Add a few more DTrace probes to the backend.

[postgresql] / src / backend / access / transam / clog.c

/*-------------------------------------------------------------------------
 *
 * clog.c
 *              PostgreSQL transaction-commit-log manager
 *
 * This module replaces the old "pg_log" access code, which treated pg_log
 * essentially like a relation, in that it went through the regular buffer
 * manager.  The problem with that was that there wasn't any good way to
 * recycle storage space for transactions so old that they'll never be
 * looked up again.  Now we use specialized access code so that the commit
 * log can be broken into relatively small, independent segments.
 *
 * XLOG interactions: this module generates an XLOG record whenever a new
 * CLOG page is initialized to zeroes.  Other writes of CLOG come from
 * recording of transaction commit or abort in xact.c, which generates its
 * own XLOG records for these events and will re-perform the status update
 * on redo; so we need make no additional XLOG entry here.      For synchronous
 * transaction commits, the XLOG is guaranteed flushed through the XLOG commit
 * record before we are called to log a commit, so the WAL rule "write xlog
 * before data" is satisfied automatically.  However, for async commits we
 * must track the latest LSN affecting each CLOG page, so that we can flush
 * XLOG that far and satisfy the WAL rule.      We don't have to worry about this
 * for aborts (whether sync or async), since the post-crash assumption would
 * be that such transactions failed anyway.
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $PostgreSQL: pgsql/src/backend/access/transam/clog.c,v 1.47 2008/08/01 13:16:08 alvherre Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/clog.h"
#include "access/slru.h"
#include "access/transam.h"
#include "pg_trace.h"
#include "postmaster/bgwriter.h"

/*
 * Defines for CLOG page sizes.  A page is the same BLCKSZ as is used
 * everywhere else in Postgres.
 *
 * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
 * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
 * and CLOG segment numbering at 0xFFFFFFFF/CLOG_XACTS_PER_SEGMENT.  We need
 * take no explicit notice of that fact in this module, except when comparing
 * segment and page numbers in TruncateCLOG (see CLOGPagePrecedes).
 */

/* We need two bits per xact, so four xacts fit in a byte */
#define CLOG_BITS_PER_XACT      2
#define CLOG_XACTS_PER_BYTE 4
#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
#define CLOG_XACT_BITMASK       ((1 << CLOG_BITS_PER_XACT) - 1)

#define TransactionIdToPage(xid)        ((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
#define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
#define TransactionIdToByte(xid)        (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
#define TransactionIdToBIndex(xid)      ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)

/* We store the latest async LSN for each group of transactions */
#define CLOG_XACTS_PER_LSN_GROUP        32      /* keep this a power of 2 */
#define CLOG_LSNS_PER_PAGE      (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)

#define GetLSNIndex(slotno, xid)        ((slotno) * CLOG_LSNS_PER_PAGE + \
        ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)


/*
 * Link to shared-memory data structures for CLOG control
 */
static SlruCtlData ClogCtlData;

#define ClogCtl (&ClogCtlData)


static int      ZeroCLOGPage(int pageno, bool writeXlog);
static bool CLOGPagePrecedes(int page1, int page2);
static void WriteZeroPageXlogRec(int pageno);
static void WriteTruncateXlogRec(int pageno);


/*
 * Record the final state of a transaction in the commit log.
 *
 * lsn must be the WAL location of the commit record when recording an async
 * commit.      For a synchronous commit it can be InvalidXLogRecPtr, since the
 * caller guarantees the commit record is already flushed in that case.  It
 * should be InvalidXLogRecPtr for abort cases, too.
 *
 * NB: this is a low-level routine and is NOT the preferred entry point
 * for most uses; TransactionLogUpdate() in transam.c is the intended caller.
 */
void
TransactionIdSetStatus(TransactionId xid, XidStatus status, XLogRecPtr lsn)
{
        int                     pageno = TransactionIdToPage(xid);
        int                     byteno = TransactionIdToByte(xid);
        int                     bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
        int                     slotno;
        char       *byteptr;
        char            byteval;

        Assert(status == TRANSACTION_STATUS_COMMITTED ||
                   status == TRANSACTION_STATUS_ABORTED ||
                   status == TRANSACTION_STATUS_SUB_COMMITTED);

        LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

        /*
         * If we're doing an async commit (ie, lsn is valid), then we must wait
         * for any active write on the page slot to complete.  Otherwise our
         * update could reach disk in that write, which will not do since we
         * mustn't let it reach disk until we've done the appropriate WAL flush.
         * But when lsn is invalid, it's OK to scribble on a page while it is
         * write-busy, since we don't care if the update reaches disk sooner than
         * we think.  Hence, pass write_ok = XLogRecPtrIsInvalid(lsn).
         */
        slotno = SimpleLruReadPage(ClogCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
        byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;

        /* Current state should be 0, subcommitted or target state */
        Assert(((*byteptr >> bshift) & CLOG_XACT_BITMASK) == 0 ||
                   ((*byteptr >> bshift) & CLOG_XACT_BITMASK) == TRANSACTION_STATUS_SUB_COMMITTED ||
                   ((*byteptr >> bshift) & CLOG_XACT_BITMASK) == status);

        /* note this assumes exclusive access to the clog page */
        byteval = *byteptr;
        byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
        byteval |= (status << bshift);
        *byteptr = byteval;

        ClogCtl->shared->page_dirty[slotno] = true;

        /*
         * Update the group LSN if the transaction completion LSN is higher.
         *
         * Note: lsn will be invalid when supplied during InRecovery processing,
         * so we don't need to do anything special to avoid LSN updates during
         * recovery. After recovery completes the next clog change will set the
         * LSN correctly.
         */
        if (!XLogRecPtrIsInvalid(lsn))
        {
                int                     lsnindex = GetLSNIndex(slotno, xid);

                if (XLByteLT(ClogCtl->shared->group_lsn[lsnindex], lsn))
                        ClogCtl->shared->group_lsn[lsnindex] = lsn;
        }

        LWLockRelease(CLogControlLock);
}

/*
 * Interrogate the state of a transaction in the commit log.
 *
 * Aside from the actual commit status, this function returns (into *lsn)
 * an LSN that is late enough to be able to guarantee that if we flush up to
 * that LSN then we will have flushed the transaction's commit record to disk.
 * The result is not necessarily the exact LSN of the transaction's commit
 * record!      For example, for long-past transactions (those whose clog pages
 * already migrated to disk), we'll return InvalidXLogRecPtr.  Also, because
 * we group transactions on the same clog page to conserve storage, we might
 * return the LSN of a later transaction that falls into the same group.
 *
 * NB: this is a low-level routine and is NOT the preferred entry point
 * for most uses; TransactionLogFetch() in transam.c is the intended caller.
 */
XidStatus
TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
{
        int                     pageno = TransactionIdToPage(xid);
        int                     byteno = TransactionIdToByte(xid);
        int                     bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
        int                     slotno;
        int                     lsnindex;
        char       *byteptr;
        XidStatus       status;

        /* lock is acquired by SimpleLruReadPage_ReadOnly */

        slotno = SimpleLruReadPage_ReadOnly(ClogCtl, pageno, xid);
        byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;

        status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;

        lsnindex = GetLSNIndex(slotno, xid);
        *lsn = ClogCtl->shared->group_lsn[lsnindex];

        LWLockRelease(CLogControlLock);

        return status;
}


/*
 * Initialization of shared memory for CLOG
 */
Size
CLOGShmemSize(void)
{
        return SimpleLruShmemSize(NUM_CLOG_BUFFERS, CLOG_LSNS_PER_PAGE);
}

void
CLOGShmemInit(void)
{
        ClogCtl->PagePrecedes = CLOGPagePrecedes;
        SimpleLruInit(ClogCtl, "CLOG Ctl", NUM_CLOG_BUFFERS, CLOG_LSNS_PER_PAGE,
                                  CLogControlLock, "pg_clog");
}

/*
 * This func must be called ONCE on system install.  It creates
 * the initial CLOG segment.  (The CLOG directory is assumed to
 * have been created by the initdb shell script, and CLOGShmemInit
 * must have been called already.)
 */
void
BootStrapCLOG(void)
{
        int                     slotno;

        LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

        /* Create and zero the first page of the commit log */
        slotno = ZeroCLOGPage(0, false);

        /* Make sure it's written out */
        SimpleLruWritePage(ClogCtl, slotno, NULL);
        Assert(!ClogCtl->shared->page_dirty[slotno]);

        LWLockRelease(CLogControlLock);
}

/*
 * Initialize (or reinitialize) a page of CLOG to zeroes.
 * If writeXlog is TRUE, also emit an XLOG record saying we did this.
 *
 * The page is not actually written, just set up in shared memory.
 * The slot number of the new page is returned.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
static int
ZeroCLOGPage(int pageno, bool writeXlog)
{
        int                     slotno;

        slotno = SimpleLruZeroPage(ClogCtl, pageno);

        if (writeXlog)
                WriteZeroPageXlogRec(pageno);

        return slotno;
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup,
 * after StartupXLOG has initialized ShmemVariableCache->nextXid.
 */
void
StartupCLOG(void)
{
        TransactionId xid = ShmemVariableCache->nextXid;
        int                     pageno = TransactionIdToPage(xid);

        LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

        /*
         * Initialize our idea of the latest page number.
         */
        ClogCtl->shared->latest_page_number = pageno;

        /*
         * Zero out the remainder of the current clog page.  Under normal
         * circumstances it should be zeroes already, but it seems at least
         * theoretically possible that XLOG replay will have settled on a nextXID
         * value that is less than the last XID actually used and marked by the
         * previous database lifecycle (since subtransaction commit writes clog
         * but makes no WAL entry).  Let's just be safe. (We need not worry about
         * pages beyond the current one, since those will be zeroed when first
         * used.  For the same reason, there is no need to do anything when
         * nextXid is exactly at a page boundary; and it's likely that the
         * "current" page doesn't exist yet in that case.)
         */
        if (TransactionIdToPgIndex(xid) != 0)
        {
                int                     byteno = TransactionIdToByte(xid);
                int                     bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
                int                     slotno;
                char       *byteptr;

                slotno = SimpleLruReadPage(ClogCtl, pageno, false, xid);
                byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;

                /* Zero so-far-unused positions in the current byte */
                *byteptr &= (1 << bshift) - 1;
                /* Zero the rest of the page */
                MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);

                ClogCtl->shared->page_dirty[slotno] = true;
        }

        LWLockRelease(CLogControlLock);
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownCLOG(void)
{
        /* Flush dirty CLOG pages to disk */
        TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(false);
        SimpleLruFlush(ClogCtl, false);
        TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(false);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointCLOG(void)
{
        /* Flush dirty CLOG pages to disk */
        TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
        SimpleLruFlush(ClogCtl, true);
        TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
}


/*
 * Make sure that CLOG has room for a newly-allocated XID.
 *
 * NB: this is called while holding XidGenLock.  We want it to be very fast
 * most of the time; even when it's not so fast, no actual I/O need happen
 * unless we're forced to write out a dirty clog or xlog page to make room
 * in shared memory.
 */
void
ExtendCLOG(TransactionId newestXact)
{
        int                     pageno;

        /*
         * No work except at first XID of a page.  But beware: just after
         * wraparound, the first XID of page zero is FirstNormalTransactionId.
         */
        if (TransactionIdToPgIndex(newestXact) != 0 &&
                !TransactionIdEquals(newestXact, FirstNormalTransactionId))
                return;

        pageno = TransactionIdToPage(newestXact);

        LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

        /* Zero the page and make an XLOG entry about it */
        ZeroCLOGPage(pageno, true);

        LWLockRelease(CLogControlLock);
}


/*
 * Remove all CLOG segments before the one holding the passed transaction ID
 *
 * Before removing any CLOG data, we must flush XLOG to disk, to ensure
 * that any recently-emitted HEAP_FREEZE records have reached disk; otherwise
 * a crash and restart might leave us with some unfrozen tuples referencing
 * removed CLOG data.  We choose to emit a special TRUNCATE XLOG record too.
 * Replaying the deletion from XLOG is not critical, since the files could
 * just as well be removed later, but doing so prevents a long-running hot
 * standby server from acquiring an unreasonably bloated CLOG directory.
 *
 * Since CLOG segments hold a large number of transactions, the opportunity to
 * actually remove a segment is fairly rare, and so it seems best not to do
 * the XLOG flush unless we have confirmed that there is a removable segment.
 */
void
TruncateCLOG(TransactionId oldestXact)
{
        int                     cutoffPage;

        /*
         * The cutoff point is the start of the segment containing oldestXact. We
         * pass the *page* containing oldestXact to SimpleLruTruncate.
         */
        cutoffPage = TransactionIdToPage(oldestXact);

        /* Check to see if there's any files that could be removed */
        if (!SlruScanDirectory(ClogCtl, cutoffPage, false))
                return;                                 /* nothing to remove */

        /* Write XLOG record and flush XLOG to disk */
        WriteTruncateXlogRec(cutoffPage);

        /* Now we can remove the old CLOG segment(s) */
        SimpleLruTruncate(ClogCtl, cutoffPage);
}


/*
 * Decide which of two CLOG page numbers is "older" for truncation purposes.
 *
 * We need to use comparison of TransactionIds here in order to do the right
 * thing with wraparound XID arithmetic.  However, if we are asked about
 * page number zero, we don't want to hand InvalidTransactionId to
 * TransactionIdPrecedes: it'll get weird about permanent xact IDs.  So,
 * offset both xids by FirstNormalTransactionId to avoid that.
 */
static bool
CLOGPagePrecedes(int page1, int page2)
{
        TransactionId xid1;
        TransactionId xid2;

        xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
        xid1 += FirstNormalTransactionId;
        xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
        xid2 += FirstNormalTransactionId;

        return TransactionIdPrecedes(xid1, xid2);
}


/*
 * Write a ZEROPAGE xlog record
 */
static void
WriteZeroPageXlogRec(int pageno)
{
        XLogRecData rdata;

        rdata.data = (char *) (&pageno);
        rdata.len = sizeof(int);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;
        (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE, &rdata);
}

/*
 * Write a TRUNCATE xlog record
 *
 * We must flush the xlog record to disk before returning --- see notes
 * in TruncateCLOG().
 */
static void
WriteTruncateXlogRec(int pageno)
{
        XLogRecData rdata;
        XLogRecPtr      recptr;

        rdata.data = (char *) (&pageno);
        rdata.len = sizeof(int);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;
        recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE, &rdata);
        XLogFlush(recptr);
}

/*
 * CLOG resource manager's routines
 */
void
clog_redo(XLogRecPtr lsn, XLogRecord *record)
{
        uint8           info = record->xl_info & ~XLR_INFO_MASK;

        if (info == CLOG_ZEROPAGE)
        {
                int                     pageno;
                int                     slotno;

                memcpy(&pageno, XLogRecGetData(record), sizeof(int));

                LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

                slotno = ZeroCLOGPage(pageno, false);
                SimpleLruWritePage(ClogCtl, slotno, NULL);
                Assert(!ClogCtl->shared->page_dirty[slotno]);

                LWLockRelease(CLogControlLock);
        }
        else if (info == CLOG_TRUNCATE)
        {
                int                     pageno;

                memcpy(&pageno, XLogRecGetData(record), sizeof(int));

                /*
                 * During XLOG replay, latest_page_number isn't set up yet; insert a
                 * suitable value to bypass the sanity test in SimpleLruTruncate.
                 */
                ClogCtl->shared->latest_page_number = pageno;

                SimpleLruTruncate(ClogCtl, pageno);
        }
        else
                elog(PANIC, "clog_redo: unknown op code %u", info);
}

void
clog_desc(StringInfo buf, uint8 xl_info, char *rec)
{
        uint8           info = xl_info & ~XLR_INFO_MASK;

        if (info == CLOG_ZEROPAGE)
        {
                int                     pageno;

                memcpy(&pageno, rec, sizeof(int));
                appendStringInfo(buf, "zeropage: %d", pageno);
        }
        else if (info == CLOG_TRUNCATE)
        {
                int                     pageno;

                memcpy(&pageno, rec, sizeof(int));
                appendStringInfo(buf, "truncate before: %d", pageno);
        }
        else
                appendStringInfo(buf, "UNKNOWN");
}