 May 2001 14:13:37 -0700
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-
-> Were you going to use WAL to get free space from old copies too?
-
-Considerable approach.
-
-> Vadim, I think I am missing something. You mentioned UNDO would be used
-> for these cases and I don't understand the purpose of adding what would
-> seem to be a pretty complex capability:
-
-Yeh, we already won title of most advanced among simple databases, -:)
-Yes, looking in list of IDs assigned to single transaction in tqual.c is much
-easy to do than UNDO. As well as couple of fsyncs is easy than WAL.
-
-> > 1. Reclaim space allocated by aborted transactions.
->
-> Is there really a lot to be saved here vs. old tuples of committed
-> transactions?
-
-Are you able to protect COPY FROM from abort/crash?
-
-Vadim
-
-
-
-From pgsql-hackers-owner+M9103@postgresql.org Sun May 20 17:33:30 2001
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-To: "Vadim Mikheev" <vmikheev@sectorbase.com>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <003701c0e16f$f3561ba0$4979583f@sectorbase.com>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com> <27745.990236257@sss.pgh.pa.us> <002d01c0e0f7$376b59a0$4c79583f@sectorbase.com> <12003.990378585@sss.pgh.pa.us> <003701c0e16f$f3561ba0$4979583f@sectorbase.com>
-Comments: In-reply-to "Vadim Mikheev" <vmikheev@sectorbase.com>
- message dated "Sun, 20 May 2001 14:00:48 -0700"
-Date: Sun, 20 May 2001 17:25:47 -0400
-Message-ID: <19770.990393947@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-"Vadim Mikheev" <vmikheev@sectorbase.com> writes:
-> Really?! Once again: WAL records give you *physical* address of tuples
-> (both heap and index ones!) to be removed and size of log to read
-> records from is not comparable with size of data files.
-
-You sure? With our current approach of dumping data pages into the WAL
-on first change since checkpoint (and doing so again after each
-checkpoint) it's not too difficult to devise scenarios where the WAL log
-is *larger* than the affected datafiles ... and can't be truncated until
-someone commits.
-
-The copied-data-page traffic is the worst problem with our current
-WAL implementation. I did some measurements last week on VACUUM of a
-test table (the accounts table from a "pg_bench -s 10" setup, which
-contains 1000000 rows; I updated 20000 rows and then vacuumed). This
-generated about 34400 8k blocks of WAL traffic, of which about 33300
-represented copied pages and the other 1100 blocks were actual WAL
-entries. That's a pretty massive I/O overhead, considering the table
-itself was under 20000 8k blocks. It was also interesting to note that
-a large fraction of the CPU time was spent calculating CRCs on the WAL
-data.
-
-Would it be possible to split the WAL traffic into two sets of files,
-one for WAL log records proper and one for copied pages? Seems like
-we could recycle the pages after each checkpoint rather than hanging
-onto them until the associated transactions commit.
-
->> Why not? Seems to me that establishing implicit savepoints is just a
->> user-interface issue; you can do it, or not do it, regardless of the
->> underlying mechanism.
-
-> Implicit savepoints are setted by server automatically before each
-> query execution - you wouldn't use transaction IDs for this.
-
-If the user asked you to, I don't see why not.
-
- regards, tom lane
-
----------------------------(end of broadcast)---------------------------
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-From pgsql-hackers-owner+M9104@postgresql.org Sun May 20 17:37:15 2001
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-Date: Sun, 20 May 2001 18:29:03 -0300 (ADT)
-From: The Hermit Hacker <scrappy@hub.org>
-To: Vadim Mikheev <vmikheev@sectorbase.com>
-cc: Tom Lane <tgl@sss.pgh.pa.us>, "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- <pgsql-hackers@postgresql.org>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <003701c0e16f$f3561ba0$4979583f@sectorbase.com>
-Message-ID: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org>
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-
-On Sun, 20 May 2001, Vadim Mikheev wrote:
-
-> > >> 1. Space reclamation via UNDO doesn't excite me a whole lot, if we can
-> > >> make lightweight VACUUM work well.
-> >
-> > > Sorry, but I'm going to consider background vacuum as temporary solution
-> > > only. As I've already pointed, original PG authors finally became
-> > > disillusioned with the same approach.
-> >
-> > How could they become disillusioned with it, when they never tried it?
-> > I know of no evidence that any version of PG has had backgroundable
-> > (non-blocking-to-other-transactions) VACUUM, still less within-relation
-> > space recycling. They may have become disillusioned with the form of
-> > VACUUM that they actually had (ie, the same one we've inherited) --- but
-> > please don't call that "the same approach" I'm proposing.
->
-> Pre-Postgres'95 (original) versions had vacuum daemon running in
-> background. I don't know if that vacuum shrinked relations or not
-> (there was no shrinking in '95 version), I know that daemon had to
-> do some extra work in moving old tuples to archival storage, but
-> anyway as you can read in old papers in the case of consistent heavy
-> load daemon was not able to cleanup storage fast enough. And the
-> reason is obvious - no matter how optimized your daemon will be
-> (in regard to blocking other transactions etc), it will have to
-> perform huge amount of IO just to find space available for reclaiming.
->
-> > Certainly, doing VACUUM this way is an experiment that may fail, or may
-> > require further work before it really works well. But I'd appreciate it
-> > if you wouldn't prejudge the results of the experiment.
->
-> Why not, Tom? Why shouldn't I say my opinion?
-> Last summer your comment about WAL, may experiment that time, was that
-> it will save just a few fsyncs. It was your right to make prejudment,
-> what's wrong with my rights? And you appealed to old papers as well, BTW.
-
-If its an "experiment", shouldn't it be done outside of the main source
-tree, with adequate testing in a high load situation, with a patch
-released to the community for further testing/comments, before it is added
-to the source tree? From reading Vadim's comment above (re:
-pre-Postgres95), this daemonized approach would cause a high I/O load on
-the server in a situation where there are *alot* of UPDATE/DELETEs
-happening to the database, which should be easily recreatable, no? Or,
-Vadim, am I misundertanding?
-
-
-
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-From pgsql-hackers-owner+M9105@postgresql.org Sun May 20 18:05:07 2001
-Return-path: <pgsql-hackers-owner+M9105@postgresql.org>
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- Sun, 20 May 2001 17:57:15 -0400 (EDT)
-To: The Hermit Hacker <scrappy@hub.org>
-cc: Vadim Mikheev <vmikheev@sectorbase.com>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org>
-References: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org>
-Comments: In-reply-to The Hermit Hacker <scrappy@hub.org>
- message dated "Sun, 20 May 2001 18:29:03 -0300"
-Date: Sun, 20 May 2001 17:57:15 -0400
-Message-ID: <19986.990395835@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-The Hermit Hacker <scrappy@hub.org> writes:
-> If its an "experiment", shouldn't it be done outside of the main source
-> tree, with adequate testing in a high load situation, with a patch
-> released to the community for further testing/comments, before it is added
-> to the source tree?
-
-Mebbe we should've handled WAL that way too ;-)
-
-Seriously, I don't think that my proposed changes need be treated with
-quite that much suspicion. The only part that is really intrusive is
-the shared-memory free-heap-space-management change. But AFAICT that
-will be a necessary component of *any* approach to getting rid of
-VACUUM. We've been arguing here, in essence, about whether a background
-or on-line approach to finding free space will be more useful; but that
-still leaves you with the question of what you do with the free space
-after you've found it. Without some kind of shared free space map,
-there's not anything you can do except have the process that found the
-space do tuple moving and file truncation --- ie, VACUUM. So even if
-I'm quite wrong about the effectiveness of a background VACUUM, the FSM
-code will still be needed: an UNDO-style approach is also going to need
-an FSM to do anything with the free space it finds. It's equally clear
-that the index AMs have to support index tuple deletion without
-exclusive lock, or we'll still have blocking problems during free-space
-cleanup, no matter what drives that cleanup. The only part of what
-I've proposed that might end up getting relegated to the scrap heap is
-the "lazy vacuum" command itself, which will be a self-contained and
-relatively small module (smaller than the present commands/vacuum.c,
-for sure).
-
-Besides which, Vadim has already said that he won't have time to do
-anything about space reclamation before 7.2. So even if background
-vacuum does end up getting superseded by something better, we're going
-to need it for a release or two ...
-
- regards, tom lane
-
----------------------------(end of broadcast)---------------------------
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-From pgsql-hackers-owner+M9110@postgresql.org Sun May 20 22:41:52 2001
-Return-path: <pgsql-hackers-owner+M9110@postgresql.org>
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-Message-ID: <002b01c0e19d$88af9fa0$4879583f@sectorbase.com>
-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "Tom Lane" <tgl@sss.pgh.pa.us>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, <pgsql-hackers@postgresql.org>
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com> <27745.990236257@sss.pgh.pa.us> <002d01c0e0f7$376b59a0$4c79583f@sectorbase.com> <12003.990378585@sss.pgh.pa.us> <003701c0e16f$f3561ba0$4979583f@sectorbase.com> <19770.990393947@sss.pgh.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sun, 20 May 2001 19:27:07 -0700
-MIME-Version: 1.0
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- charset="windows-1251"
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-
-> > Really?! Once again: WAL records give you *physical* address of tuples
-> > (both heap and index ones!) to be removed and size of log to read
-> > records from is not comparable with size of data files.
->
-> You sure? With our current approach of dumping data pages into the WAL
-> on first change since checkpoint (and doing so again after each
-> checkpoint) it's not too difficult to devise scenarios where the WAL log
-> is *larger* than the affected datafiles ... and can't be truncated until
-> someone commits.
-
-Yes, but note mine "size of log to read records from" - each log record
-has pointer to previous record made by same transaction: rollback must
-not read entire log file to get all records of specific transaction.
-
-> >> Why not? Seems to me that establishing implicit savepoints is just a
-> >> user-interface issue; you can do it, or not do it, regardless of the
-> >> underlying mechanism.
->
-> > Implicit savepoints are setted by server automatically before each
-> > query execution - you wouldn't use transaction IDs for this.
->
-> If the user asked you to, I don't see why not.
-
-Example of one of implicit savepoint usage: skipping duplicate key insertion.
-Using transaction IDs when someone want to insert a few thousand records?
-
-Vadim
-
-
-
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-From vmikheev@sectorbase.com Sun May 20 22:57:50 2001
-Return-path: <vmikheev@sectorbase.com>
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-Message-ID: <004301c0e1a1$d1e7fd80$4879583f@sectorbase.com>
-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "The Hermit Hacker" <scrappy@hub.org>
-cc: "Tom Lane" <tgl@sss.pgh.pa.us>, "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- <pgsql-hackers@postgresql.org>
-References: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sun, 20 May 2001 19:57:48 -0700
-MIME-Version: 1.0
-Content-Type: text/plain;
- charset="iso-8859-1"
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-Status: OR
-
-> If its an "experiment", shouldn't it be done outside of the main source
-> tree, with adequate testing in a high load situation, with a patch
-> released to the community for further testing/comments, before it is added
-> to the source tree? From reading Vadim's comment above (re:
-> pre-Postgres95), this daemonized approach would cause a high I/O load on
-> the server in a situation where there are *alot* of UPDATE/DELETEs
-> happening to the database, which should be easily recreatable, no? Or,
-> Vadim, am I misundertanding?
-
-It probably will not cause more IO than vacuum does right now.
-But unfortunately it will not reduce that IO. Cleanup work will be spreaded
-in time and users will not experience long lockouts but average impact
-on overall system throughput will be same (or maybe higher).
-My point is that we'll need in dynamic cleanup anyway and UNDO is
-what should be implemented for dynamic cleanup of aborted changes.
-Plus UNDO gives us natural implementation of savepoints and some
-abilities in transaction IDs management, which we may use or not
-(though, 4. - pg_log size management - is really good thing).
-
-Vadim
-
-
-
-From pgsql-hackers-owner+M9112@postgresql.org Sun May 20 23:14:28 2001
-Return-path: <pgsql-hackers-owner+M9112@postgresql.org>
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- for <pgman@candle.pha.pa.us>; Sun, 20 May 2001 23:14:28 -0400 (EDT)
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-Message-ID: <004f01c0e1a3$0024bb60$4879583f@sectorbase.com>
-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "The Hermit Hacker" <scrappy@hub.org>, "Tom Lane" <tgl@sss.pgh.pa.us>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, <pgsql-hackers@postgresql.org>
-References: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org> <19986.990395835@sss.pgh.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Sun, 20 May 2001 20:06:15 -0700
-MIME-Version: 1.0
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-
-> Seriously, I don't think that my proposed changes need be treated with
-> quite that much suspicion. The only part that is really intrusive is
-
-Agreed. I fight for UNDO, not against background vacuum -:)
-
-> the shared-memory free-heap-space-management change. But AFAICT that
-> will be a necessary component of *any* approach to getting rid of
-> VACUUM. We've been arguing here, in essence, about whether a background
-> or on-line approach to finding free space will be more useful; but that
-> still leaves you with the question of what you do with the free space
-> after you've found it. Without some kind of shared free space map,
-> there's not anything you can do except have the process that found the
-> space do tuple moving and file truncation --- ie, VACUUM. So even if
-> I'm quite wrong about the effectiveness of a background VACUUM, the FSM
-> code will still be needed: an UNDO-style approach is also going to need
-> an FSM to do anything with the free space it finds. It's equally clear
-
-Unfortunately, I think that we'll need in on-disk FSM and that FSM is
-actually the most complex thing to do in "space reclamation" project.
-
-> Besides which, Vadim has already said that he won't have time to do
-> anything about space reclamation before 7.2. So even if background
-> vacuum does end up getting superseded by something better, we're going
-> to need it for a release or two ...
-
-Yes.
-
-Vadim
-
-
-
----------------------------(end of broadcast)---------------------------
-TIP 4: Don't 'kill -9' the postmaster
-
-From pgsql-hackers-owner+M9113@postgresql.org Mon May 21 00:43:11 2001
-Return-path: <pgsql-hackers-owner+M9113@postgresql.org>
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- for <pgman@candle.pha.pa.us>; Mon, 21 May 2001 00:43:11 -0400 (EDT)
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- Mon, 21 May 2001 00:32:42 -0400 (EDT)
-To: "Vadim Mikheev" <vmikheev@sectorbase.com>
-cc: "The Hermit Hacker" <scrappy@hub.org>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <004f01c0e1a3$0024bb60$4879583f@sectorbase.com>
-References: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org> <19986.990395835@sss.pgh.pa.us> <004f01c0e1a3$0024bb60$4879583f@sectorbase.com>
-Comments: In-reply-to "Vadim Mikheev" <vmikheev@sectorbase.com>
- message dated "Sun, 20 May 2001 20:06:15 -0700"
-Date: Mon, 21 May 2001 00:32:42 -0400
-Message-ID: <20821.990419562@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-"Vadim Mikheev" <vmikheev@sectorbase.com> writes:
-> Unfortunately, I think that we'll need in on-disk FSM and that FSM is
-> actually the most complex thing to do in "space reclamation" project.
-
-I hope we can avoid on-disk FSM. Seems to me that that would create
-problems both for performance (lots of extra disk I/O) and reliability
-(what happens if FSM is corrupted? A restart won't fix it).
-
-But, if we do need it, most of the work needed to install FSM APIs
-should carry over. So I still don't see an objection to doing
-in-memory FSM as a first step.
-
-
-BTW, I was digging through the old Postgres papers this afternoon,
-to refresh my memory about what they actually said about VACUUM.
-I was interested to discover that at one time the tuple-insertion
-algorithm went as follows:
- 1. Pick a page at random in the relation, read it in, and see if it
- has enough free space. Repeat up to three times.
- 2. If #1 fails to find space, append tuple at end.
-When they got around to doing some performance measurement, they
-discovered that step #1 was a serious loser, and dropped it in favor
-of pure #2 (which is what we still have today). Food for thought.
-
- regards, tom lane
-
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-From tgl@sss.pgh.pa.us Mon May 21 13:38:41 2001
-Return-path: <tgl@sss.pgh.pa.us>
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-To: "Vadim Mikheev" <vmikheev@SECTORBASE.COM>
-cc: "The Hermit Hacker" <scrappy@hub.org>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <004301c0e1a1$d1e7fd80$4879583f@sectorbase.com>
-References: <Pine.BSF.4.33.0105201826150.3057-100000@mobile.hub.org> <004301c0e1a1$d1e7fd80$4879583f@sectorbase.com>
-Comments: In-reply-to "Vadim Mikheev" <vmikheev@sectorbase.com>
- message dated "Sun, 20 May 2001 19:57:48 -0700"
-Date: Mon, 21 May 2001 10:06:35 -0400
-Message-ID: <24896.990453995@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: OR
-
-"Vadim Mikheev" <vmikheev@sectorbase.com> writes:
-> It probably will not cause more IO than vacuum does right now.
-> But unfortunately it will not reduce that IO.
-
-Uh ... what? Certainly it will reduce the total cost of vacuum,
-because it won't bother to try to move tuples to fill holes.
-The index cleanup method I've proposed should be substantially
-more efficient than the existing code, as well.
-
-> My point is that we'll need in dynamic cleanup anyway and UNDO is
-> what should be implemented for dynamic cleanup of aborted changes.
-
-UNDO might offer some other benefits, but I doubt that it will allow
-us to eliminate VACUUM completely. To do that, you would need to
-keep track of free space using exact, persistent (on-disk) bookkeeping
-data structures. The overhead of that will be very substantial: more,
-I predict, than the approximate approach I proposed.
-
- regards, tom lane
-
-From pgsql-hackers-owner+M9138@postgresql.org Mon May 21 14:27:34 2001
-Return-path: <pgsql-hackers-owner+M9138@postgresql.org>
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-Message-ID: <3705826352029646A3E91C53F7189E32016630@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- Tom Lane
- <tgl@sss.pgh.pa.us>
-cc: "'Bruce Momjian'" <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 09:31:15 -0700
-MIME-Version: 1.0
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-
-> > Really?! Once again: WAL records give you *physical*
-> > address of tuples (both heap and index ones!) to be
-> > removed and size of log to read records from is not
-> > comparable with size of data files.
->
-> So how about a background "vacuum like" process, that reads
-> the WAL and does the cleanup ? Seems that would be great,
-> since it then does not need to scan, and does not make
-> forground cleanup necessary.
->
-> Problem is when cleanup can not keep up with cleaning WAL
-> files, that already want to be removed. I would envision a
-> config, that sais how many Mb of WAL are allowed to queue
-> up before clients are blocked.
-
-Yes, some daemon could read logs and gather cleanup info.
-We could activate it when switching to new log file segment
-and synchronization with checkpointer is not big deal. That
-daemon would also archive log files for WAL-based BAR,
-if archiving is ON.
-
-But this will be useful only with on-disk FSM.
-
-Vadim
-
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-From vmikheev@SECTORBASE.COM Mon May 21 13:36:13 2001
-Return-path: <vmikheev@SECTORBASE.COM>
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-Message-ID: <3705826352029646A3E91C53F7189E32016631@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Tom Lane'" <tgl@sss.pgh.pa.us>
-cc: The Hermit Hacker <scrappy@hub.org>,
- "'Bruce Momjian'"
- <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 09:53:35 -0700
-MIME-Version: 1.0
-X-Mailer: Internet Mail Service (5.5.2653.19)
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-
-> > It probably will not cause more IO than vacuum does right now.
-> > But unfortunately it will not reduce that IO.
->
-> Uh ... what? Certainly it will reduce the total cost of vacuum,
-> because it won't bother to try to move tuples to fill holes.
-
-Oh, you're right here, but daemon will most likely read data files
-again and again with in-memory FSM. Also, if we'll do partial table
-scans then we'll probably re-read indices > 1 time.
-
-> The index cleanup method I've proposed should be substantially
-> more efficient than the existing code, as well.
-
-Not in IO area.
-
-> > My point is that we'll need in dynamic cleanup anyway and UNDO is
-> > what should be implemented for dynamic cleanup of aborted changes.
->
-> UNDO might offer some other benefits, but I doubt that it will allow
-> us to eliminate VACUUM completely. To do that, you would need to
-
-I never told this -:)
-
-> keep track of free space using exact, persistent (on-disk) bookkeeping
-> data structures. The overhead of that will be very substantial: more,
-> I predict, than the approximate approach I proposed.
-
-I doubt that "big guys" use in-memory FSM. If they were able to do this...
-
-Vadim
-
-From pgsql-hackers-owner+M9136@postgresql.org Mon May 21 14:25:59 2001
-Return-path: <pgsql-hackers-owner+M9136@postgresql.org>
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- for <pgman@candle.pha.pa.us>; Mon, 21 May 2001 14:25:59 -0400 (EDT)
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-Message-ID: <3705826352029646A3E91C53F7189E32016632@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Tom Lane'" <tgl@sss.pgh.pa.us>
-cc: The Hermit Hacker <scrappy@hub.org>,
- "'Bruce Momjian'"
- <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 09:55:40 -0700
-MIME-Version: 1.0
-X-Mailer: Internet Mail Service (5.5.2653.19)
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-
-> I hope we can avoid on-disk FSM. Seems to me that that would create
-> problems both for performance (lots of extra disk I/O) and reliability
-> (what happens if FSM is corrupted? A restart won't fix it).
-
-We can use WAL for FSM.
-
-Vadim
-
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-From janwieck@Yahoo.com Mon May 21 13:36:06 2001
-Return-path: <janwieck@Yahoo.com>
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- by jupiter.jw.home (8.9.3/8.9.3) id NAA14283;
- Mon, 21 May 2001 13:13:55 -0400
-From: Jan Wieck <JanWieck@Yahoo.com>
-Message-ID: <200105211713.NAA14283@jupiter.jw.home>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <20821.990419562@sss.pgh.pa.us> from Tom Lane at "May 21, 2001 00:32:42
- am"
-To: Tom Lane <tgl@sss.pgh.pa.us>
-Date: Mon, 21 May 2001 13:13:55 -0400 (EDT)
-cc: Vadim Mikheev <vmikheev@SECTORBASE.COM>,
- The Hermit Hacker <scrappy@hub.org>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- pgsql-hackers@postgresql.orgrg.us.greatbridge.com
-X-Mailer: ELM [version 2.4ME+ PL68 (25)]
-MIME-Version: 1.0
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-Status: OR
-
-Tom Lane wrote:
-> "Vadim Mikheev" <vmikheev@sectorbase.com> writes:
-> > Unfortunately, I think that we'll need in on-disk FSM and that FSM is
-> > actually the most complex thing to do in "space reclamation" project.
->
-> I hope we can avoid on-disk FSM. Seems to me that that would create
-> problems both for performance (lots of extra disk I/O) and reliability
-> (what happens if FSM is corrupted? A restart won't fix it).
->
-> But, if we do need it, most of the work needed to install FSM APIs
-> should carry over. So I still don't see an objection to doing
-> in-memory FSM as a first step.
->
->
-> BTW, I was digging through the old Postgres papers this afternoon,
-> to refresh my memory about what they actually said about VACUUM.
-> I was interested to discover that at one time the tuple-insertion
-> algorithm went as follows:
-> 1. Pick a page at random in the relation, read it in, and see if it
-> has enough free space. Repeat up to three times.
-> 2. If #1 fails to find space, append tuple at end.
-> When they got around to doing some performance measurement, they
-> discovered that step #1 was a serious loser, and dropped it in favor
-> of pure #2 (which is what we still have today). Food for thought.
-
- No surprise to me, because without removing dead tuples (plus
- their index entries) and compacting pages, there's VERY
- unlikely freespace on a randomly selected page. And AFAIR
- these steps haven't been done by those versions.
-
- I think the in-shared-mem FSM could have some max-per-table
- limit and the background VACUUM just skips the entire table
- as long as nobody reused any space. Also it might only
- compact pages that lead to 25 or more percent of freespace in
- the first place. That makes it more likely that if someone
- looks for a place to store a tuple that it'll fit into that
- block (remember that the toaster tries to keep main tuples
- below BLKSZ/4).
-
-
-Jan
-
---
-
-#======================================================================#
-# It's easier to get forgiveness for being wrong than for being right. #
-# Let's break this rule - forgive me. #
-#================================================== JanWieck@Yahoo.com #
-
-
-
-_________________________________________________________
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-Get your free @yahoo.com address at http://mail.yahoo.com
-
-
-From vmikheev@SECTORBASE.COM Mon May 21 13:36:05 2001
-Return-path: <vmikheev@SECTORBASE.COM>
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-Message-ID: <3705826352029646A3E91C53F7189E32016634@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Jan Wieck'" <JanWieck@Yahoo.com>, Tom Lane <tgl@sss.pgh.pa.us>
-cc: The Hermit Hacker <scrappy@hub.org>,
- "'Bruce Momjian'"
- <pgman@candle.pha.pa.us>,
- pgsql-hackers@postgresql.orgrg.us.greatbridge.com
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 10:22:49 -0700
-MIME-Version: 1.0
-X-Mailer: Internet Mail Service (5.5.2653.19)
-Content-Type: text/plain;
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-
-> I think the in-shared-mem FSM could have some max-per-table
-> limit and the background VACUUM just skips the entire table
-> as long as nobody reused any space. Also it might only
-> compact pages that lead to 25 or more percent of freespace in
-> the first place. That makes it more likely that if someone
-> looks for a place to store a tuple that it'll fit into that
-> block (remember that the toaster tries to keep main tuples
-> below BLKSZ/4).
-
-This should be configurable parameter like PCFREE (or something
-like that) in Oracle: consider page for insertion only if it's
-PCFREE % empty.
-
-Vadim
-
-From pgsql-hackers-owner+M9142@postgresql.org Mon May 21 16:02:27 2001
-Return-path: <pgsql-hackers-owner+M9142@postgresql.org>
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- for <pgman@candle.pha.pa.us>; Mon, 21 May 2001 16:02:26 -0400 (EDT)
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- Mon, 21 May 2001 13:22:53 -0400 (EDT)
-To: Jan Wieck <JanWieck@yahoo.com>
-cc: Vadim Mikheev <vmikheev@sectorbase.com>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <200105211713.NAA14283@jupiter.jw.home>
-References: <200105211713.NAA14283@jupiter.jw.home>
-Comments: In-reply-to Jan Wieck <JanWieck@yahoo.com>
- message dated "Mon, 21 May 2001 13:13:55 -0400"
-Date: Mon, 21 May 2001 13:22:53 -0400
-Message-ID: <29285.990465773@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-Jan Wieck <JanWieck@yahoo.com> writes:
-> I think the in-shared-mem FSM could have some max-per-table
-> limit and the background VACUUM just skips the entire table
-> as long as nobody reused any space.
-
-I was toying with the notion of trying to use Vadim's "MNMB" idea
-(see his description of the work he did for Perlstein last year);
-that is, keep track of the lowest block number of any modified block
-within each relation since the last VACUUM. Then VACUUM would only
-have to scan from there to the end. This covers the totally-untouched-
-relation case nicely, and also helps a lot for large rels that you're
-mostly just adding to or perhaps updating recent additions.
-
-The FSM could probably keep track of such info fairly easily, since
-it will already be aware of which blocks it's told backends to try
-to insert into. But it would have to be told about deletes too,
-which would mean more FSM access traffic and more lock contention.
-Another problem (given my current view of how FSM should work) is that
-rels not being used at all would not be in FSM, or would age out of it,
-and so you wouldn't know that you didn't need to vacuum them.
-So I'm not sure yet if it's a good idea.
-
- regards, tom lane
-
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-From pgsql-hackers-owner+M9141@postgresql.org Mon May 21 14:55:40 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Tom Lane'" <tgl@sss.pgh.pa.us>
-cc: pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 10:52:28 -0700
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-> > We could keep share buffer lock (or add some other kind of lock)
-> > untill tuple projected - after projection we need not to read data
-> > for fetched tuple from shared buffer and time between fetching
-> > tuple and projection is very short, so keeping lock on buffer will
-> > not impact concurrency significantly.
->
-> Or drop the pin on the buffer to show we no longer have a pointer to it.
-
-This is not good for seqscans which will return to that buffer anyway.
-
-> > Or we could register callback cleanup function with buffer so bufmgr
-> > would call it when refcnt drops to 0.
->
-> Hmm ... might work. There's no guarantee that the refcnt
-> would drop to zero before the current backend exits, however.
-> Perhaps set a flag in the shared buffer header, and the last guy
-> to drop his pin is supposed to do the cleanup?
-
-This is what I've meant - set (register) some pointer in buffer header
-to cleanup function.
-
-> But then you'd be pushing VACUUM's work into productive transactions,
-> which is probably not the way to go.
-
-Not big work - I wouldn't worry about it.
-
-> > Two ways: hold index page lock untill heap tuple is checked
-> > or (rough schema) store info in shmem (just IndexTupleData.t_tid
-> > and flag) that an index tuple is used by some scan so cleaner could
-> > change stored TID (get one from prev index tuple) and set flag to
-> > help scan restore its current position on return.
->
-> Another way is to mark the index tuple "gone but not forgotten", so to
-> speak --- mark it dead without removing it. (We could know that we need
-> to do that if we see someone else has a buffer pin on the index page.)
-
-Register cleanup function just like with heap above.
-
-> None of these seem real clean though. Needs more thought.
-
-Vadim
-
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-From vmikheev@SECTORBASE.COM Mon May 21 14:02:52 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>
-cc: Tom Lane <tgl@sss.pgh.pa.us>, "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 11:01:45 -0700
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-> > My point is that we'll need in dynamic cleanup anyway and UNDO is
-> > what should be implemented for dynamic cleanup of aborted changes.
->
-> I do not yet understand why you want to handle aborts different than
-> outdated tuples.
-
-Maybe because of aborted tuples have shorter Time-To-Live.
-And probability to find pages for them in buffer pool is higher.
-
-> The ratio in a well tuned system should well favor outdated tuples.
-> If someone ever adds "dirty read" it is also not the case that it
-> is guaranteed, that nobody accesses the tuple you currently want
-> to undo. So I really miss to see the big difference.
-
-It will not be guaranteed anyway as soon as we start removing
-tuples without exclusive access to relation.
-
-And, I cannot say that I would implement UNDO because of
-1. (cleanup) OR 2. (savepoints) OR 4. (pg_log management)
-but because of ALL of 1., 2., 4.
-
-Vadim
-
-From pgsql-hackers-owner+M9152@postgresql.org Mon May 21 16:18:57 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Tom Lane'" <tgl@sss.pgh.pa.us>
-cc: pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 13:01:29 -0700
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-> > We could keep share buffer lock (or add some other kind of lock)
-> > untill tuple projected - after projection we need not to read data
-> > for fetched tuple from shared buffer and time between fetching
-> > tuple and projection is very short, so keeping lock on buffer will
-> > not impact concurrency significantly.
->
-> Or drop the pin on the buffer to show we no longer have a pointer
-> to it. I'm not sure that the time to do projection is short though
-> --- what if there are arbitrary user-defined functions in the quals
-> or the projection targetlist?
-
-Well, while we are on this subject I finally should say about issue
-bothered me for long time: only "simple" functions should be allowed
-to deal with data in shared buffers directly. "Simple" means: no SQL
-queries there. Why? One reason: we hold shlock on buffer while doing
-seqscan qual - what if qual' SQL queries will try to acquire exclock
-on the same buffer? Another reason - concurrency. I think that such
-"heavy" functions should be provided with copy of data.
-
-Vadim
-
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-From pgsql-hackers-owner+M9160@postgresql.org Mon May 21 20:38:44 2001
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-Date: Mon, 21 May 2001 17:18:18 -0700
-From: Barry Lind <barry@xythos.com>
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-To: pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <3705826352029646A3E91C53F7189E3201662E@sectorbase2.sectorbase.com>
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-
-
-Mikheev, Vadim wrote:
-
->
-> Ok, last reminder -:))
->
-> On transaction abort, read WAL records and undo (rollback)
-> changes made in storage. Would allow:
->
-> 1. Reclaim space allocated by aborted transactions.
-> 2. Implement SAVEPOINTs.
-> Just to remind -:) - in the event of error discovered by server
-> - duplicate key, deadlock, command mistyping, etc, - transaction
-> will be rolled back to the nearest implicit savepoint setted
-> just before query execution; - or transaction can be aborted by
-> ROLLBACK TO <savepoint_name> command to some explicit savepoint
-> setted by user. Transaction rolled back to savepoint may be continued.
-> 3. Reuse transaction IDs on postmaster restart.
-> 4. Split pg_log into small files with ability to remove old ones (which
-> do not hold statuses for any running transactions).
->
-> Vadim
-
-This is probably not a good thread to add my two cents worth, but here
-goes anyway.
-
-The biggest issue I see with the proposed UNDO using WAL is the issue of
-large/long lasting transactions. While it might be possible to solve
-this problem with some extra work. However keep in mind that different
-types of transactions (i.e. normal vs bulk loads) require different
-amounts of time and/or UNDO. To solve this problem, you really need per
-transaction limits which seems difficult to implement.
-
-I have no doubt that UNDO with WAL can be done. But is there some other
-way of doing UNDO that might be just as good or better?
-
-Part of what I see in this thread reading between the lines is that some
-believe the solution to many problems in the long term is to implement
-an overwriting storage manager. Implementing UNDO via WAL is a
-necessary step in that direction. While others seem to believe that the
-non-overwriting storage manager has some life in it yet, and may even be
-the storage manager for many releases to come. I don't know enough
-about the internals to have any say in that discussion, however the
-grass isn't always greener on the other side of the fence (i.e. an
-overwriting storage manager will come with its own set of problems/issues).
-
-So let me throw out an idea for UNDO using the current storage manager.
-First let me state that UNDO is a bit of a misnomer, since undo for
-transactions is already implemented. That is what pg_log is all about.
-The part of UNDO that is missing is savepoints (either explicit or
-implicit), because pg_log doesn't capture the information for each
-command in a transaction. So the question really becomes, how to
-implement savepoints with current storage manager?
-
-I am going to lay out one assumption that I am making:
-1) Most transactions are either completely successful or completely
-rolled back
- (If this weren't true, i.e. you really needed savepoints to partially
-rollback changes, you couldn't be using the existing version of
-postgresql at all)
-
-My proposal is:
- 1) create a new relation to store 'failed commands' for transactions.
- This is similar to pg_log for transactions, but takes it to the
-command level. And since it records only failed commands (or ranges of
-failed commands), thus most transactions will not have any information
-in this relation per the assumption above.
- 2) Use the unused pg_log status (3 = unused, 2 = commit, 1 = abort, 0
-= inprocess) to mean that the transaction was commited but some commands
-were rolled back (i.e. partial commit)
- Again for the majority of transactions nothing will need to change,
-since they will still be marked as committed or aborted.
- 3) Code that determines whether or not a tuple is committed or not
-needs to be aware of this new pg_log status, and look in the new
-relation to see if the particular command was rolled back or not to
-determine the commited status of the tuple. This subtly changes the
-meaning of HEAP_XMIN_COMMITTED and related flags to reflect the
-transaction and command status instead of just the transaction status.
-
-The runtime cost of this shouldn't be too high, since the committed
-state is cached in HEAP_XMIN_COMMITTED et al, it is only the added cost
-for the pass that needs to set these flags, and then there is only added
-cost in the case that the transaction wasn't completely sucessful (again
-my assumption above).
-
-Now I have know idea if what I am proposing is really doable or not. I
-am just throwing this out as an alternative to WAL based
-UNDO/savepoints. The reason I am doing this is that to me it seems to
-leverage much of the existing infrastructure already in place that
-performs undo for rolledback transactions (all the tmin, tmax, cmin,
-cmax stuff as well as vacuum). Also it doesn't come with the large WAL
-log file problem for large transactions.
-
-Now having said all of this I realize that this doesn't solve the 4
-billion transaction id limit problem, or the large size of the pg_log
-file with large numbers of transactions.
-
-thanks,
---Barry
-
-
->
-
-
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-From pgsql-hackers-owner+M9208@postgresql.org Tue May 22 14:02:04 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105221732.f4MHWef08905@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3705826352029646A3E91C53F7189E32016637@sectorbase2.sectorbase.com>
- "from Mikheev, Vadim at May 21, 2001 11:01:45 am"
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-Date: Tue, 22 May 2001 13:32:40 -0400 (EDT)
-cc: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
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-[ Charset ISO-8859-1 unsupported, converting... ]
-> > > My point is that we'll need in dynamic cleanup anyway and UNDO is
-> > > what should be implemented for dynamic cleanup of aborted changes.
-> >
-> > I do not yet understand why you want to handle aborts different than
-> > outdated tuples.
->
-> Maybe because of aborted tuples have shorter Time-To-Live.
-> And probability to find pages for them in buffer pool is higher.
-
-This brings up an idea I had about auto-vacuum. I wonder if autovacuum
-could do most of its work by looking at the buffer cache pages and
-commit xids. Seems it would be quite easy record freespace in pages
-already in the buffer and collect that information for other backends to
-use. It could also move tuples between cache pages with little
-overhead.
-
-There wouldn't be an I/O overhead, and frequently used tables are
-already in the cache.
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-From pgsql-hackers-owner+M9209@postgresql.org Tue May 22 14:42:27 2001
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-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105221747.f4MHlF409586@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3705826352029646A3E91C53F7189E32016637@sectorbase2.sectorbase.com>
- "from Mikheev, Vadim at May 21, 2001 11:01:45 am"
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-Date: Tue, 22 May 2001 13:47:15 -0400 (EDT)
-cc: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
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-
-> > The ratio in a well tuned system should well favor outdated tuples.
-> > If someone ever adds "dirty read" it is also not the case that it
-> > is guaranteed, that nobody accesses the tuple you currently want
-> > to undo. So I really miss to see the big difference.
->
-> It will not be guaranteed anyway as soon as we start removing
-> tuples without exclusive access to relation.
->
-> And, I cannot say that I would implement UNDO because of
-> 1. (cleanup) OR 2. (savepoints) OR 4. (pg_log management)
-> but because of ALL of 1., 2., 4.
-
-OK, I understand your reasoning here, but I want to make a comment.
-
-Looking at the previous features you added, like subqueries, MVCC, or
-WAL, these were major features that greatly enhanced the system's
-capabilities.
-
-Now, looking at UNDO, I just don't see it in the same league as those
-other additions. Of course, you can work on whatever you want, but I
-was hoping to see another major feature addition for 7.2. We know we
-badly need auto-vacuum, improved replication, and point-in-time recover.
-
-I can see UNDO improving row reuse, and making subtransactions and
-pg_log compression easier, but these items do not require UNDO.
-
-In fact, I am unsure why we would want an UNDO way of reusing rows of
-aborted transactions and an autovacuum way of reusing rows from
-committed transactions, expecially because aborted transactions account
-for <5% of all transactions. It would be better to put work into one
-mechanism that would reuse all tuples.
-
-If UNDO came with no limitations, it may be a good option, but the need
-to carry tuples until transaction commit does add an extra burden on
-programmers and administrators, and I just don't see what we are getting
-for it.
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-Message-ID: <3705826352029646A3E91C53F7189E32016648@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-cc: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 14:33:38 -0700
-MIME-Version: 1.0
-X-Mailer: Internet Mail Service (5.5.2653.19)
-Content-Type: text/plain;
- charset="iso-8859-1"
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-Status: OR
-
-> > And, I cannot say that I would implement UNDO because of
-> > 1. (cleanup) OR 2. (savepoints) OR 4. (pg_log management)
-> > but because of ALL of 1., 2., 4.
->
-> OK, I understand your reasoning here, but I want to make a comment.
->
-> Looking at the previous features you added, like subqueries, MVCC, or
-> WAL, these were major features that greatly enhanced the system's
-> capabilities.
->
-> Now, looking at UNDO, I just don't see it in the same league as those
-> other additions. Of course, you can work on whatever you want, but I
-> was hoping to see another major feature addition for 7.2. We know we
-> badly need auto-vacuum, improved replication, and point-in-time recover.
-
-I don't like auto-vacuum approach in long term, WAL-based BAR is too easy
-to do -:) (and you know that there is man who will do it, probably),
-bidirectional sync replication is good to work on, but I'm more
-interested in storage/transaction management now. And I'm not sure
-if I'll have enough time for "another major feature in 7.2" anyway.
-
-> It would be better to put work into one mechanism that would
-> reuse all tuples.
-
-This is what we're discussing now -:)
-If community will not like UNDO then I'll probably try to implement
-dead space collector which will read log files and so on. Easy to
-#ifdef it in 7.2 to use in 7.3 (or so) with on-disk FSM. Also, I have
-to implement logging for non-btree indices (anyway required for UNDO,
-WAL-based BAR, WAL-based space reusing).
-
-Vadim
-
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-From vmikheev@SECTORBASE.COM Tue May 22 17:34:52 2001
-Return-path: <vmikheev@SECTORBASE.COM>
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-Message-ID: <3705826352029646A3E91C53F7189E32016648@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-cc: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 14:33:38 -0700
-MIME-Version: 1.0
-X-Mailer: Internet Mail Service (5.5.2653.19)
-Content-Type: text/plain;
- charset="iso-8859-1"
-Status: OR
-
-> > And, I cannot say that I would implement UNDO because of
-> > 1. (cleanup) OR 2. (savepoints) OR 4. (pg_log management)
-> > but because of ALL of 1., 2., 4.
->
-> OK, I understand your reasoning here, but I want to make a comment.
->
-> Looking at the previous features you added, like subqueries, MVCC, or
-> WAL, these were major features that greatly enhanced the system's
-> capabilities.
->
-> Now, looking at UNDO, I just don't see it in the same league as those
-> other additions. Of course, you can work on whatever you want, but I
-> was hoping to see another major feature addition for 7.2. We know we
-> badly need auto-vacuum, improved replication, and point-in-time recover.
-
-I don't like auto-vacuum approach in long term, WAL-based BAR is too easy
-to do -:) (and you know that there is man who will do it, probably),
-bidirectional sync replication is good to work on, but I'm more
-interested in storage/transaction management now. And I'm not sure
-if I'll have enough time for "another major feature in 7.2" anyway.
-
-> It would be better to put work into one mechanism that would
-> reuse all tuples.
-
-This is what we're discussing now -:)
-If community will not like UNDO then I'll probably try to implement
-dead space collector which will read log files and so on. Easy to
-#ifdef it in 7.2 to use in 7.3 (or so) with on-disk FSM. Also, I have
-to implement logging for non-btree indices (anyway required for UNDO,
-WAL-based BAR, WAL-based space reusing).
-
-Vadim
-
-From Inoue@tpf.co.jp Tue May 22 20:49:08 2001
-Return-path: <Inoue@tpf.co.jp>
-Received: from sd.tpf.co.jp (IDENT:qmailr@sd.tpf.co.jp [210.161.239.34])
- by candle.pha.pa.us (8.10.1/8.10.1) with SMTP id f4N0n6Q16869
- for <pgman@candle.pha.pa.us>; Tue, 22 May 2001 20:49:06 -0400 (EDT)
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- by viscomail.tpf.co.jp (8.8.8+Sun/8.8.8) with ESMTP id JAA10910;
- Wed, 23 May 2001 09:48:57 +0900 (JST)
-Message-ID: <3B0B091D.A5AF412E@tpf.co.jp>
-Date: Wed, 23 May 2001 09:49:33 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
-X-Mailer: Mozilla 4.73 [ja] (Windows NT 5.0; U)
-X-Accept-Language: ja
-MIME-Version: 1.0
-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105221747.f4MHlF409586@candle.pha.pa.us>
-Content-Type: text/plain; charset=iso-2022-jp
-Content-Transfer-Encoding: 7bit
-Status: ORr
-
-Bruce Momjian wrote:
->
-> > > The ratio in a well tuned system should well favor outdated tuples.
-> > > If someone ever adds "dirty read" it is also not the case that it
-> > > is guaranteed, that nobody accesses the tuple you currently want
-> > > to undo. So I really miss to see the big difference.
-> >
-> > It will not be guaranteed anyway as soon as we start removing
-> > tuples without exclusive access to relation.
-> >
-> > And, I cannot say that I would implement UNDO because of
-> > 1. (cleanup) OR 2. (savepoints) OR 4. (pg_log management)
-> > but because of ALL of 1., 2., 4.
->
-> OK, I understand your reasoning here, but I want to make a comment.
->
-> Looking at the previous features you added, like subqueries, MVCC, or
-> WAL, these were major features that greatly enhanced the system's
-> capabilities.
->
-> Now, looking at UNDO, I just don't see it in the same league as those
-> other additions.
-
-Hmm hasn't it been an agreement ? I know UNDO was planned
-for 7.0 and I've never heard objections about it until
-recently. People also have referred to an overwriting smgr
-easily. Please tell me how to introduce an overwriting smgr
-without UNDO.
-
-regards,
-Hiroshi Inoue
-
-From pgsql-hackers-owner+M9233@postgresql.org Tue May 22 21:11:29 2001
-Return-path: <pgsql-hackers-owner+M9233@postgresql.org>
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- by candle.pha.pa.us (8.10.1/8.10.1) with ESMTP id f4N1BSQ24335
- for <pgman@candle.pha.pa.us>; Tue, 22 May 2001 21:11:28 -0400 (EDT)
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- Tue, 22 May 2001 20:53:23 -0400 (EDT)
-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105230053.f4N0rNY17041@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3B0B091D.A5AF412E@tpf.co.jp> "from Hiroshi Inoue at May 23, 2001
- 09:49:33 am"
-To: Hiroshi Inoue <Inoue@tpf.co.jp>
-Date: Tue, 22 May 2001 20:53:23 -0400 (EDT)
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-X-Mailer: ELM [version 2.4ME+ PL90 (25)]
-MIME-Version: 1.0
-Content-Transfer-Encoding: 7bit
-Content-Type: text/plain; charset=US-ASCII
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-> > Looking at the previous features you added, like subqueries, MVCC, or
-> > WAL, these were major features that greatly enhanced the system's
-> > capabilities.
-> >
-> > Now, looking at UNDO, I just don't see it in the same league as those
-> > other additions.
->
-> Hmm hasn't it been an agreement ? I know UNDO was planned
-> for 7.0 and I've never heard objections about it until
-> recently. People also have referred to an overwriting smgr
-> easily. Please tell me how to introduce an overwriting smgr
-> without UNDO.
-
-I guess that is the question. Are we heading for an overwriting storage
-manager? I didn't see that in Vadim's list of UNDO advantages, but
-maybe that is his final goal. If so UNDO may make sense, but then the
-question is how do we keep MVCC with an overwriting storage manager?
-
-The only way I can see doing it is to throw the old tuples into the WAL
-and have backends read through that for MVCC info.
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
----------------------------(end of broadcast)---------------------------
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-From Inoue@tpf.co.jp Tue May 22 23:04:52 2001
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-Message-ID: <3B0B28F3.47F70E0F@tpf.co.jp>
-Date: Wed, 23 May 2001 12:05:23 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
-X-Mailer: Mozilla 4.73 [ja] (Windows NT 5.0; U)
-X-Accept-Language: ja
-MIME-Version: 1.0
-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us>
-Content-Type: text/plain; charset=iso-2022-jp
-Content-Transfer-Encoding: 7bit
-Status: OR
-
-
-
-Bruce Momjian wrote:
->
-> > > Looking at the previous features you added, like subqueries, MVCC, or
-> > > WAL, these were major features that greatly enhanced the system's
-> > > capabilities.
-> > >
-> > > Now, looking at UNDO, I just don't see it in the same league as those
-> > > other additions.
-> >
-> > Hmm hasn't it been an agreement ? I know UNDO was planned
-> > for 7.0 and I've never heard objections about it until
-> > recently. People also have referred to an overwriting smgr
-> > easily. Please tell me how to introduce an overwriting smgr
-> > without UNDO.
->
-> I guess that is the question. Are we heading for an overwriting storage
-> manager?
-
-I've never heard that it was given up. So there seems to be
-at least a possibility to introduce it in the future.
-PostgreSQL could have lived without UNDO due to its no
-overwrite smgr. I don't know if avoiding UNDO is possible
-to implement partial rollback(I don't think it's easy
-anyway). However it seems harmful for the future
-implementation of an overwriting smgr if we would
-introduce it.
-
-> I didn't see that in Vadim's list of UNDO advantages, but
-> maybe that is his final goal.
-> If so UNDO may make sense, but then the
-> question is how do we keep MVCC with an overwriting storage manager?
->
-
-It doesn't seem easy. ISTM it's one of the main reason we
-couldn't introduce an overwriting smgr in 7.2.
-
-regards,
-Hiroshi Inoue
-
-From pgsql-hackers-owner+M9241@postgresql.org Tue May 22 23:20:08 2001
-Return-path: <pgsql-hackers-owner+M9241@postgresql.org>
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- by candle.pha.pa.us (8.10.1/8.10.1) with ESMTP id f4N3K7Q00337
- for <pgman@candle.pha.pa.us>; Tue, 22 May 2001 23:20:07 -0400 (EDT)
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- Wed, 23 May 2001 12:04:47 +0900 (JST)
-Message-ID: <3B0B28F3.47F70E0F@tpf.co.jp>
-Date: Wed, 23 May 2001 12:05:23 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
-X-Mailer: Mozilla 4.73 [ja] (Windows NT 5.0; U)
-X-Accept-Language: ja
-MIME-Version: 1.0
-To: Bruce Momjian <pgman@candle.pha.pa.us>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us>
-Content-Type: text/plain; charset=iso-2022-jp
-Content-Transfer-Encoding: 7bit
-Precedence: bulk
-Sender: pgsql-hackers-owner@postgresql.org
-Status: OR
-
-
-
-Bruce Momjian wrote:
->
-> > > Looking at the previous features you added, like subqueries, MVCC, or
-> > > WAL, these were major features that greatly enhanced the system's
-> > > capabilities.
-> > >
-> > > Now, looking at UNDO, I just don't see it in the same league as those
-> > > other additions.
-> >
-> > Hmm hasn't it been an agreement ? I know UNDO was planned
-> > for 7.0 and I've never heard objections about it until
-> > recently. People also have referred to an overwriting smgr
-> > easily. Please tell me how to introduce an overwriting smgr
-> > without UNDO.
->
-> I guess that is the question. Are we heading for an overwriting storage
-> manager?
-
-I've never heard that it was given up. So there seems to be
-at least a possibility to introduce it in the future.
-PostgreSQL could have lived without UNDO due to its no
-overwrite smgr. I don't know if avoiding UNDO is possible
-to implement partial rollback(I don't think it's easy
-anyway). However it seems harmful for the future
-implementation of an overwriting smgr if we would
-introduce it.
-
-> I didn't see that in Vadim's list of UNDO advantages, but
-> maybe that is his final goal.
-> If so UNDO may make sense, but then the
-> question is how do we keep MVCC with an overwriting storage manager?
->
-
-It doesn't seem easy. ISTM it's one of the main reason we
-couldn't introduce an overwriting smgr in 7.2.
-
-regards,
-Hiroshi Inoue
-
----------------------------(end of broadcast)---------------------------
-TIP 3: if posting/reading through Usenet, please send an appropriate
-subscribe-nomail command to majordomo@postgresql.org so that your
-message can get through to the mailing list cleanly
-
-From pjw@rhyme.com.au Wed May 23 05:01:44 2001
-Return-path: <pjw@rhyme.com.au>
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- for <pgman@candle.pha.pa.us>; Wed, 23 May 2001 05:01:42 -0400 (EDT)
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-Date: Wed, 23 May 2001 18:58:58 +1000
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-From: Philip Warner <pjw@rhyme.com.au>
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-cc: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-In-Reply-To: <3705826352029646A3E91C53F7189E32016648@sectorbase2.sectorb
- ase.com>
-MIME-Version: 1.0
-Content-Type: text/plain; charset="us-ascii"
-Status: OR
-
-At 14:33 22/05/01 -0700, Mikheev, Vadim wrote:
->
->If community will not like UNDO then I'll probably try to implement
->dead space collector which will read log files and so on.
-
-I'd vote for UNDO; in terms of usability & friendliness it's a big win.
-Tom's plans for FSM etc are, at least, going to get us some useful data,
-and at best will mean we can hang of WAL based FSM for a few versions.
-
-
-
-
-----------------------------------------------------------------
-Philip Warner | __---_____
-Albatross Consulting Pty. Ltd. |----/ - \
-(A.B.N. 75 008 659 498) | /(@) ______---_
-Tel: (+61) 0500 83 82 81 | _________ \
-Fax: (+61) 0500 83 82 82 | ___________ |
-Http://www.rhyme.com.au | / \|
- | --________--
-PGP key available upon request, | /
-and from pgp5.ai.mit.edu:11371 |/
-
-From pgsql-hackers-owner+M9249@postgresql.org Wed May 23 06:18:40 2001
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-To: Hiroshi Inoue <Inoue@tpf.co.jp>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105221747.f4MHlF409586@candle.pha.pa.us> <3B0B091D.A5AF412E@tpf.co.jp>
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-
-Hiroshi Inoue wrote:
->
-> People also have referred to an overwriting smgr easily.
-
-I am all for an overwriting smgr, but as a feature that can be selected
-on a table-by table basis (or at least in compile time), not as an
-overall change
-
-> Please tell me how to introduce an overwriting smgr
-> without UNDO.
-
-I would much more like a dead-space-reusing smgr on top of MVCC which
-does
-not touch live transactions.
-
-------------------
-Hannu
-
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-From tgl@sss.pgh.pa.us Wed May 23 15:31:29 2001
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-To: Hiroshi Inoue <Inoue@tpf.co.jp>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev,
- Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3B0B28F3.47F70E0F@tpf.co.jp>
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us> <3B0B28F3.47F70E0F@tpf.co.jp>
-Comments: In-reply-to Hiroshi Inoue <Inoue@tpf.co.jp>
- message dated "Wed, 23 May 2001 12:05:23 +0900"
-Date: Wed, 23 May 2001 15:31:10 -0400
-Message-ID: <16593.990646270@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Status: OR
-
-Hiroshi Inoue <Inoue@tpf.co.jp> writes:
->> I guess that is the question. Are we heading for an overwriting storage
->> manager?
-
-> I've never heard that it was given up. So there seems to be
-> at least a possibility to introduce it in the future.
-
-Unless we want to abandon MVCC (which I don't), I think an overwriting
-smgr is impractical. We need a more complex space-reuse scheme than
-that.
-
- regards, tom lane
-
-From Inoue@tpf.co.jp Wed May 23 19:14:31 2001
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-Message-ID: <3B0C447A.E6EF4AF3@tpf.co.jp>
-Date: Thu, 24 May 2001 08:15:06 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
-X-Mailer: Mozilla 4.73 [ja] (Windows NT 5.0; U)
-X-Accept-Language: ja
-MIME-Version: 1.0
-To: Tom Lane <tgl@sss.pgh.pa.us>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us> <3B0B28F3.47F70E0F@tpf.co.jp> <16593.990646270@sss.pgh.pa.us>
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-Status: OR
-
-Tom Lane wrote:
->
-> Hiroshi Inoue <Inoue@tpf.co.jp> writes:
-> >> I guess that is the question. Are we heading for an overwriting storage
-> >> manager?
->
-> > I've never heard that it was given up. So there seems to be
-> > at least a possibility to introduce it in the future.
->
-> Unless we want to abandon MVCC (which I don't), I think an overwriting
-> smgr is impractical.
-
-Impractical ? Oracle does it.
-
-> We need a more complex space-reuse scheme than
-> that.
->
-
-IMHO we have to decide which to go now.
-As I already mentioned, changing current handling
-of transactionId/CommandId to avoid UNDO is not
-only useless but also harmful for an overwriting
-smgr.
-
-regards,
-Hiroshi Inoue
-
-From dhogaza@pacifier.com Wed May 23 19:25:51 2001
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-Date: Wed, 23 May 2001 16:24:48 -0700
-To: Hiroshi Inoue <Inoue@tpf.co.jp>, Tom Lane <tgl@sss.pgh.pa.us>
-From: Don Baccus <dhogaza@pacifier.com>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
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-
-At 08:15 AM 5/24/01 +0900, Hiroshi Inoue wrote:
-
->> Unless we want to abandon MVCC (which I don't), I think an overwriting
->> smgr is impractical.
->
->Impractical ? Oracle does it.
-
-It's not easy, though ... the current PG scheme has the advantage of being
-relatively simple and probably more efficient than scanning logs like
-Oracle has to do (assuming your datafiles aren't thoroughly clogged with
-old dead tuples).
-
-Has anyone looked at InterBase for hints for space-reusing strategies?
-
-As I understand it, they have a tuple-versioning scheme similar to PG's.
-
-If nothing else, something might be learned as to the efficiency and
-effectiveness of one particular approach to solving the problem.
-
-
-
-- Don Baccus, Portland OR <dhogaza@pacifier.com>
- Nature photos, on-line guides, Pacific Northwest
- Rare Bird Alert Service and other goodies at
- http://donb.photo.net.
-
-From pgsql-hackers-owner+M9291@postgresql.org Wed May 23 22:29:59 2001
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-Message-ID: <3B0C7048.902DD407@tpf.co.jp>
-Date: Thu, 24 May 2001 11:22:00 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
-X-Mailer: Mozilla 4.73 [ja] (Windows NT 5.0; U)
-X-Accept-Language: ja
-MIME-Version: 1.0
-To: Don Baccus <dhogaza@pacifier.com>
-cc: Tom Lane <tgl@sss.pgh.pa.us>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us>
- <3B0B28F3.47F70E0F@tpf.co.jp>
- <16593.990646270@sss.pgh.pa.us> <3.0.1.32.20010523162448.01797330@mail.pacifier.com>
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-Status: OR
-
-Don Baccus wrote:
->
-> At 08:15 AM 5/24/01 +0900, Hiroshi Inoue wrote:
->
-> >> Unless we want to abandon MVCC (which I don't), I think an overwriting
-> >> smgr is impractical.
-> >
-> >Impractical ? Oracle does it.
->
-> It's not easy, though ... the current PG scheme has the advantage of being
-> relatively simple and probably more efficient than scanning logs like
-> Oracle has to do (assuming your datafiles aren't thoroughly clogged with
-> old dead tuples).
->
-
-I think so too. I've never said that an overwriting smgr
-is easy and I don't love it particularily.
-
-What I'm objecting is to avoid UNDO without giving up
-an overwriting smgr. We shouldn't be noncommittal now.
-
-regards,
-Hiroshi Inoue
-
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-From Inoue@tpf.co.jp Wed May 23 22:21:26 2001
-Return-path: <Inoue@tpf.co.jp>
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-Date: Thu, 24 May 2001 11:22:00 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
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-X-Accept-Language: ja
-MIME-Version: 1.0
-To: Don Baccus <dhogaza@pacifier.com>
-cc: Tom Lane <tgl@sss.pgh.pa.us>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us>
- <3B0B28F3.47F70E0F@tpf.co.jp>
- <16593.990646270@sss.pgh.pa.us> <3.0.1.32.20010523162448.01797330@mail.pacifier.com>
-Content-Type: text/plain; charset=iso-2022-jp
-Content-Transfer-Encoding: 7bit
-Status: OR
-
-Don Baccus wrote:
->
-> At 08:15 AM 5/24/01 +0900, Hiroshi Inoue wrote:
->
-> >> Unless we want to abandon MVCC (which I don't), I think an overwriting
-> >> smgr is impractical.
-> >
-> >Impractical ? Oracle does it.
->
-> It's not easy, though ... the current PG scheme has the advantage of being
-> relatively simple and probably more efficient than scanning logs like
-> Oracle has to do (assuming your datafiles aren't thoroughly clogged with
-> old dead tuples).
->
-
-I think so too. I've never said that an overwriting smgr
-is easy and I don't love it particularily.
-
-What I'm objecting is to avoid UNDO without giving up
-an overwriting smgr. We shouldn't be noncommittal now.
-
-regards,
-Hiroshi Inoue
-
-From dhogaza@pacifier.com Thu May 24 08:55:51 2001
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-Date: Wed, 23 May 2001 21:42:43 -0700
-To: Tom Lane <tgl@sss.pgh.pa.us>, Hiroshi Inoue <Inoue@tpf.co.jp>
-From: Don Baccus <dhogaza@pacifier.com>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-In-Reply-To: <26782.990673338@sss.pgh.pa.us>
-References: <3B0C447A.E6EF4AF3@tpf.co.jp>
- <200105230053.f4N0rNY17041@candle.pha.pa.us>
- <3B0B28F3.47F70E0F@tpf.co.jp>
- <16593.990646270@sss.pgh.pa.us>
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-
-At 11:02 PM 5/23/01 -0400, Tom Lane wrote:
->Hiroshi Inoue <Inoue@tpf.co.jp> writes:
->> Tom Lane wrote:
->>> Unless we want to abandon MVCC (which I don't), I think an overwriting
->>> smgr is impractical.
->
->> Impractical ? Oracle does it.
->
->Oracle has MVCC?
-
-With restrictions, yes. You didn't know that? Vadim did ...
-
-
-
-- Don Baccus, Portland OR <dhogaza@pacifier.com>
- Nature photos, on-line guides, Pacific Northwest
- Rare Bird Alert Service and other goodies at
- http://donb.photo.net.
-
-From pgsql-hackers-owner+M9319@postgresql.org Thu May 24 13:21:55 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'"
- <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 24 May 2001 10:00:31 -0700
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-
-> >> Impractical ? Oracle does it.
-> >
-> >Oracle has MVCC?
->
-> With restrictions, yes.
-
-What restrictions? Rollback segments size?
-Non-overwriting smgr can eat all disk space...
-
-> You didn't know that? Vadim did ...
-
-Didn't I mention a few times that I was
-inspired by Oracle? -:)
-
-Vadim
-
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-From pgsql-hackers-owner+M9322@postgresql.org Thu May 24 13:49:18 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>, Hiroshi Inoue <Inoue@tpf.co.jp>
-cc: The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 24 May 2001 10:30:39 -0700
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-
-> If PostgreSQL wants to stay MVCC, then we should imho forget
-> "overwriting smgr" very fast.
->
-> Let me try to list the pros and cons that I can think of:
-> Pro:
-> no index modification if key stays same
-> no search for free space for update (if tuple still
-> fits into page)
-> no pg_log
-> Con:
-> additional IO to write "before image" to rollback segment
-> (every before image, not only first after checkpoint)
-> (also before image of every index page that is updated !)
-
-I don't think that Oracle writes entire page as before image - just
-tuple data and some control info. As for additional IO - we'll do it
-anyway to remove "before image" (deleted tuple data) from data files.
-
-Vadim
-
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-From vmikheev@SECTORBASE.COM Thu May 24 13:31:55 2001
-Return-path: <vmikheev@SECTORBASE.COM>
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>, Hiroshi Inoue <Inoue@tpf.co.jp>
-cc: The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 24 May 2001 10:30:39 -0700
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-
-> If PostgreSQL wants to stay MVCC, then we should imho forget
-> "overwriting smgr" very fast.
->
-> Let me try to list the pros and cons that I can think of:
-> Pro:
-> no index modification if key stays same
-> no search for free space for update (if tuple still
-> fits into page)
-> no pg_log
-> Con:
-> additional IO to write "before image" to rollback segment
-> (every before image, not only first after checkpoint)
-> (also before image of every index page that is updated !)
-
-I don't think that Oracle writes entire page as before image - just
-tuple data and some control info. As for additional IO - we'll do it
-anyway to remove "before image" (deleted tuple data) from data files.
-
-Vadim
-
-From pgsql-hackers-owner+M9327@postgresql.org Thu May 24 14:23:44 2001
-Return-path: <pgsql-hackers-owner+M9327@postgresql.org>
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-Message-ID: <3705826352029646A3E91C53F7189E32016652@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Hiroshi Inoue'" <Inoue@tpf.co.jp>, Don Baccus <dhogaza@pacifier.com>
-cc: Tom Lane <tgl@sss.pgh.pa.us>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 24 May 2001 10:57:19 -0700
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-
-> I think so too. I've never said that an overwriting smgr
-> is easy and I don't love it particularily.
->
-> What I'm objecting is to avoid UNDO without giving up
-> an overwriting smgr. We shouldn't be noncommittal now.
-
-Why not? We could decide to do overwriting smgr later
-and implement UNDO then. For the moment we could just
-change checkpointer to use checkpoint.redo instead of
-checkpoint.undo when defining what log files should be
-deleted - it's a few minutes deal, and so is changing it
-back.
-
-Vadim
-
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-From dhogaza@pacifier.com Mon May 28 10:42:51 2001
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-Date: Thu, 24 May 2001 11:16:46 -0700
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>
-From: Don Baccus <dhogaza@pacifier.com>
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-In-Reply-To: <3705826352029646A3E91C53F7189E32016650@sectorbase2.sectorb
- ase.com>
-MIME-Version: 1.0
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-
-At 10:00 AM 5/24/01 -0700, Mikheev, Vadim wrote:
->> >> Impractical ? Oracle does it.
->> >
->> >Oracle has MVCC?
->>
->> With restrictions, yes.
->
->What restrictions? Rollback segments size?
->Non-overwriting smgr can eat all disk space...
-
-Actually, the restriction I'm thinking about isn't MVCC related, per
-se, but a within-transaction restriction. The infamous "mutating table"
-error.
-
->> You didn't know that? Vadim did ...
->
->Didn't I mention a few times that I was
->inspired by Oracle? -:)
-
-Yes, you most certainly have!
-
-
-
-- Don Baccus, Portland OR <dhogaza@pacifier.com>
- Nature photos, on-line guides, Pacific Northwest
- Rare Bird Alert Service and other goodies at
- http://donb.photo.net.
-
-From pgsql-hackers-owner+M9344@postgresql.org Thu May 24 20:00:27 2001
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-Date: Fri, 25 May 2001 01:51:06 +0500
-From: Hannu Krosing <hannu@tm.ee>
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-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
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-
-"Mikheev, Vadim" wrote:
->
-> > >> Impractical ? Oracle does it.
-> > >
-> > >Oracle has MVCC?
-> >
-> > With restrictions, yes.
->
-> What restrictions? Rollback segments size?
-> Non-overwriting smgr can eat all disk space...
-
-Is'nt the same true for an overwriting smgr ? ;)
-
-> > You didn't know that? Vadim did ...
->
-> Didn't I mention a few times that I was
-> inspired by Oracle? -:)
-
-How does it do MVCC with an overwriting storage manager ?
-
-Could it possibly be a Postgres-inspired bolted-on hack
-needed for better concurrency ?
-
-
-BTW, are you aware how Interbase does its MVCC - is it more
-like Oracle's way or like PostgreSQL's ?
-
-----------------
-Hannu
-
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-From pgsql-hackers-owner+M9345@postgresql.org Thu May 24 20:14:28 2001
-Return-path: <pgsql-hackers-owner+M9345@postgresql.org>
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- by candle.pha.pa.us (8.10.1/8.10.1) with ESMTP id f4P0ERt20188
- for <pgman@candle.pha.pa.us>; Thu, 24 May 2001 20:14:27 -0400 (EDT)
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- Thu, 24 May 2001 20:13:12 -0400 (EDT)
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-Date: Fri, 25 May 2001 02:05:19 +0500
-From: Hannu Krosing <hannu@tm.ee>
-X-Mailer: Mozilla 4.76 [en] (X11; U; Linux 2.4.2-2 i686)
-X-Accept-Language: en, ru, et
-MIME-Version: 1.0
-To: Don Baccus <dhogaza@pacifier.com>
-cc: Hiroshi Inoue <Inoue@tpf.co.jp>, Tom Lane <tgl@sss.pgh.pa.us>,
- Bruce Momjian <pgman@candle.pha.pa.us>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <200105230053.f4N0rNY17041@candle.pha.pa.us>
- <3B0B28F3.47F70E0F@tpf.co.jp>
- <16593.990646270@sss.pgh.pa.us> <3.0.1.32.20010523162448.01797330@mail.pacifier.com>
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-
-Don Baccus wrote:
->
-> At 08:15 AM 5/24/01 +0900, Hiroshi Inoue wrote:
->
-> >> Unless we want to abandon MVCC (which I don't), I think an overwriting
-> >> smgr is impractical.
-> >
-> >Impractical ? Oracle does it.
->
-> It's not easy, though ... the current PG scheme has the advantage of being
-> relatively simple and probably more efficient than scanning logs like
-> Oracle has to do (assuming your datafiles aren't thoroughly clogged with
-> old dead tuples).
->
-> Has anyone looked at InterBase for hints for space-reusing strategies?
->
-> As I understand it, they have a tuple-versioning scheme similar to PG's.
->
-> If nothing else, something might be learned as to the efficiency and
-> effectiveness of one particular approach to solving the problem.
-
-It may also be beneficial to study SapDB (which is IIRC a branch-off of
-Adabas) although they claim at http://www.sapdb.org/ in features
-section:
-
-NOT supported features:
-
- Collations
-
- Result sets that are created within a stored procedure and
-fetched outside. This feature is planned to be
- offered in one of the coming releases.
- Meanwhile, use temporary tables.
- see Reference Manual: SAP DB 7.2 and 7.3 -> Data
-definition -> CREATE TABLE statement: Owner of a
- table
-
- Multi version concurrency for OLTP
- It is available with the object extension of SAPDB only.
-
- Hot stand by
- This feature is planned to be offered in one of the coming
-releases.
-
-So MVCC seems to be a bolt-on feature there.
-
----------------------
-Hannu
-
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-From pgsql-hackers-owner+M9343@postgresql.org Thu May 24 19:58:01 2001
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-Date: Fri, 25 May 2001 08:51:44 +0900
-From: Hiroshi Inoue <Inoue@tpf.co.jp>
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-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: Don Baccus <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <3705826352029646A3E91C53F7189E32016652@sectorbase2.sectorbase.com>
-Content-Type: text/plain; charset=iso-2022-jp
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-
-"Mikheev, Vadim" wrote:
->
-> > I think so too. I've never said that an overwriting smgr
-> > is easy and I don't love it particularily.
-> >
-> > What I'm objecting is to avoid UNDO without giving up
-> > an overwriting smgr. We shouldn't be noncommittal now.
->
-> Why not? We could decide to do overwriting smgr later
-> and implement UNDO then.
-
-What I'm refering to is the discussion about the handling
-of subtransactions in order to introduce the savepoints
-functionality. Or do we postpone *savepoints* again ?
-
-I realize now few people have had the idea how to switch
-to an overwriting smgr. I don't think an overwriting smgr
-could be achived at once and we have to prepare one by one
-for it. AFAIK there's no idea how to introduce an overwriting
-smgr without UNDO. If we avoid UNDO now when overwriting smgr
-would appear ? I also think that the problems Andreas has
-specified are pretty serious. I also have known the problems
-and I've expected that people have the idea to solve it but
-... I'm inclined to give up an overwriting smgr now.
-
-regards,
-Hiroshi Inoue
-
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-From pgsql-hackers-owner+M9347@postgresql.org Thu May 24 20:31:27 2001
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-Date: Fri, 25 May 2001 10:21:37 +1000
-To: Hannu Krosing <hannu@tm.ee>, "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-From: Philip Warner <pjw@rhyme.com.au>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-In-Reply-To: <3B0D743A.B57B76A0@tm.ee>
-References: <3705826352029646A3E91C53F7189E32016650@sectorbase2.sectorbase.com>
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-
-At 01:51 25/05/01 +0500, Hannu Krosing wrote:
->
->How does it do MVCC with an overwriting storage manager ?
->
-
-I don't know about Oracle, but Dec/RDB also does overwriting and MVCC. It
-does this by taking a snapshot of pages that are participating in both RW
-and RO transactions (De/RDB has the options on SET TRANSACTION that specify
-if the TX will do updates or not). It has the disadvantage that the
-snapshot will grow quite large for bulk loads. Typically they are about
-10-20% of DB size. Pages are freed from the snapshot as active TXs finish.
-
-Note that the snapshots are separate from the journalling (WAL) and
-rollback files.
-
-
-----------------------------------------------------------------
-Philip Warner | __---_____
-Albatross Consulting Pty. Ltd. |----/ - \
-(A.B.N. 75 008 659 498) | /(@) ______---_
-Tel: (+61) 0500 83 82 81 | _________ \
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-From pgsql-hackers-owner+M9346@postgresql.org Thu May 24 20:30:01 2001
-Return-path: <pgsql-hackers-owner+M9346@postgresql.org>
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-Message-ID: <3705826352029646A3E91C53F7189E32016655@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Hannu Krosing'" <hannu@tm.ee>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 24 May 2001 17:23:19 -0700
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-> > > >Oracle has MVCC?
-> > >
-> > > With restrictions, yes.
-> >
-> > What restrictions? Rollback segments size?
-> > Non-overwriting smgr can eat all disk space...
->
-> Is'nt the same true for an overwriting smgr ? ;)
-
-Removing dead records from rollback segments should
-be faster than from datafiles.
-
-> > > You didn't know that? Vadim did ...
-> >
-> > Didn't I mention a few times that I was
-> > inspired by Oracle? -:)
->
-> How does it do MVCC with an overwriting storage manager ?
-
-1. System Change Number (SCN) is used: system increments it
- on each transaction commit.
-2. When scan meets data block with SCN > SCN as it was when
- query/transaction started, old block image is restored
- using rollback segments.
-
-> Could it possibly be a Postgres-inspired bolted-on hack
-> needed for better concurrency ?
-
--:)) Oracle has MVCC for years, probably from the beginning
-and for sure before Postgres.
-
-> BTW, are you aware how Interbase does its MVCC - is it more
-> like Oracle's way or like PostgreSQL's ?
-
-Like ours.
-
-Vadim
-
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-From pgsql-hackers-owner+M9348@postgresql.org Thu May 24 21:13:34 2001
-Return-path: <pgsql-hackers-owner+M9348@postgresql.org>
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- Thu, 24 May 2001 21:00:08 -0400 (EDT)
-From: Bruce Momjian <pgman@candle.pha.pa.us>
-Message-ID: <200105250100.f4P108j23173@candle.pha.pa.us>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3B0D9E90.8DB98EFC@tpf.co.jp> "from Hiroshi Inoue at May 25, 2001
- 08:51:44 am"
-To: Hiroshi Inoue <Inoue@tpf.co.jp>
-Date: Thu, 24 May 2001 21:00:08 -0400 (EDT)
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- Don Baccus <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
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-
-> What I'm refering to is the discussion about the handling
-> of subtransactions in order to introduce the savepoints
-> functionality. Or do we postpone *savepoints* again ?
->
-> I realize now few people have had the idea how to switch
-> to an overwriting smgr. I don't think an overwriting smgr
-> could be achived at once and we have to prepare one by one
-> for it. AFAIK there's no idea how to introduce an overwriting
-> smgr without UNDO. If we avoid UNDO now when overwriting smgr
-> would appear ? I also think that the problems Andreas has
-> specified are pretty serious. I also have known the problems
-> and I've expected that people have the idea to solve it but
-> ... I'm inclined to give up an overwriting smgr now.
-
-Now that everyone has commented on the UNDO issue, I thought I would try
-to summarize the comments so we can come to some kind of conclusion.
-
-Here are the issues as I see them:
-
----------------------------------------------------------------------------
-
-Do we want to keep MVCC?
-
-Yes. No one has said otherwise.
-
----------------------------------------------------------------------------
-
-Do we want to head for an overwriting storage manager?
-
-Not sure.
-
-Advantages: UPDATE has easy space reuse because usually done in-place,
-no index change on UPDATE unless key is changed.
-
-Disadvantages: Old records have to be stored somewhere for MVCC use.
-Could limit transaction size.
-
----------------------------------------------------------------------------
-
-Do we want UNDO _if_ we are heading for an overwriting storage manager?
-
-Everyone seems to say yes.
-
----------------------------------------------------------------------------
-
-Do we want UNDO if we are _not_ heading for an overwriting storage
-manager?
-
-This is the tough one. UNDO advantages are:
-
- Make subtransactions easier by rolling back aborted subtransaction.
- Workaround is using a new transactions id for each subtransaction.
-
- Easy space reuse for aborted transactions.
-
- Reduce size of pg_log.
-
-UNDO disadvantages are:
-
- Limit size of transactions to log storage size.
-
----------------------------------------------------------------------------
-
-If we are heading for an overwriting storage manager, we may as well get
-UNDO now. If we are not, then we have to decide if we can solve the
-problems that UNDO would fix. Basically, can we solve those problems
-easier without UNDO, or are the disadvanges of UNDO too great?
-
---
- Bruce Momjian | http://candle.pha.pa.us
- pgman@candle.pha.pa.us | (610) 853-3000
- + If your life is a hard drive, | 830 Blythe Avenue
- + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
-
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-From pgsql-hackers-owner+M9356@postgresql.org Fri May 25 05:15:44 2001
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-Date: Fri, 25 May 2001 11:01:53 +0500
-From: Hannu Krosing <hannu@tm.ee>
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-X-Accept-Language: en, ru, et
-MIME-Version: 1.0
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <3705826352029646A3E91C53F7189E32016655@sectorbase2.sectorbase.com>
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-
-"Mikheev, Vadim" wrote:
->
-> > > > >Oracle has MVCC?
-> > > >
-> > > > With restrictions, yes.
-> > >
-> > > What restrictions? Rollback segments size?
-> > > Non-overwriting smgr can eat all disk space...
-> >
-> > Is'nt the same true for an overwriting smgr ? ;)
->
-> Removing dead records from rollback segments should
-> be faster than from datafiles.
-
-Is it for better locality or are they stored in a different way ?
-
-Do you think that there is some fundamental performance advantage
-in making a copy to rollback segment and then deleting it from
-there vs. reusing space in datafiles ?
-
-One thing (not having to updata non-changing index entries) can be
-quite substantial under some scenarios, but there are probably ways
-to at least speed up part of this by doing other compromizes, perhaps
-by saving more info in index leaf (trading lookup speed for space
-and insert speed) or chaining data pages (trading insert speed for
-(some) space and lookup speed)
-
-> > > > You didn't know that? Vadim did ...
-> > >
-> > > Didn't I mention a few times that I was
-> > > inspired by Oracle? -:)
-> >
-> > How does it do MVCC with an overwriting storage manager ?
->
-> 1. System Change Number (SCN) is used: system increments it
-> on each transaction commit.
-> 2. When scan meets data block with SCN > SCN as it was when
-> query/transaction started, old block image is restored
-> using rollback segments.
-
-You mean it is restored in session that is running the transaction ?
-
-I guess thet it could be slower than our current way of doing it.
-
-> > Could it possibly be a Postgres-inspired bolted-on hack
-> > needed for better concurrency ?
->
-> -:)) Oracle has MVCC for years, probably from the beginning
-> and for sure before Postgres.
-
-In that case we can claim thet their way is more primitive ;) ;)
-
------------------
-Hannu
-
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-From vmikheev@SECTORBASE.COM Fri May 25 12:38:38 2001
-Return-path: <vmikheev@SECTORBASE.COM>
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-Message-ID: <3705826352029646A3E91C53F7189E32016656@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>
-cc: Don Baccus <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 25 May 2001 09:37:16 -0700
-MIME-Version: 1.0
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-
-> Do we want to head for an overwriting storage manager?
->
-> Not sure.
->
-> Advantages: UPDATE has easy space reuse because usually done
-> in-place, no index change on UPDATE unless key is changed.
->
-> Disadvantages: Old records have to be stored somewhere for MVCC use.
-> Could limit transaction size.
-
-Really? Why is it assumed that we *must* limit size of rollback segments?
-We can let them grow without bounds, as we do now keeping old records in
-datafiles and letting them eat all of disk space.
-
-> UNDO disadvantages are:
->
-> Limit size of transactions to log storage size.
-
-Don't be kidding - in any system transactions size is limitted
-by available storage. So we should tell that more disk space
-is required for UNDO. From my POV, putting $100 to buy 30Gb
-disk is not big deal, keeping in mind that PGSQL requires
-$ZERO to be used.
-
-Vadim
-
-From pgsql-hackers-owner+M9365@postgresql.org Fri May 25 13:11:43 2001
-Return-path: <pgsql-hackers-owner+M9365@postgresql.org>
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>
-cc: Don Baccus <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 25 May 2001 09:37:16 -0700
-MIME-Version: 1.0
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-
-> Do we want to head for an overwriting storage manager?
->
-> Not sure.
->
-> Advantages: UPDATE has easy space reuse because usually done
-> in-place, no index change on UPDATE unless key is changed.
->
-> Disadvantages: Old records have to be stored somewhere for MVCC use.
-> Could limit transaction size.
-
-Really? Why is it assumed that we *must* limit size of rollback segments?
-We can let them grow without bounds, as we do now keeping old records in
-datafiles and letting them eat all of disk space.
-
-> UNDO disadvantages are:
->
-> Limit size of transactions to log storage size.
-
-Don't be kidding - in any system transactions size is limitted
-by available storage. So we should tell that more disk space
-is required for UNDO. From my POV, putting $100 to buy 30Gb
-disk is not big deal, keeping in mind that PGSQL requires
-$ZERO to be used.
-
-Vadim
-
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-From vmikheev@SECTORBASE.COM Fri May 25 13:06:50 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- "'Don Baccus'"
- <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker
- <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 25 May 2001 10:05:31 -0700
-MIME-Version: 1.0
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-
-> > > >Oracle has MVCC?
-> > >
-> > > With restrictions, yes.
-> >
-> > What restrictions? Rollback segments size?
->
-> No, that is not the whole story. The problem with their
-> "rollback segment approach" is, that they do not guard against
-> overwriting a tuple version in the rollback segment.
-> They simply recycle each segment in a wrap around manner.
-> Thus there could be an open transaction that still wanted to
-> see a tuple version that was already overwritten, leading to the
-> feared "snapshot too old" error.
->
-> Copying their "rollback segment" approach is imho the last
-> thing we want to do.
-
-So, they limit size of rollback segments and we don't limit
-how big our datafiles may grow if there is some long running
-transaction in serializable mode. We could allow our rollback
-segments to grow without limits as well.
-
-> > Non-overwriting smgr can eat all disk space...
-> >
-> > > You didn't know that? Vadim did ...
-> >
-> > Didn't I mention a few times that I was inspired by Oracle? -:)
->
-> Looking at what they supply in the feature area is imho good.
-> Copying their technical architecture is not so good in general.
-
-Copying is not inspiration -:)
-
-Vadim
-
-From pgsql-hackers-owner+M9367@postgresql.org Fri May 25 14:01:43 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Hannu Krosing'" <hannu@tm.ee>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 25 May 2001 10:52:17 -0700
-MIME-Version: 1.0
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-> > Removing dead records from rollback segments should
-> > be faster than from datafiles.
->
-> Is it for better locality or are they stored in a different way ?
-
-Locality - all dead data would be localized in one place.
-
-> Do you think that there is some fundamental performance advantage
-> in making a copy to rollback segment and then deleting it from
-> there vs. reusing space in datafiles ?
-
-As it showed by WAL additional writes don't mean worse performance.
-As for deleting from RS (rollback segment) - we could remove or reuse
-RS files as whole.
-
-> > > How does it do MVCC with an overwriting storage manager ?
-> >
-> > 1. System Change Number (SCN) is used: system increments it
-> > on each transaction commit.
-> > 2. When scan meets data block with SCN > SCN as it was when
-> > query/transaction started, old block image is restored
-> > using rollback segments.
->
-> You mean it is restored in session that is running the transaction ?
->
-> I guess thet it could be slower than our current way of doing it.
-
-Yes, for older transactions which *really* need in *particular*
-old data, but not for newer ones. Look - now transactions have to read
-dead data again and again, even if some of them (newer) need not to see
-those data at all, and we keep dead data as long as required for other
-old transactions *just for the case* they will look there.
-But who knows?! Maybe those old transactions will not read from table
-with big amount of dead data at all! So - why keep dead data in datafiles
-for long time? This obviously affects overall system performance.
-
-Vadim
-
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-From pgsql-hackers-owner+M9387@postgresql.org Sun May 27 04:42:32 2001
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-From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-cc: "Don Baccus" <dhogaza@pacifier.com>, "Tom Lane" <tgl@sss.pgh.pa.us>,
- "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- "The Hermit Hacker" <scrappy@hub.org>, <pgsql-hackers@postgresql.org>
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Sun, 27 May 2001 17:32:54 +0900
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-> -----Original Message-----
-> From: Mikheev, Vadim [mailto:vmikheev@SECTORBASE.COM]
->
-> > Do we want to head for an overwriting storage manager?
-> >
-> > Not sure.
-> >
-> > Advantages: UPDATE has easy space reuse because usually done
-> > in-place, no index change on UPDATE unless key is changed.
-> >
-> > Disadvantages: Old records have to be stored somewhere for MVCC use.
-> > Could limit transaction size.
->
-> Really? Why is it assumed that we *must* limit size of rollback segments?
-> We can let them grow without bounds, as we do now keeping old records in
-> datafiles and letting them eat all of disk space.
->
-
-Is it proper/safe for a DBMS to allow the system eating all disk
-space ? For example, could we expect to recover the database
-even when no disk space available ?
-
-1) even before WAL
- Is 'deleting records and vacuum' always possible ?
- I saw the cases that indexes grow by vacuum.
-
-2) under WAL(current)
- If DROP or VACUUM is done after a checkpoint, wouldn't
- REDO recovery add the pages drop/truncated by the
- DROP/VACUUM ?
-
-3) with rollback data
- Shouldn't WAL log UNDO operations either ?
- If so, UNDO requires an extra disk space which could
- be unlimitedly big.
-
-There's another serious problem. Once UNDO is required
-with a biiiig rollback data, it would take a veeery long time
-to undo. It's quite different from the current behavior. Even
-though people want to cancel the UNDO, there's no way
-unfortunately(under an overwriting smgr).
-
-regards,
-Hiroshi Inoue
-
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-
-From vmikheev@sectorbase.com Mon May 28 13:11:10 2001
-Return-path: <vmikheev@sectorbase.com>
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-Message-ID: <007001c0e799$321dcc00$4a79583f@sectorbase.com>
-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "Zeugswetter Andreas SB" <ZeugswetterA@wien.spardat.at>,
- "'Hannu Krosing'" <hannu@tm.ee>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, "Tom Lane" <tgl@sss.pgh.pa.us>,
- "Hiroshi Inoue" <Inoue@tpf.co.jp>, "Bruce Momjian" <pgman@candle.pha.pa.us>,
- "The Hermit Hacker" <scrappy@hub.org>, <pgsql-hackers@postgresql.org>
-References: <11C1E6749A55D411A9670001FA6879633682F5@sdexcsrv1.f000.d0188.sd.spardat.at>
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 28 May 2001 10:11:10 -0700
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-
-> Yes, that is a good description. And old version is only required in the following
-> two cases:
->
-> 1. the txn that modified this tuple is still open (reader in default committed read)
-> 2. reader is in serializable transaction isolation and has earlier xtid
->
-> Seems overwrite smgr has mainly advantages in terms of speed for operations
-> other than rollback.
-
-... And rollback is required for < 5% transactions ...
-
-Vadim
-
-
-
-From hannu@tm.ee Mon May 28 13:37:50 2001
-Return-path: <hannu@tm.ee>
-Received: from taru.tm.ee (taru.tm.ee [194.204.62.23])
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- Mon, 28 May 2001 19:41:40 +0200
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-Date: Mon, 28 May 2001 19:41:40 +0200
-From: Hannu Krosing <hannu@tm.ee>
-X-Mailer: Mozilla 4.76 [en] (X11; U; Linux 2.4.2-2 i686)
-X-Accept-Language: et, en, ru
-MIME-Version: 1.0
-To: Vadim Mikheev <vmikheev@sectorbase.com>
-cc: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <11C1E6749A55D411A9670001FA6879633682F5@sdexcsrv1.f000.d0188.sd.spardat.at> <007001c0e799$321dcc00$4a79583f@sectorbase.com>
-Content-Type: text/plain; charset=us-ascii
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-
-Vadim Mikheev wrote:
->
-> > Yes, that is a good description. And old version is only required in the following
-> > two cases:
-> >
-> > 1. the txn that modified this tuple is still open (reader in default committed read)
-> > 2. reader is in serializable transaction isolation and has earlier xtid
-> >
-> > Seems overwrite smgr has mainly advantages in terms of speed for operations
-> > other than rollback.
->
-> ... And rollback is required for < 5% transactions ...
-
-This obviously depends on application.
-
-I know people who rollback most of their transactions (actually they use
-it to
-emulate temp tables when reporting).
-
-OTOH it is possible to do without rolling back at all as MySQL folks
-have
-shown us ;)
-
-Also, IIRC, pgbench does no rollbacks. I think that we have no
-performance test that does.
-
------------------
-Hannu
-
-From pgsql-hackers-owner+M9464@postgresql.org Tue May 29 16:40:30 2001
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-Date: Tue, 29 May 2001 22:07:16 +0500
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-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-cc: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Plans for solving the VACUUM problem
-References: <3705826352029646A3E91C53F7189E3201665D@sectorbase2.sectorbase.com>
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-"Mikheev, Vadim" wrote:
->
-> > I know people who rollback most of their transactions
-> > (actually they use it to emulate temp tables when reporting).
->
-> Shouldn't they use TEMP tables? -:)
-
-They probably should.
-
-Actually they did it on Oracle, so it shows that it can be done
-even with O-smgr ;)
-
-> > OTOH it is possible to do without rolling back at all as
-> > MySQL folks have shown us ;)
->
-> Not with SDB tables which support transactions.
-
-My point was that MySQL was used quite a long time without it
-and still quite many useful applications were produced.
-
-BTW, do you know what strategy is used by BSDDB/SDB for
-rollback/undo ?
-
----------------
-Hannu
-
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-From vmikheev@SECTORBASE.COM Tue May 29 13:50:48 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Hannu Krosing'" <hannu@tm.ee>
-cc: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- "'Don Baccus'"
- <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 29 May 2001 10:49:12 -0700
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-> > > Seems overwrite smgr has mainly advantages in terms of
-> > > speed for operations other than rollback.
-> >
-> > ... And rollback is required for < 5% transactions ...
->
-> This obviously depends on application.
-
-Small number of aborted transactions was used to show
-useless of UNDO in terms of space cleanup - that's why
-I use same argument to show usefulness of O-smgr -:)
-
-> I know people who rollback most of their transactions
-> (actually they use it to emulate temp tables when reporting).
-
-Shouldn't they use TEMP tables? -:)
-
-> OTOH it is possible to do without rolling back at all as
-> MySQL folks have shown us ;)
-
-Not with SDB tables which support transactions.
-
-Vadim
-
-From pgsql-hackers-owner+M9462@postgresql.org Tue May 29 14:12:23 2001
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-Date: Tue, 29 May 2001 10:55:33 -0700
-To: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Hannu Krosing'" <hannu@tm.ee>
-From: Don Baccus <dhogaza@pacifier.com>
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-cc: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- Tom Lane <tgl@sss.pgh.pa.us>, Hiroshi Inoue <Inoue@tpf.co.jp>,
- Bruce Momjian <pgman@candle.pha.pa.us>, The Hermit Hacker <scrappy@hub.org>,
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-
-At 10:49 AM 5/29/01 -0700, Mikheev, Vadim wrote:
-
->> I know people who rollback most of their transactions
->> (actually they use it to emulate temp tables when reporting).
->
->Shouldn't they use TEMP tables? -:)
-
-Which is a very good point. Pandering to poor practice at the
-expense of good performance for better-designed applications
-isn't a good idea.
-
-
-
-- Don Baccus, Portland OR <dhogaza@pacifier.com>
- Nature photos, on-line guides, Pacific Northwest
- Rare Bird Alert Service and other goodies at
- http://donb.photo.net.
-
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-From pgsql-hackers-owner+M9466@postgresql.org Tue May 29 17:15:46 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Hannu Krosing'" <hannu@tm.ee>
-cc: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- "'Don Baccus'"
- <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: RE: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 29 May 2001 13:37:03 -0700
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-
-> > > OTOH it is possible to do without rolling back at all as
-> > > MySQL folks have shown us ;)
-> >
-> > Not with SDB tables which support transactions.
->
-> My point was that MySQL was used quite a long time without it
-> and still quite many useful applications were produced.
-
-And my point was that needless to talk about rollbacks in
-non-transaction system and in transaction system one has to
-implement rollback somehow.
-
-> BTW, do you know what strategy is used by BSDDB/SDB for
-> rollback/undo ?
-
-AFAIR, they use O-smgr => UNDO is required.
-
-Vadim
-
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
- "'Bruce Momjian'"
- <pgman@candle.pha.pa.us>
-cc: "'Tom Lane'" <tgl@sss.pgh.pa.us>, pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 16:44:42 +0200
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-
-> > Vadim, can you remind me what UNDO is used for?
-> 4. Split pg_log into small files with ability to remove old ones (which
-> do not hold statuses for any running transactions).
-
-They are already small (16Mb). Or do you mean even smaller ?
-This imposes one huge risk, that is already a pain in other db's. You need
-all logs of one transaction online. For a GigaByte transaction like a bulk
-insert this can be very inconvenient.
-Imho there should be some limit where you can choose whether you want
-to continue without the feature (no savepoint) or are automatically aborted.
-
-In any case, imho some thought should be put into this :-)
-
-Another case where this is a problem is a client that starts a tx, does one little
-insert or update on his private table, and then sits and waits for a day.
-
-Both cases currently impose no problem whatsoever.
-
-Andreas
-
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-From ZeugswetterA@wien.spardat.at Mon May 21 13:37:56 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Vadim Mikheev'" <vmikheev@SECTORBASE.COM>,
- The Hermit Hacker
- <scrappy@hub.org>
-cc: Tom Lane <tgl@sss.pgh.pa.us>, "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 18:11:16 +0200
-MIME-Version: 1.0
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-
-
-> My point is that we'll need in dynamic cleanup anyway and UNDO is
-> what should be implemented for dynamic cleanup of aborted changes.
-
-I do not yet understand why you want to handle aborts different than outdated
-tuples. The ratio in a well tuned system should well favor outdated tuples.
-If someone ever adds "dirty read" it is also not the case that it is guaranteed,
-that nobody accesses the tuple you currently want to undo. So I really miss to see
-the big difference.
-
-Andreas
-
-From pgsql-hackers-owner+M9153@postgresql.org Mon May 21 16:27:39 2001
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-Date: Mon, 21 May 2001 13:19:08 -0700
-From: Barry Lind <barry@xythos.com>
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-MIME-Version: 1.0
-To: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- pgsql-hackers@postgresql.org
-Subject: Re: AW: [HACKERS] Plans for solving the VACUUM problem
-References: <11C1E6749A55D411A9670001FA6879633682DA@sdexcsrv1.f000.d0188.sd.spardat.at>
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-
-Zeugswetter Andreas SB wrote:
-
->>> Vadim, can you remind me what UNDO is used for?
->>
->> 4. Split pg_log into small files with ability to remove old ones (which
->> do not hold statuses for any running transactions).
->
->
-> They are already small (16Mb). Or do you mean even smaller ?
-> This imposes one huge risk, that is already a pain in other db's. You need
-> all logs of one transaction online. For a GigaByte transaction like a bulk
-> insert this can be very inconvenient.
-> Imho there should be some limit where you can choose whether you want
-> to continue without the feature (no savepoint) or are automatically aborted.
->
-> In any case, imho some thought should be put into this :-)
->
-> Another case where this is a problem is a client that starts a tx, does one little
-> insert or update on his private table, and then sits and waits for a day.
->
-> Both cases currently impose no problem whatsoever.
-
-Correct me if I am wrong, but both cases do present a problem currently
-in 7.1. The WAL log will not remove any WAL files for transactions that
-are still open (even after a checkpoint occurs). Thus if you do a bulk
-insert of gigabyte size you will require a gigabyte sized WAL
-directory. Also if you have a simple OLTP transaction that the user
-started and walked away from for his one week vacation, then no WAL log
-files can be deleted until that user returns from his vacation and ends
-his transaction.
-
---Barry
-
->
->
-> Andreas
->
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-From pgsql-hackers-owner+M9154@postgresql.org Mon May 21 16:45:07 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Barry Lind'" <barry@xythos.com>,
- Zeugswetter Andreas SB
- <ZeugswetterA@wien.spardat.at>,
- pgsql-hackers@postgresql.org
-Subject: RE: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 21 May 2001 13:29:03 -0700
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-> Correct me if I am wrong, but both cases do present a problem
-> currently in 7.1. The WAL log will not remove any WAL files
-> for transactions that are still open (even after a checkpoint
-> occurs). Thus if you do a bulk insert of gigabyte size you will
-> require a gigabyte sized WAL directory. Also if you have a simple
-> OLTP transaction that the user started and walked away from for
-> his one week vacation, then no WAL log files can be deleted until
-> that user returns from his vacation and ends his transaction.
-
-Todo:
-
-1. Compact log files after checkpoint (save records of uncommitted
- transactions and remove/archive others).
-2. Abort long running transactions.
-
-Vadim
-
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-From pgsql-hackers-owner+M9155@postgresql.org Mon May 21 17:01:45 2001
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-From: Jan Wieck <JanWieck@Yahoo.com>
-Message-ID: <200105212041.QAA15136@jupiter.jw.home>
-Subject: Re: AW: [HACKERS] Plans for solving the VACUUM problem
-In-Reply-To: <3B09783C.1080508@xythos.com> from Barry Lind at "May 21, 2001 01:19:08
- pm"
-To: Barry Lind <barry@xythos.com>
-Date: Mon, 21 May 2001 16:41:33 -0400 (EDT)
-cc: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- pgsql-hackers@postgresql.org
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-
-Barry Lind wrote:
->
->
-> Zeugswetter Andreas SB wrote:
->
-> >>> Vadim, can you remind me what UNDO is used for?
-> >>
-> >> 4. Split pg_log into small files with ability to remove old ones (which
-> >> do not hold statuses for any running transactions).
-> >
-> >
-> > They are already small (16Mb). Or do you mean even smaller ?
-> > This imposes one huge risk, that is already a pain in other db's. You need
-> > all logs of one transaction online. For a GigaByte transaction like a bulk
-> > insert this can be very inconvenient.
-> > Imho there should be some limit where you can choose whether you want
-> > to continue without the feature (no savepoint) or are automatically aborted.
-> >
-> > In any case, imho some thought should be put into this :-)
-> >
-> > Another case where this is a problem is a client that starts a tx, does one little
-> > insert or update on his private table, and then sits and waits for a day.
-> >
-> > Both cases currently impose no problem whatsoever.
->
-> Correct me if I am wrong, but both cases do present a problem currently
-> in 7.1. The WAL log will not remove any WAL files for transactions that
-> are still open (even after a checkpoint occurs). Thus if you do a bulk
-> insert of gigabyte size you will require a gigabyte sized WAL
-> directory. Also if you have a simple OLTP transaction that the user
-> started and walked away from for his one week vacation, then no WAL log
-> files can be deleted until that user returns from his vacation and ends
-> his transaction.
-
- As a rule of thumb, online applications that hold open
- transactions during user interaction are considered to be
- Broken By Design (tm). So I'd slap the programmer/design
- team with - let's use the server box since it doesn't contain
- anything useful.
-
-
-Jan
-
---
-
-#======================================================================#
-# It's easier to get forgiveness for being wrong than for being right. #
-# Let's break this rule - forgive me. #
-#================================================== JanWieck@Yahoo.com #
-
-
-
-_________________________________________________________
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-Get your free @yahoo.com address at http://mail.yahoo.com
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-
-From pgsql-hackers-owner+M9174@postgresql.org Tue May 22 04:34:56 2001
-Return-path: <pgsql-hackers-owner+M9174@postgresql.org>
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Barry Lind'" <barry@xythos.com>, Tom Lane <tgl@sss.pgh.pa.us>
-cc: pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 10:16:10 +0200
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-
-> REDO in oracle is done by something known as a 'rollback segment'.
-
-You are not seriously saying that you like the "rollback segments" in Oracle.
-They only cause trouble:
-1. configuration (for every different workload you need a different config)
-2. snapshot too old
-3. tx abort because rollback segments are full
-4. They use up huge amounts of space (e.g. 20 Gb rollback seg for a 120 Gb SAP)
-
-If I read the papers correctly Version 9 gets rid of Point 1 but the rest ...
-
-Andreas
-
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-From pgsql-hackers-owner+M9206@postgresql.org Tue May 22 13:26:46 2001
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-Date: Tue, 22 May 2001 10:11:50 -0700
-From: Barry Lind <barry@xythos.com>
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-To: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-cc: pgsql-hackers@postgresql.org
-Subject: Re: AW: [HACKERS] Plans for solving the VACUUM problem
-References: <11C1E6749A55D411A9670001FA6879633682E8@sdexcsrv1.f000.d0188.sd.spardat.at>
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-
-Actually I don't like the problems with rollback segments in oracle at
-all. I am just concerned that using WAL for UNDO will have all of the
-same problems if it isn't designed carefully. At least in oracle's
-rollback segments there are multiple of them, in WAL there is just one,
-thus you will potentially have that 20Gig all in your single log
-directory. People are already reporting the log directory growing to a
-gig or more when running vacuum in 7.1.
-
-Of the points you raised about oracle's rollback segment problems:
-
-1. configuration (for every different workload you need a different config)
-
-Postgres should be able to do a better job here.
-
-
-2. snapshot too old
-
-Shouldn't be a problem as long as postgres continues to use a non-overwriting storage manager. However under an overwriting storage manager, you need to keep all of the changes in the UNDO records to satisfy the query snapshot, thus if you want to limit the size of UNDO you may need to kill long running queries.
-
-3. tx abort because rollback segments are full
-If you want to limit the size of the UNDO, then this is a corresponding
-byproduct. I believe a mail note was sent out yesterday suggesting that
-limits like this be added to the todo list.
-
-4. They use up huge amounts of space (e.g. 20 Gb rollback seg for a 120 Gb SAP)
-You need to store the UNDO information somewhere. And on active
-databases that can amount to alot of information, especially for bulk
-loads or massive updates.
-
-thanks,
---Barry
-
-
-Zeugswetter Andreas SB wrote:
-
->
->
->> REDO in oracle is done by something known as a 'rollback segment'.
->
->
-> You are not seriously saying that you like the "rollback segments" in Oracle.
-> They only cause trouble:
-> 1. configuration (for every different workload you need a different config)
-> 2. snapshot too old
-> 3. tx abort because rollback segments are full
-> 4. They use up huge amounts of space (e.g. 20 Gb rollback seg for a 120 Gb SAP)
->
-> If I read the papers correctly Version 9 gets rid of Point 1 but the rest ...
->
-> Andreas
->
->
-
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-From pgsql-hackers-owner+M9210@postgresql.org Tue May 22 14:56:54 2001
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-Message-ID: <3705826352029646A3E91C53F7189E32016642@sectorbase2.sectorbase.com>
-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- "'Tom Lane'"
- <tgl@sss.pgh.pa.us>
-cc: pgsql-hackers@postgresql.org
-Subject: RE: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 11:20:37 -0700
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-> > 1. Compact log files after checkpoint (save records of uncommitted
-> > transactions and remove/archive others).
->
-> On the grounds that undo is not guaranteed anyway (concurrent
-> heap access), why not simply forget it,
-
-We can set flag in ItemData and register callback function in
-buffer header regardless concurrent heap/index access. So buffer
-will be cleaned before throwing it out from buffer pool
-(little optimization: if at the time when pin drops to 0 buffer
-is undirty then we shouldn't really clean it up to avoid unnecessary
-write - we can save info in FSM that space is available and clean it
-up on first pin/read).
-So, only ability of *immediate reusing* is not guaranteed. But this is
-general problem of space reusing till we assume that buffer pin is
-enough to access data.
-
-> since above sounds rather expensive ?
-
-I'm not sure. For non-overwriting smgr required UNDO info is pretty
-small because of we're not required to keep tuple data, unlike
-Oracle & Co. We can even store UNDO info in non-WAL format to avoid
-log record header overhead. UNDO files would be kind of Oracle rollback
-segments but muuuuch smaller. But yeh - additional log reads.
-
-> The downside would only be, that long running txn's cannot
-> [easily] rollback to savepoint.
-
-We should implement savepoints for all or none transactions, no?
-
-> > 2. Abort long running transactions.
->
-> This is imho "the" big downside of UNDO, and should not
-> simply be put on the TODO without thorow research. I think it
-> would be better to forget UNDO for long running transactions
-> before aborting them.
-
-Abort could be configurable.
-
-Vadim
-
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-From pgsql-hackers-owner+M9243@postgresql.org Wed May 23 04:12:50 2001
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- for <pgsql-hackers@postgresql.org>; Wed, 23 May 2001 09:35:35 +0200
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-Message-ID: <11C1E6749A55D411A9670001FA6879633682ED@sdexcsrv1.f000.d0188.sd.spardat.at>
-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
- "'Bruce Momjian'"
- <pgman@candle.pha.pa.us>
-cc: The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Wed, 23 May 2001 09:35:22 +0200
-MIME-Version: 1.0
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-
-> If community will not like UNDO then I'll probably try to implement
-
-Imho UNDO would be great under the following circumstances:
- 1. The undo is only registered for some background work process
- and not done in the client's backend (or only if it is a small txn).
- 2. The same mechanism should also be used for outdated tuples
- (the only difference beeing, that some tuples need to wait longer
- because of an active xid)
-
-The reasoning to not do it in the client's backend is not only that the client
-does not need to wait, but that the nervous dba tends to kill them if after one hour
-of forward work the backend seemingly does not respond anymore (because it is
-busy with undo).
-
-> dead space collector which will read log files and so on.
-
-Which would then only be a possible implementation variant of above :-)
-First step probably would be to separate the physical log to reduce WAL size.
-
-> to implement logging for non-btree indices (anyway required for UNDO,
-> WAL-based BAR, WAL-based space reusing).
-
-Imho it would be great to implement a generic (albeit more expensive)
-redo for all possible index types, that would be used in absence of a physical
-redo for that particular index type (which is currently available for btree).
-
-The prerequisites would be a physical log that saves the page before
-modification. The redo could then be done (with the info from the heap tuple log record)
-with the same index interface, that is used during normal operation.
-
-Imho implementing a new index type is difficult enough as is without the need
-to write a redo and undo.
-
-Andreas
-
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-From pgsql-hackers-owner+M9245@postgresql.org Wed May 23 04:41:13 2001
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-Message-ID: <11C1E6749A55D411A9670001FA6879633682EE@sdexcsrv1.f000.d0188.sd.spardat.at>
-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Bruce Momjian'" <pgman@candle.pha.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>
-cc: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- The Hermit Hacker
- <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Wed, 23 May 2001 10:26:26 +0200
-MIME-Version: 1.0
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-> > People also have referred to an overwriting smgr
-> > easily. Please tell me how to introduce an overwriting smgr
-> > without UNDO.
-
-There is no way. Although undo for an overwriting smgr would involve a
-very different approach than with non-overwriting. See Vadim's post about what
-info suffices for undo in non overwriting smgr (file and ctid).
-
-> I guess that is the question. Are we heading for an overwriting storage
-> manager? I didn't see that in Vadim's list of UNDO advantages, but
-> maybe that is his final goal. If so UNDO may make sense, but then the
-> question is how do we keep MVCC with an overwriting storage manager?
->
-> The only way I can see doing it is to throw the old tuples into the WAL
-> and have backends read through that for MVCC info.
-
-If PostgreSQL wants to stay MVCC, then we should imho forget "overwriting smgr"
-very fast.
-
-Let me try to list the pros and cons that I can think of:
-Pro:
- no index modification if key stays same
- no search for free space for update (if tuple still fits into page)
- no pg_log
-Con:
- additional IO to write "before image" to rollback segment
- (every before image, not only first after checkpoint)
- (also before image of every index page that is updated !)
- need a rollback segment that imposes all sorts of contention problems
- active rollback, that needs to do a lot of undo work
-
-Andreas
-
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-From ZeugswetterA@wien.spardat.at Wed May 23 05:25:30 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Philip Warner'" <pjw@rhyme.com.au>,
- "Mikheev, Vadim"
- <vmikheev@SECTORBASE.COM>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-cc: The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Wed, 23 May 2001 11:25:12 +0200
-MIME-Version: 1.0
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-> >If community will not like UNDO then I'll probably try to implement
-> >dead space collector which will read log files and so on.
->
-> I'd vote for UNDO; in terms of usability & friendliness it's a big win.
-
-Could you please try it a little more verbose ? I am very interested in
-the advantages you see in "UNDO for rollback only".
-
-pg_log is a very big argument, but couldn't we try to change the format
-to something that only stores ranges of aborted txn's in a btree like format ?
-Now that we have WAL, that should be possible.
-
-Andreas
-
-From pjw@rhyme.com.au Wed May 23 06:45:18 2001
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-Date: Wed, 23 May 2001 20:43:24 +1000
-To: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>,
- "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>,
- "'Bruce Momjian'" <pgman@candle.pha.pa.us>
-From: Philip Warner <pjw@rhyme.com.au>
-Subject: Re: AW: [HACKERS] Plans for solving the VACUUM problem
-cc: The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-In-Reply-To: <11C1E6749A55D411A9670001FA6879633682F0@sdexcsrv1.f000.d018
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-
-At 11:25 23/05/01 +0200, Zeugswetter Andreas SB wrote:
->
->> >If community will not like UNDO then I'll probably try to implement
->> >dead space collector which will read log files and so on.
->>
->> I'd vote for UNDO; in terms of usability & friendliness it's a big win.
->
->Could you please try it a little more verbose ? I am very interested in
->the advantages you see in "UNDO for rollback only".
-
-I have not been paying strict attention to this thread, so it may have
-wandered into a narrower band than I think we are in, but my understanding
-is that UNDO is required for partial rollback in the case of failed
-commands withing a single TX. Specifically,
-
-- A simple typo in psql can currently cause a forced rollback of the entire
-TX. UNDO should avoid this.
-
-- It is not uncommon for application in other databases to handle errors
-from the database (eg. missing FKs), and continue a TX.
-
-- Similarly, when we get a new error reporting system, general constraint
-(or other) failures should be able to be handled in one TX.
-
-
-----------------------------------------------------------------
-Philip Warner | __---_____
-Albatross Consulting Pty. Ltd. |----/ - \
-(A.B.N. 75 008 659 498) | /(@) ______---_
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-
-From vmikheev@SECTORBASE.COM Thu May 24 14:07:24 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>,
- "'Philip Warner'" <pjw@rhyme.com.au>,
- "'Bruce Momjian'"
- <pgman@candle.pha.pa.us>
-cc: The Hermit Hacker <scrappy@hub.org>, Tom Lane <tgl@sss.pgh.pa.us>,
- pgsql-hackers@postgresql.org
-Subject: RE: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Thu, 24 May 2001 11:06:08 -0700
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-> > - A simple typo in psql can currently cause a forced
-> > rollback of the entire TX. UNDO should avoid this.
->
-> Yes, I forgot to mention this very big advantage, but undo is
-> not the only possible way to implement savepoints. Solutions
-> using CommandCounter have been discussed.
-
-This would be hell.
-
-Vadim
-
-From ZeugswetterA@wien.spardat.at Fri May 25 03:44:30 2001
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-Message-ID: <11C1E6749A55D411A9670001FA6879633682F3@sdexcsrv1.f000.d0188.sd.spardat.at>
-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
- "'Don Baccus'"
- <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker
- <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 25 May 2001 09:44:14 +0200
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-
-
-> > >> Impractical ? Oracle does it.
-> > >
-> > >Oracle has MVCC?
-> >
-> > With restrictions, yes.
->
-> What restrictions? Rollback segments size?
-
-No, that is not the whole story. The problem with their "rollback segment approach" is,
-that they do not guard against overwriting a tuple version in the rollback segment.
-They simply recycle each segment in a wrap around manner.
-Thus there could be an open transaction that still wanted to see a tuple version
-that was already overwritten, leading to the feared "snapshot too old" error.
-
-Copying their "rollback segment" approach is imho the last thing we want to do.
-
-> Non-overwriting smgr can eat all disk space...
->
-> > You didn't know that? Vadim did ...
->
-> Didn't I mention a few times that I was inspired by Oracle? -:)
-
-Looking at what they supply in the feature area is imho good.
-Copying their technical architecture is not so good in general.
-
-Andreas
-
-From pgsql-hackers-owner+M9354@postgresql.org Fri May 25 04:10:48 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
- "'Don Baccus'"
- <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue
- <Inoue@tpf.co.jp>
-cc: Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker
- <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Fri, 25 May 2001 09:44:14 +0200
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-> > >> Impractical ? Oracle does it.
-> > >
-> > >Oracle has MVCC?
-> >
-> > With restrictions, yes.
->
-> What restrictions? Rollback segments size?
-
-No, that is not the whole story. The problem with their "rollback segment approach" is,
-that they do not guard against overwriting a tuple version in the rollback segment.
-They simply recycle each segment in a wrap around manner.
-Thus there could be an open transaction that still wanted to see a tuple version
-that was already overwritten, leading to the feared "snapshot too old" error.
-
-Copying their "rollback segment" approach is imho the last thing we want to do.
-
-> Non-overwriting smgr can eat all disk space...
->
-> > You didn't know that? Vadim did ...
->
-> Didn't I mention a few times that I was inspired by Oracle? -:)
-
-Looking at what they supply in the feature area is imho good.
-Copying their technical architecture is not so good in general.
-
-Andreas
-
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-From pgsql-hackers-owner+M9402@postgresql.org Mon May 28 04:14:09 2001
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-Message-ID: <11C1E6749A55D411A9670001FA6879633682F5@sdexcsrv1.f000.d0188.sd.spardat.at>
-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
- "'Hannu Krosing'"
- <hannu@tm.ee>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 28 May 2001 10:02:17 +0200
-MIME-Version: 1.0
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-> > You mean it is restored in session that is running the transaction ?
-
-Depends on what you mean with restored. It first reads the heap page,
-sees that it needs an older version and thus reads it from the "rollback segment".
-
-> >
-> > I guess thet it could be slower than our current way of doing it.
->
-> Yes, for older transactions which *really* need in *particular*
-> old data, but not for newer ones. Look - now transactions have to read
-> dead data again and again, even if some of them (newer) need not to see
-> those data at all, and we keep dead data as long as required for other
-> old transactions *just for the case* they will look there.
-> But who knows?! Maybe those old transactions will not read from table
-> with big amount of dead data at all! So - why keep dead data in datafiles
-> for long time? This obviously affects overall system performance.
-
-Yes, that is a good description. And old version is only required in the following
-two cases:
-
-1. the txn that modified this tuple is still open (reader in default committed read)
-2. reader is in serializable transaction isolation and has earlier xtid
-
-Seems overwrite smgr has mainly advantages in terms of speed for operations
-other than rollback.
-
-Andreas
-
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-From pgsql-hackers-owner+M9403@postgresql.org Mon May 28 05:16:44 2001
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-Date: Mon, 28 May 2001 11:11:46 +0200
-From: Hannu Krosing <hannu@tm.ee>
-X-Mailer: Mozilla 4.76 [en] (X11; U; Linux 2.4.2-2 i686)
-X-Accept-Language: et, en, ru
-MIME-Version: 1.0
-To: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-cc: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
- "'Don Baccus'" <dhogaza@pacifier.com>, Tom Lane <tgl@sss.pgh.pa.us>,
- Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
- The Hermit Hacker <scrappy@hub.org>, pgsql-hackers@postgresql.org
-Subject: Re: AW: [HACKERS] Plans for solving the VACUUM problem
-References: <11C1E6749A55D411A9670001FA6879633682F5@sdexcsrv1.f000.d0188.sd.spardat.at>
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-Zeugswetter Andreas SB wrote:
->
-> > > You mean it is restored in session that is running the transaction ?
->
-> Depends on what you mean with restored. It first reads the heap page,
-> sees that it needs an older version and thus reads it from the "rollback segment".
-
-So are whole pages stored in rollback segments or just the modified data
-?
-
-Storing whole pages could be very wasteful for tables with small records
-that
-are often modified.
-
----------------
-Hannu
-
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-From vmikheev@sectorbase.com Mon May 28 13:15:16 2001
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-From: "Vadim Mikheev" <vmikheev@sectorbase.com>
-To: "Hannu Krosing" <hannu@tm.ee>,
- "Zeugswetter Andreas SB" <ZeugswetterA@wien.spardat.at>
-cc: "'Don Baccus'" <dhogaza@pacifier.com>, "Tom Lane" <tgl@sss.pgh.pa.us>,
- "Hiroshi Inoue" <Inoue@tpf.co.jp>, "Bruce Momjian" <pgman@candle.pha.pa.us>,
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-References: <11C1E6749A55D411A9670001FA6879633682F5@sdexcsrv1.f000.d0188.sd.spardat.at> <3B121652.C4DCE1F6@tm.ee>
-Subject: Re: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Mon, 28 May 2001 10:15:17 -0700
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-> > > > You mean it is restored in session that is running the transaction ?
-> >
-> > Depends on what you mean with restored. It first reads the heap page,
-> > sees that it needs an older version and thus reads it from the "rollback segment".
->
-> So are whole pages stored in rollback segments or just the modified data?
-
-This is implementation dependent. Storing whole pages is much easy to do,
-but obviously it's better to store just modified data.
-
-Vadim
-
-
-
-From pgsql-hackers-owner+M9458@postgresql.org Tue May 29 13:49:27 2001
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-From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
-To: "'Zeugswetter Andreas SB'" <ZeugswetterA@wien.spardat.at>
-cc: "'pgsql-hackers@postgresql.org'" <pgsql-hackers@postgresql.org>
-Subject: RE: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 29 May 2001 10:39:59 -0700
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-> > > So are whole pages stored in rollback segments or just
-> > > the modified data?
-> >
-> > This is implementation dependent. Storing whole pages is
-> > much easy to do, but obviously it's better to store just
-> > modified data.
->
-> I am not sure it is necessarily better. Seems to be a tradeoff here.
-> pros of whole pages:
-> a possible merge with physical log (for first
-> modification of a page after checkpoint
-> there would be no overhead compared to current
-> since it is already written now)
-
-Using WAL as RS data storage is questionable.
-
-> in a clever implementation a page already in the
-> "rollback segment" might satisfy the
-> modification of another row on that page, and
-> thus would not need any additional io.
-
-This would be possible only if there was no commit (same SCN)
-between two modifications.
-
-But, aren't we too deep on overwriting smgr (O-smgr) implementation?
-It's doable. It has advantages in terms of IO active transactions
-must do to follow MVCC. It has drawback in terms of required
-disk space (and, oh yeh, it's not easy to implement -:)).
-So, any other opinions about value of O-smgr?
-
-Vadim
-
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-From pgsql-hackers-owner+M9173@postgresql.org Tue May 22 04:27:18 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Barry Lind'" <barry@xythos.com>, pgsql-hackers@postgresql.org
-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 09:54:21 +0200
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-> Correct me if I am wrong, but both cases do present a problem currently
-> in 7.1. The WAL log will not remove any WAL files for transactions that
-> are still open (even after a checkpoint occurs). Thus if you do a bulk
-> insert of gigabyte size you will require a gigabyte sized WAL
-> directory. Also if you have a simple OLTP transaction that the user
-> started and walked away from for his one week vacation, then no WAL log
-> files can be deleted until that user returns from his vacation and ends
-> his transaction.
-
-I am not sure, it might be so implemented. But there is no technical reason
-to keep them beyond checkpoint without UNDO.
-
-Andreas
-
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-From pgsql-hackers-owner+M9170@postgresql.org Tue May 22 04:20:39 2001
-Return-path: <pgsql-hackers-owner+M9170@postgresql.org>
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-Message-ID: <11C1E6749A55D411A9670001FA6879633682E7@sdexcsrv1.f000.d0188.sd.spardat.at>
-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Jan Wieck'" <JanWieck@yahoo.com>, Barry Lind <barry@xythos.com>
-cc: pgsql-hackers@postgresql.org
-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 10:06:54 +0200
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-> As a rule of thumb, online applications that hold open
-> transactions during user interaction are considered to be
-> Broken By Design (tm). So I'd slap the programmer/design
-> team with - let's use the server box since it doesn't contain
-> anything useful.
-
-We have a database system here, and not an OLTP helper app.
-A database system must support all sorts of mixed usage from simple
-OLTP to OLAP. Imho the usual separation on different servers gives more
-headaches than are necessary.
-
-Thus above statement can imho be true for one OLTP application, but not
-for all applications on one db server.
-
-Andreas
-
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-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
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- <tgl@sss.pgh.pa.us>
-cc: pgsql-hackers@postgresql.org
-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 22 May 2001 11:27:19 +0200
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-> Todo:
->
-> 1. Compact log files after checkpoint (save records of uncommitted
-> transactions and remove/archive others).
-
-On the grounds that undo is not guaranteed anyway (concurrent heap access),
-why not simply forget it, since above sounds rather expensive ?
-The downside would only be, that long running txn's cannot [easily] rollback
-to savepoint.
-
-> 2. Abort long running transactions.
-
-This is imho "the" big downside of UNDO, and should not simply be put on
-the TODO without thorow research. I think it would be better to forget UNDO for long
-running transactions before aborting them.
-
-Andreas
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-From pgsql-hackers-owner+M9246@postgresql.org Wed May 23 04:58:55 2001
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-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>,
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-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Wed, 23 May 2001 10:45:17 +0200
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-> > The downside would only be, that long running txn's cannot
-> > [easily] rollback to savepoint.
->
-> We should implement savepoints for all or none transactions, no?
-
-We should not limit transaction size to online available disk space for WAL.
-Imho that is much more important. With guaranteed undo we would need
-diskspace for more than 2x new data size (+ at least space for 1x all modified
-pages unless physical log is separated from WAL).
-
-Imho a good design should involve only little more than 1x new data size.
-
->
-> > > 2. Abort long running transactions.
-> >
-> > This is imho "the" big downside of UNDO, and should not
-> > simply be put on the TODO without thorow research. I think it
-> > would be better to forget UNDO for long running transactions
-> > before aborting them.
->
-> Abort could be configurable.
-
-The point is, that you need to abort before WAL runs out of disk space
-regardless of configuration.
-
-Andreas
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-From pgsql-hackers-owner+M9252@postgresql.org Wed May 23 07:17:03 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Philip Warner'" <pjw@rhyme.com.au>,
- "Mikheev, Vadim"
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-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Wed, 23 May 2001 13:08:28 +0200
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-> - A simple typo in psql can currently cause a forced rollback of the entire
-> TX. UNDO should avoid this.
-
-Yes, I forgot to mention this very big advantage, but undo is not the only possible way
-to implement savepoints. Solutions using CommandCounter have been discussed.
-Although the pg_log mechanism would become more complex, a background
-"vacuum-like" process could put highest priority on removing such rolled back parts
-of transactions.
-
-Andreas
-
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-From pgsql-hackers-owner+M9443@postgresql.org Tue May 29 04:21:45 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Vadim Mikheev'" <vmikheev@sectorbase.com>
-cc: "'pgsql-hackers@postgresql.org'" <pgsql-hackers@postgresql.org>
-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Tue, 29 May 2001 09:35:01 +0200
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-> > > > > You mean it is restored in session that is running the transaction ?
-> > >
-> > > Depends on what you mean with restored. It first reads the heap page,
-> > > sees that it needs an older version and thus reads it from the "rollback segment".
-> >
-> > So are whole pages stored in rollback segments or just the modified data?
->
-> This is implementation dependent. Storing whole pages is much easy to do,
-> but obviously it's better to store just modified data.
-
-I am not sure it is necessarily better. Seems to be a tradeoff here.
-pros of whole pages:
- a possible merge with physical log (for first modification of a page after checkpoint
- there would be no overhead compared to current since it is already written now)
- in a clever implementation a page already in the "rollback segment" might satisfy the
- modification of another row on that page, and thus would not need any additional io.
-
-Andreas
-
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-From pgsql-hackers-owner+M9473@postgresql.org Wed May 30 06:30:34 2001
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-From: Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at>
-To: "'Mikheev, Vadim'" <vmikheev@SECTORBASE.COM>
-cc: "'pgsql-hackers@postgresql.org'" <pgsql-hackers@postgresql.org>
-Subject: AW: AW: [HACKERS] Plans for solving the VACUUM problem
-Date: Wed, 30 May 2001 12:18:07 +0200
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-> > > > So are whole pages stored in rollback segments or just
-> > > > the modified data?
-> > >
-> > > This is implementation dependent. Storing whole pages is
-> > > much easy to do, but obviously it's better to store just
-> > > modified data.
-> >
-> > I am not sure it is necessarily better. Seems to be a tradeoff here.
-> > pros of whole pages:
-> > a possible merge with physical log (for first
-> > modification of a page after checkpoint
-> > there would be no overhead compared to current
-> > since it is already written now)
->
-> Using WAL as RS data storage is questionable.
-
-No, I meant the other way around. Move the physical log pages away from WAL
-files to the "rollback segment" (imho "snapshot area" would be a better name)
-
-> > in a clever implementation a page already in the
-> > "rollback segment" might satisfy the
-> > modification of another row on that page, and
-> > thus would not need any additional io.
->
-> This would be possible only if there was no commit (same SCN)
-> between two modifications.
-
-I don't think someone else's commit matters unless it touches the same page.
-In that case a reader would possibly need to chain back to an older version
-inside the snapshot area, and then it gets complicated even in the whole page
-case. A good concept could probably involve both whole page and change
-only, and let the optimizer decide what to do.
-
-> But, aren't we too deep on overwriting smgr (O-smgr) implementation?
-
-Yes, but some understanding of the possibilities needs to be sorted out
-to allow good decicsions, no ?
-
-Andreas
-
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