-<!--
-$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.65 2005/04/19 01:39:50 momjian Exp $
--->
+<!-- doc/src/sgml/backup.sgml -->
+
<chapter id="backup">
<title>Backup and Restore</title>
<para>
As with everything that contains valuable data, <productname>PostgreSQL</>
databases should be backed up regularly. While the procedure is
- essentially simple, it is important to have a basic understanding of
+ essentially simple, it is important to have a clear understanding of
the underlying techniques and assumptions.
</para>
<itemizedlist>
<listitem><para><acronym>SQL</> dump</para></listitem>
<listitem><para>File system level backup</para></listitem>
- <listitem><para>On-line backup</para></listitem>
+ <listitem><para>Continuous archiving</para></listitem>
</itemizedlist>
- Each has its own strengths and weaknesses.
+ Each has its own strengths and weaknesses; each is discussed in turn
+ in the following sections.
</para>
<sect1 id="backup-dump">
<title><acronym>SQL</> Dump</title>
<para>
- The idea behind the SQL-dump method is to generate a text file with SQL
+ The idea behind this dump method is to generate a text file with SQL
commands that, when fed back to the server, will recreate the
database in the same state as it was at the time of the dump.
<productname>PostgreSQL</> provides the utility program
<synopsis>
pg_dump <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">outfile</replaceable>
</synopsis>
- As you see, <application>pg_dump</> writes its results to the
+ As you see, <application>pg_dump</> writes its result to the
standard output. We will see below how this can be useful.
</para>
<para>
<application>pg_dump</> is a regular <productname>PostgreSQL</>
client application (albeit a particularly clever one). This means
- that you can do this backup procedure from any remote host that has
+ that you can perform this backup procedure from any remote host that has
access to the database. But remember that <application>pg_dump</>
does not operate with special permissions. In particular, it must
have read access to all tables that you want to back up, so in
</para>
<para>
-
As any other <productname>PostgreSQL</> client application,
+
Like any other <productname>PostgreSQL</> client application,
<application>pg_dump</> will by default connect with the database
user name that is equal to the current operating system user name. To override
this, either specify the <option>-U</option> option or set the
linkend="client-authentication">).
</para>
+ <para>
+ An important advantage of <application>pg_dump</> over the other backup
+ methods described later is that <application>pg_dump</>'s output can
+ generally be re-loaded into newer versions of <productname>PostgreSQL</>,
+ whereas file-level backups and continuous archiving are both extremely
+ server-version-specific. <application>pg_dump</> is also the only method
+ that will work when transferring a database to a different machine
+ architecture, such as going from a 32-bit to a 64-bit server.
+ </para>
+
<para>
Dumps created by <application>pg_dump</> are internally consistent,
- that is, updates to the database while <application>pg_dump</> is
-
running will not be in the dump. <application>pg_dump</> does not
+ meaning, the dump represents a snapshot of the database at the time
+
<application>pg_dump</> began running. <application>pg_dump</> does not
block other operations on the database while it is working.
(Exceptions are those operations that need to operate with an
- exclusive lock, such as <command>VACUUM FULL</command>.)
+ exclusive lock, such as most forms of <command>ALTER TABLE</command>.)
</para>
<important>
<para>
- When your database schema relies on OIDs (for instance as foreign
+ If your database schema relies on OIDs (for instance, as foreign
keys) you must instruct <application>pg_dump</> to dump the OIDs
- as well. To do this, use the <option>-o</option> command line
- option. <quote>Large objects</> are not dumped by default,
- either. See <xref linkend="app-pgdump">'s reference page if you
- use large objects.
+ as well. To do this, use the <option>-o</option> command-line
+ option.
</para>
</important>
<synopsis>
psql <replaceable class="parameter">dbname</replaceable> < <replaceable class="parameter">infile</replaceable>
</synopsis>
- where <replaceable class="parameter">infile</replaceable> is what
- you used as <replaceable class="parameter">outfile</replaceable>
- for the <application>pg_dump</> command. The database <replaceable
+ where <replaceable class="parameter">infile</replaceable> is the
+ file output by the <application>pg_dump</> command. The database <replaceable
class="parameter">dbname</replaceable> will not be created by this
- command, you must create it yourself from <literal>template0</> before executing
- <application>psql</> (e.g., with <literal>createdb -T template0
- <replaceable class="parameter">dbname</></literal>).
- <application>psql</> supports options similar to <application>pg_dump</>
- for controlling the database server location and the user name. See
- <xref linkend="app-psql">'s reference page for more information.
+ command, so you must create it yourself from <literal>template0</>
+ before executing <application>psql</> (e.g., with
+ <literal>createdb -T template0 <replaceable
+ class="parameter">dbname</></literal>). <application>psql</>
+ supports options similar to <application>pg_dump</> for specifying
+ the database server to connect to and the user name to use. See
+ the <xref linkend="app-psql"> reference page for more information.
</para>
<para>
- Not only must the target database already exist before starting to
- run the restore, but so must all the users who own objects in the
- dumped database or were granted permissions on the objects. If they
- do not, then the restore will fail to recreate the objects with the
- original ownership and/or permissions. (Sometimes this is what you want,
- but usually it is not.)
+ Before restoring an SQL dump, all the users who own objects or were
+ granted permissions on objects in the dumped database must already
+ exist. If they do not, the restore will fail to recreate the
+ objects with the original ownership and/or permissions.
+ (Sometimes this is what you want, but usually it is not.)
</para>
<para>
- Once restored, it is wise to run <xref linkend="sql-analyze"
- endterm="sql-analyze-title"> on each database so the optimizer has
- useful statistics. An easy way to do this is to run
- <command>vacuumdb -a -z</> to
- <command>VACUUM ANALYZE</> all databases; this is equivalent to
- running <command>VACUUM ANALYZE</command> manually.
+ By default, the <application>psql</> script will continue to
+ execute after an SQL error is encountered. You might wish to run
+ <application>psql</application> with
+ the <literal>ON_ERROR_STOP</> variable set to alter that
+ behavior and have <application>psql</application> exit with an
+ exit status of 3 if an SQL error occurs:
+<programlisting>
+psql --set ON_ERROR_STOP=on dbname < infile
+</programlisting>
+ Either way, you will only have a partially restored database.
+ Alternatively, you can specify that the whole dump should be
+ restored as a single transaction, so the restore is either fully
+ completed or fully rolled back. This mode can be specified by
+ passing the <option>-1</> or <option>--single-transaction</>
+ command-line options to <application>psql</>. When using this
+ mode, be aware that even a minor error can rollback a
+ restore that has already run for many hours. However, that might
+ still be preferable to manually cleaning up a complex database
+ after a partially restored dump.
</para>
<para>
The ability of <application>pg_dump</> and <application>psql</> to
write to or read from pipes makes it possible to dump a database
- directly from one server to another; for example:
+ directly from one server to another, for example:
<programlisting>
pg_dump -h <replaceable>host1</> <replaceable>dbname</> | psql -h <replaceable>host2</> <replaceable>dbname</>
</programlisting>
<para>
The dumps produced by <application>pg_dump</> are relative to
<literal>template0</>. This means that any languages, procedures,
- etc. added to <literal>template1</> will also be dumped by
+ etc. added via <literal>template1</> will also be dumped by
<application>pg_dump</>. As a result, when restoring, if you are
using a customized <literal>template1</>, you must create the
empty database from <literal>template0</>, as in the example
</important>
<para>
- For advice on how to load large amounts of data into
- <productname>PostgreSQL</productname> efficiently, refer to <xref
+ After restoring a backup, it is wise to run <xref
+ linkend="sql-analyze"> on each
+ database so the query optimizer has useful statistics;
+ see <xref linkend="vacuum-for-statistics">
+ and <xref linkend="autovacuum"> for more information.
+ For more advice on how to load large amounts of data
+ into <productname>PostgreSQL</> efficiently, refer to <xref
linkend="populate">.
</para>
</sect2>
<title>Using <application>pg_dumpall</></title>
<para>
- The above mechanism is cumbersome and inappropriate when backing
- up an entire database cluster. For this reason the <xref
- linkend="app-pg-dumpall"> program is provided.
+ <application>pg_dump</> dumps only a single database at a time,
+ and it does not dump information about roles or tablespaces
+ (because those are cluster-wide rather than per-database).
+ To support convenient dumping of the entire contents of a database
+ cluster, the <xref linkend="app-pg-dumpall"> program is provided.
<application>pg_dumpall</> backs up each database in a given
- cluster, and also preserves cluster-wide data such as users and
- groups. The basic usage of this command is:
+ cluster, and also preserves cluster-wide data such as role and
+ tablespace definitions. The basic usage of this command is:
<synopsis>
pg_dumpall > <replaceable>outfile</>
</synopsis>
The resulting dump can be restored with <application>psql</>:
<synopsis>
-psql -f <replaceable class="parameter">infile</replaceable> template1
+psql -f <replaceable class="parameter">infile</replaceable> postgres
</synopsis>
(Actually, you can specify any existing database name to start from,
- but if you are reloading in an empty cluster then <literal>template1</>
- is the only available choice.) It is always necessary to have
+ but if you are loading into an empty cluster then <literal>postgres</>
+ should usually be used.) It is always necessary to have
database superuser access when restoring a <application>pg_dumpall</>
- dump, as that is required to restore the user and group information.
+ dump, as that is required to restore the role and tablespace information.
+ If you use tablespaces, make sure that the tablespace paths in the
+ dump are appropriate for the new installation.
+ </para>
+
+ <para>
+ <application>pg_dumpall</> works by emitting commands to re-create
+ roles, tablespaces, and empty databases, then invoking
+ <application>pg_dump</> for each database. This means that while
+ each database will be internally consistent, the snapshots of
+ different databases might not be exactly in-sync.
</para>
</sect2>
<title>Handling large databases</title>
<para>
- Since <productname>PostgreSQL</productname> allows tables larger
- than the maximum file size on your system, it can be problematic
- to dump such a table to a file, since the resulting file will likely
- be larger than the maximum size allowed by your system. Since
- <application>pg_dump</> can write to the standard output, you can
- just use standard Unix tools to work around this possible problem.
+ Some operating systems have maximum file size limits that cause
+ problems when creating large <application>pg_dump</> output files.
+ Fortunately, <application>pg_dump</> can write to the standard
+ output, so you can use standard Unix tools to work around this
+ potential problem. There are several possible methods:
</para>
<formalpara>
<title>Use compressed dumps.</title>
<para>
You can use your favorite compression program, for example
- <application>gzip</application>.
+ <application>gzip</application>:
<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | gzip > <replaceable class="parameter">filename</replaceable>.gz
</programlisting>
- Reload with
+ Reload with:
<programlisting>
-createdb <replaceable class="parameter">dbname</replaceable>
gunzip -c <replaceable class="parameter">filename</replaceable>.gz | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
- or
+ or:
<programlisting>
cat <replaceable class="parameter">filename</replaceable>.gz | gunzip | psql <replaceable class="parameter">dbname</replaceable>
<title>Use <command>split</>.</title>
<para>
The <command>split</command> command
- allows you to split the output into pieces that are
+ allows you to split the output into smaller files that are
acceptable in size to the underlying file system. For example, to
make chunks of 1 megabyte:
pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 1m - <replaceable class="parameter">filename</replaceable>
</programlisting>
- Reload with
+ Reload with:
<programlisting>
-createdb <replaceable class="parameter">dbname</replaceable>
cat <replaceable class="parameter">filename</replaceable>* | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
</para>
</formalpara>
<formalpara>
- <title>Use the custom dump format.</title>
+ <title>Use <application>pg_dump</>'s custom dump format.</title>
<para>
If <productname>PostgreSQL</productname> was built on a system with the
<application>zlib</> compression library installed, the custom dump
</programlisting>
A custom-format dump is not a script for <application>psql</>, but
- instead must be restored with <application>pg_restore</>.
+ instead must be restored with <application>pg_restore</>, for example:
+
+<programlisting>
+pg_restore -d <replaceable class="parameter">dbname</replaceable> <replaceable class="parameter">filename</replaceable>
+</programlisting>
+
See the <xref linkend="app-pgdump"> and <xref
linkend="app-pgrestore"> reference pages for details.
</para>
</formalpara>
- </sect2>
-
- <sect2 id="backup-dump-caveats">
- <title>Caveats</title>
-
<para>
- For reasons of backward compatibility, <application>pg_dump</>
- does not dump large objects by default.<indexterm><primary>large
- object</primary><secondary>backup</secondary></indexterm> To dump
- large objects you must use either the custom or the tar output
- format, and use the <option>-b</> option in
- <application>pg_dump</>. See the <xref linkend="app-pgdump"> reference
- page for details. The
- directory <filename>contrib/pg_dumplo</> of the
- <productname>PostgreSQL</> source tree also contains a program
- that can dump large objects.
+ For very large databases, you might need to combine <command>split</>
+ with one of the other two approaches.
</para>
- <para>
- Please familiarize yourself with the <xref linkend="app-pgdump">
- reference page.
- </para>
</sect2>
</sect1>
<sect1 id="backup-file">
- <title>File system level backup</title>
+ <title>File System Level Backup</title>
<para>
An alternative backup strategy is to directly copy the files that
- <productname>PostgreSQL</> uses to store the data in the database. In
- <xref linkend="creating-cluster"> it is explained where these files
- are located, but you have probably found them already if you are
- interested in this method. You can use whatever method you prefer
- for doing usual file system backups, for example
+ <productname>PostgreSQL</> uses to store the data in the database;
+ <xref linkend="creating-cluster"> explains where these files
+ are located. You can use whatever method you prefer
+ for doing file system backups; for example:
<programlisting>
tar -cf backup.tar /usr/local/pgsql/data
The database server <emphasis>must</> be shut down in order to
get a usable backup. Half-way measures such as disallowing all
connections will <emphasis>not</emphasis> work
- (mainly because <command>tar</command> and similar tools do not take an
- atomic snapshot of the state of the file system at a point in
- time). Information about stopping the server can be found in
- <xref linkend="postmaster-shutdown">. Needless to say that you
+ (in part because <command>tar</command> and similar tools do not take
+ an atomic snapshot of the state of the file system,
+ but also because of internal buffering within the server).
+ Information about stopping the server can be found in
+ <xref linkend="server-shutdown">. Needless to say, you
also need to shut down the server before restoring the data.
</para>
</listitem>
<listitem>
<para>
If you have dug into the details of the file system layout of the
- database, you may be tempted to try to back up or restore only certain
+ database, you might be tempted to try to back up or restore only certain
individual tables or databases from their respective files or
directories. This will <emphasis>not</> work because the
- information contained in these files contains only half the
- truth. The other half is in the commit log files
+ information contained in these files is not usable without
+ the commit log files,
<filename>pg_clog/*</filename>, which contain the commit status of
all transactions. A table file is only usable with this
information. Of course it is also impossible to restore only a
table and the associated <filename>pg_clog</filename> data
because that would render all other tables in the database
cluster useless. So file system backups only work for complete
- restoration of an entire database cluster.
+ backup and restoration of an entire database cluster.
</para>
</listitem>
</orderedlist>
above) from the snapshot to a backup device, then release the frozen
snapshot. This will work even while the database server is running.
However, a backup created in this way saves
- the database files in a state where the database server was not
+ the database files in a state as if the database server was not
properly shut down; therefore, when you start the database server
- on the backed-up data, it will think the server had crashed
- and replay the WAL log. This is not a problem, just be aware of
- it (and be sure to include the WAL files in your backup).
+ on the backed-up data, it will think the previous server instance
+ crashed and will replay the WAL log. This is not a problem; just
+ be aware of it (and be sure to include the WAL files in your backup).
+ You can perform a <command>CHECKPOINT</command> before taking the
+ snapshot to reduce recovery time.
</para>
<para>
- If your database is spread across multiple file systems, there may not
- be any way to obtain exactly-simultaneous frozen snapshots of all
+ If your database is spread across multiple file systems, there might not
+ be any way to obtain exactly-simultaneous frozen snapshots of all
the volumes. For example, if your data files and WAL log are on different
disks, or if tablespaces are on different file systems, it might
- not be possible to use snapshot backup because the snapshots must be
- simultaneous.
+ not be possible to use snapshot backup because the snapshots
+ <emphasis>must</> be simultaneous.
Read your file system documentation very carefully before trusting
- to the consistent-snapshot technique in such situations. The safest
- approach is to shut down the database server for long enough to
- establish all the frozen snapshots.
+ the consistent-snapshot technique in such situations.
+ </para>
+
+ <para>
+ If simultaneous snapshots are not possible, one option is to shut down
+ the database server long enough to establish all the frozen snapshots.
+ Another option is perform a continuous archiving base backup (<xref
+ linkend="backup-base-backup">) because such backups are immune to file
+ system changes during the backup. This requires enabling continuous
+ archiving just during the backup process; restore is done using
+ continuous archive recovery (<xref linkend="backup-pitr-recovery">).
</para>
<para>
</para>
<para>
- Note that a file system backup will not necessarily be
- smaller than an SQL dump. On the contrary, it will most likely be
- larger. (<application>pg_dump</application> does not need to dump
+ Note that a file system backup will typically be larger
+ than an SQL dump. (<application>pg_dump</application> does not need to dump
the contents of indexes for example, just the commands to recreate
- them.)
+ them.) However, taking a file system backup might be faster.
</para>
</sect1>
- <sect1 id="backup-online">
- <title>On-line backup and point-in-time recovery (PITR)</title>
+ <sect1 id="continuous-archiving">
+ <title>Continuous Archiving and Point-In-Time Recovery (PITR)</title>
<indexterm zone="backup">
- <primary>on-line backup</primary>
+ <primary>continuous archiving</primary>
</indexterm>
<indexterm zone="backup">
<para>
At all times, <productname>PostgreSQL</> maintains a
<firstterm>write ahead log</> (WAL) in the <filename>pg_xlog/</>
- subdirectory of the cluster's data directory. The log describes
+ subdirectory of the cluster's data directory. The log records
every change made to the database's data files. This log exists
primarily for crash-safety purposes: if the system crashes, the
database can be restored to consistency by <quote>replaying</> the
log entries made since the last checkpoint. However, the existence
of the log makes it possible to use a third strategy for backing up
databases: we can combine a file-system-level backup with backup of
- the WAL files. If recovery is needed, we restore the backup and
- then replay from the backed-up WAL files to bring the backup up to
- current time. This approach is more complex to administer than
+ the WAL files. If recovery is needed, we restore the file system backup and
+ then replay from the backed-up WAL files to bring the system to a
+ current state. This approach is more complex to administer than
either of the previous approaches, but it has some significant
benefits:
<itemizedlist>
<listitem>
<para>
- We do not need a perfectly consistent backup as the starting point.
+ We do not need a perfectly consistent file system backup as the starting point.
Any internal inconsistency in the backup will be corrected by log
replay (this is not significantly different from what happens during
- crash recovery). So we don't need file system snapshot capability,
+ crash recovery). So we do not need a file system snapshot capability,
just <application>tar</> or a similar archiving tool.
</para>
</listitem>
<listitem>
<para>
- Since we can string together an indefinitely long sequence of WAL files
+ Since we can combine an indefinitely long sequence of WAL files
for replay, continuous backup can be achieved simply by continuing to archive
the WAL files. This is particularly valuable for large databases, where
- it may not be convenient to take a full backup frequently.
+ it might not be convenient to take a full backup frequently.
</para>
</listitem>
<listitem>
<para>
- There is nothing that says we have to replay the WAL entries all the
+ It is not necessary to replay the WAL entries all the
way to the end. We could stop the replay at any point and have a
consistent snapshot of the database as it was at that time. Thus,
this technique supports <firstterm>point-in-time recovery</>: it is
<para>
If we continuously feed the series of WAL files to another
machine that has been loaded with the same base backup file, we
- have a <quote>hot standby</> system: at any point we can bring up
+ have a <firstterm>warm standby</> system: at any point we can bring up
the second machine and it will have a nearly-current copy of the
database.
</para>
</itemizedlist>
</para>
+ <note>
+ <para>
+ <application>pg_dump</application> and
+ <application>pg_dumpall</application> do not produce file-system-level
+ backups and cannot be used as part of a continuous-archiving solution.
+ Such dumps are <emphasis>logical</> and do not contain enough
+ information to be used by WAL replay.
+ </para>
+ </note>
+
<para>
As with the plain file-system-backup technique, this method can only
support restoration of an entire database cluster, not a subset.
- Also, it requires a lot of archival storage: the base backup may be bulky,
+ Also, it requires a lot of archival storage: the base backup might be bulky,
and a busy system will generate many megabytes of WAL traffic that
have to be archived. Still, it is the preferred backup technique in
many situations where high reliability is needed.
</para>
<para>
- To recover successfully using an on-line backup, you need a continuous
+ To recover successfully using continuous archiving (also called
+ <quote>online backup</> by many database vendors), you need a continuous
sequence of archived WAL files that extends back at least as far as the
start time of your backup. So to get started, you should set up and test
your procedure for archiving WAL files <emphasis>before</> you take your
In an abstract sense, a running <productname>PostgreSQL</> system
produces an indefinitely long sequence of WAL records. The system
physically divides this sequence into WAL <firstterm>segment
- files</>, which are normally 16MB apiece (although the size can be
- altered when building <productname>PostgreSQL</>). The segment
+ files</>, which are normally 16MB apiece (although the segment size
+
can be altered when building <productname>PostgreSQL</>). The segment
files are given numeric names that reflect their position in the
abstract WAL sequence. When not using WAL archiving, the system
normally creates just a few segment files and then
<quote>recycles</> them by renaming no-longer-needed segment files
- to higher segment numbers. It's assumed that a segment file whose
- contents precede the checkpoint-before-last is no longer of
+ to higher segment numbers. It's assumed that segment files whose
+ contents precede the checkpoint-before-last are no longer of
interest and can be recycled.
</para>
<para>
- When archiving WAL data, we want to capture the contents of each segment
+ When archiving WAL data, we need to capture the contents of each segment
file once it is filled, and save that data somewhere before the segment
file is recycled for reuse. Depending on the application and the
available hardware, there could be many different ways of <quote>saving
the data somewhere</>: we could copy the segment files to an NFS-mounted
directory on another machine, write them onto a tape drive (ensuring that
- you have a way of restoring the file with its original file name), or batch
+ you have a way of identifying the original name of each file), or batch
them together and burn them onto CDs, or something else entirely. To
- provide the database administrator with as much flexibility as possible,
- <productname>PostgreSQL</> tries not to make any assumptions about how
+ provide the database administrator with flexibility,
+ <productname>PostgreSQL</> tries not to make any assumptions about how
the archiving will be done. Instead, <productname>PostgreSQL</> lets
the administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command could be
- as simple as a <application>cp</>, or it could invoke a complex shell
+ as simple as a <literal>cp</>, or it could invoke a complex shell
script — it's all up to you.
</para>
<para>
- The shell command to use is specified by the <xref
- linkend="guc-archive-command"> configuration parameter, which in practice
- will always be placed in the <filename>postgresql.conf</filename> file.
- In this string,
- any <literal>%p</> is replaced by the absolute path of the file to
- archive, while any <literal>%f</> is replaced by the file name only.
- Write <literal>%%</> if you need to embed an actual <literal>%</>
+ To enable WAL archiving, set the <xref linkend="guc-wal-level">
+ configuration parameter to <literal>archive</> (or <literal>hot_standby</>),
+ <xref linkend="guc-archive-mode"> to <literal>on</>,
+ and specify the shell command to use in the <xref
+ linkend="guc-archive-command"> configuration parameter. In practice
+ these settings will always be placed in the
+ <filename>postgresql.conf</filename> file.
+ In <varname>archive_command</>,
+ <literal>%p</> is replaced by the path name of the file to
+ archive, while <literal>%f</> is replaced by only the file name.
+ (The path name is relative to the current working directory,
+ i.e., the cluster's data directory.)
+ Use <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is something
- like
+ like:
<programlisting>
-archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'
+archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null' # Unix
+archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
</programlisting>
which will copy archivable WAL segments to the directory
- <filename>/mnt/server/archivedir</>. (This is an example, not a
- recommendation, and may not work on all platforms.)
+ <filename>/mnt/server/archivedir</>. (This is an example, not a
+ recommendation, and might not work on all platforms.) After the
+ <literal>%p</> and <literal>%f</> parameters have been replaced,
+ the actual command executed might look like this:
+<programlisting>
+cp -i pg_xlog/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065 </dev/null
+</programlisting>
+ A similar command will be generated for each new file to be archived.
</para>
<para>
<para>
It is important that the archive command return zero exit status if and
- only if it succeeded. Upon getting a zero result,
- <productname>PostgreSQL</> will assume that the WAL segment file has been
- successfully archived, and will remove or recycle it.
- However, a nonzero status tells
- <productname>PostgreSQL</> that the file was not archived; it will try
- again periodically until it succeeds.
+ only if it succeeds. Upon getting a zero result,
+ <productname>PostgreSQL</> will assume that the file has been
+ successfully archived, and will remove or recycle it. However, a nonzero
+ status tells <productname>PostgreSQL</> that the file was not archived;
+ it will try again periodically until it succeeds.
</para>
<para>
directory).
It is advisable to test your proposed archive command to ensure that it
indeed does not overwrite an existing file, <emphasis>and that it returns
- nonzero status in this case</>. We have found that <literal>cp -i</> does
- this correctly on some platforms but not others. If the chosen command
- does not itself handle this case correctly, you should add a command
- to test for pre-existence of the archive file. For example, something
- like
+ nonzero status in this case</>. On many Unix platforms, <command>cp
+ -i</> causes copy to prompt before overwriting a file, and
+ <literal>< /dev/null</> causes the prompt (and overwriting) to
+ fail. If your platform does not support this behavior, you should
+ add a command to test for the existence of the archive file. For
+ example, something like:
<programlisting>
-archive_command = 'test ! -f .../%f && cp %p .../%f'
+archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'
</programlisting>
works correctly on most Unix variants.
</para>
<para>
While designing your archiving setup, consider what will happen if
- the archive command fails repeatedly because some aspect requires
+ the archive command fails repeatedly because some aspect requires
operator intervention or the archive runs out of space. For example, this
- could occur if you write to tape without an autochanger; when the tape
+ could occur if you write to tape without an autochanger; when the tape
fills, nothing further can be archived until the tape is swapped.
You should ensure that any error condition or request to a human operator
- is reported appropriately so that the situation can be
- resolved relatively quickly. The <filename>pg_xlog/</> directory will
+ is reported appropriately so that the situation can be
+ resolved reasonably quickly. The <filename>pg_xlog/</> directory will
continue to fill with WAL segment files until the situation is resolved.
+ (If the file system containing <filename>pg_xlog/</> fills up,
+ <productname>PostgreSQL</> will do a PANIC shutdown. No committed
+ transactions will be lost, but the database will remain offline until
+ you free some space.)
</para>
<para>
- The speed of the archiving command is not important, so long as it can keep up
+ The speed of the archiving command is unimportant as long as it can keep up
with the average rate at which your server generates WAL data. Normal
operation continues even if the archiving process falls a little behind.
If archiving falls significantly behind, this will increase the amount of
</para>
<para>
- If you are concerned about being able to recover right up to the
- current instant, you may want to take additional steps to ensure that
- the current, partially-filled WAL segment is also copied someplace.
- This is particularly important if your server generates only little WAL
- traffic (or has slack periods where it does so), since it could take a
- long time before a WAL segment file is completely filled and ready to
- archive. One possible way to handle this is to set up a
- <application>cron</> job that periodically (once a minute, perhaps)
- identifies the current WAL segment file and saves it someplace safe.
- Then the combination of the archived WAL segments and the saved current
- segment will be enough to ensure you can always restore to within a
- minute of current time. This behavior is not presently built into
- <productname>PostgreSQL</> because we did not want to complicate the
- definition of the <xref linkend="guc-archive-command"> by requiring it
- to keep track of successively archived, but different, copies of the
- same WAL file. The <xref linkend="guc-archive-command"> is only
- invoked on completed WAL segments. Except in the case of retrying a
- failure, it will be called only once for any given file name.
- </para>
-
- <para>
- In writing your archive command, you should assume that the filenames to
- be archived may be up to 64 characters long and may contain any
+ In writing your archive command, you should assume that the file names to
+ be archived can be up to 64 characters long and can contain any
combination of ASCII letters, digits, and dots. It is not necessary to
- remember the original full path (<literal>%p</>) but it is necessary to
- remember the file name (<literal>%f</>).
+ preserve the original relative path (<literal>%p</>) but it is necessary to
+ preserve the file name (<literal>%f</>).
</para>
<para>
Note that although WAL archiving will allow you to restore any
- modifications made to the data in your <productname>PostgreSQL</> database
+ modifications made to the data in your <productname>PostgreSQL</> database,
it will not restore changes made to configuration files (that is,
<filename>postgresql.conf</>, <filename>pg_hba.conf</> and
<filename>pg_ident.conf</>), since those are edited manually rather
than through SQL operations.
- You may wish to keep the configuration files in a location that will
+ You might wish to keep the configuration files in a location that will
be backed up by your regular file system backup procedures. See
<xref linkend="runtime-config-file-locations"> for how to relocate the
configuration files.
</para>
+
+ <para>
+ The archive command is only invoked on completed WAL segments. Hence,
+ if your server generates only little WAL traffic (or has slack periods
+ where it does so), there could be a long delay between the completion
+ of a transaction and its safe recording in archive storage. To put
+ a limit on how old unarchived data can be, you can set
+ <xref linkend="guc-archive-timeout"> to force the server to switch
+ to a new WAL segment file at least that often. Note that archived
+ files that are archived early due to a forced switch are still the same
+ length as completely full files. It is therefore unwise to set a very
+ short <varname>archive_timeout</> — it will bloat your archive
+ storage. <varname>archive_timeout</> settings of a minute or so are
+ usually reasonable.
+ </para>
+
+ <para>
+ Also, you can force a segment switch manually with
+ <function>pg_switch_xlog</> if you want to ensure that a
+ just-finished transaction is archived as soon as possible. Other utility
+ functions related to WAL management are listed in <xref
+ linkend="functions-admin-backup-table">.
+ </para>
+
+ <para>
+ When <varname>wal_level</> is <literal>minimal</> some SQL commands
+ are optimized to avoid WAL logging, as described in <xref
+ linkend="populate-pitr">. If archiving or streaming replication were
+ turned on during execution of one of these statements, WAL would not
+ contain enough information for archive recovery. (Crash recovery is
+ unaffected.) For this reason, <varname>wal_level</> can only be changed at
+ server start. However, <varname>archive_command</> can be changed with a
+ configuration file reload. If you wish to temporarily stop archiving,
+ one way to do it is to set <varname>archive_command</> to the empty
+ string (<literal>''</>).
+ This will cause WAL files to accumulate in <filename>pg_xlog/</> until a
+ working <varname>archive_command</> is re-established.
+ </para>
</sect2>
<sect2 id="backup-base-backup">
</listitem>
<listitem>
<para>
- Connect to the database as a superuser, and issue the command
+ Connect to the database as a superuser and issue the command:
<programlisting>
SELECT pg_start_backup('label');
</programlisting>
full path where you intend to put the backup dump file.)
<function>pg_start_backup</> creates a <firstterm>backup label</> file,
called <filename>backup_label</>, in the cluster directory with
- information about your backup.
+ information about your backup, including the start time and label
+ string.
</para>
<para>
- It does not matter which database within the cluster you connect to to
+ It does not matter which database within the cluster you connect to to
issue this command. You can ignore the result returned by the function;
but if it reports an error, deal with that before proceeding.
</para>
+
+ <para>
+ By default, <function>pg_start_backup</> can take a long time to finish.
+ This is because it performs a checkpoint, and the I/O
+ required for the checkpoint will be spread out over a significant
+ period of time, by default half your inter-checkpoint interval
+ (see the configuration parameter
+ <xref linkend="guc-checkpoint-completion-target">). This is
+ usually what you want, because it minimizes the impact on query
+ processing. If you want to start the backup as soon as
+ possible, use:
+<programlisting>
+SELECT pg_start_backup('label', true);
+</programlisting>
+ This forces the checkpoint to be done as quickly as possible.
+ </para>
</listitem>
<listitem>
<para>
Perform the backup, using any convenient file-system-backup tool
- such as <application>tar</> or <application>cpio</>. It is neither
+ such as <application>tar</> or <application>cpio</> (not
+ <application>pg_dump</application> or
+ <application>pg_dumpall</application>). It is neither
necessary nor desirable to stop normal operation of the database
while you do this.
</para>
</listitem>
<listitem>
<para>
- Again connect to the database as a superuser, and issue the command
+ Again connect to the database as a superuser, and issue the command:
<programlisting>
SELECT pg_stop_backup();
</programlisting>
- This should return successfully.
+ This terminates the backup mode and performs an automatic switch to
+ the next WAL segment. The reason for the switch is to arrange for
+ the last WAL segment file written during the backup interval to be
+ ready to archive.
</para>
</listitem>
<listitem>
<para>
- Once the WAL segment files used during the backup are archived as part
- of normal database activity, you are done.
+ Once the WAL segment files active during the backup are archived, you are
+ done. The file identified by <function>pg_stop_backup</>'s result is
+ the last segment that is required to form a complete set of backup files.
+ If <varname>archive_mode</> is enabled,
+ <function>pg_stop_backup</> does not return until the last segment has
+ been archived.
+ Archiving of these files happens automatically since you have
+ already configured <varname>archive_command</>. In most cases this
+ happens quickly, but you are advised to monitor your archive
+ system to ensure there are no delays.
+ If the archive process has fallen behind
+ because of failures of the archive command, it will keep retrying
+ until the archive succeeds and the backup is complete.
+ If you wish to place a time limit on the execution of
+ <function>pg_stop_backup</>, set an appropriate
+ <varname>statement_timeout</varname> value.
</para>
</listitem>
</orderedlist>
</para>
<para>
- It is not necessary to be very concerned about the amount of time elapsed
+ You can also use the <xref linkend="app-pgbasebackup"> tool to take
+ the backup, instead of manually copying the files. This tool will take
+ care of the <function>pg_start_backup()</>, copy and
+ <function>pg_stop_backup()</> steps automatically, and transfers the
+ backup over a regular <productname>PostgreSQL</productname> connection
+ using the replication protocol, instead of requiring filesystem level
+ access.
+ </para>
+
+ <para>
+ Some file system backup tools emit warnings or errors
+ if the files they are trying to copy change while the copy proceeds.
+ When taking a base backup of an active database, this situation is normal
+ and not an error. However, you need to ensure that you can distinguish
+ complaints of this sort from real errors. For example, some versions
+ of <application>rsync</> return a separate exit code for
+ <quote>vanished source files</>, and you can write a driver script to
+ accept this exit code as a non-error case. Also, some versions of
+ GNU <application>tar</> return an error code indistinguishable from
+ a fatal error if a file was truncated while <application>tar</> was
+ copying it. Fortunately, GNU <application>tar</> versions 1.16 and
+ later exit with <literal>1</> if a file was changed during the backup,
+ and <literal>2</> for other errors.
+ </para>
+
+ <para>
+ It is not necessary to be concerned about the amount of time elapsed
between <function>pg_start_backup</> and the start of the actual backup,
nor between the end of the backup and <function>pg_stop_backup</>; a
- few minutes' delay won't hurt anything. You
- must however be quite sure that these operations are carried out in
- sequence and do not overlap.
+ few minutes' delay won't hurt anything. (However, if you normally run the
+ server with <varname>full_page_writes</> disabled, you might notice a drop
+ in performance between <function>pg_start_backup</> and
+ <function>pg_stop_backup</>, since <varname>full_page_writes</> is
+ effectively forced on during backup mode.) You must ensure that these
+ steps are carried out in sequence, without any possible
+ overlap, or you will invalidate the backup.
</para>
<para>
- Be certain that your backup dump includes all of the files underneath
+ Be certain that your backup dump includes all of the files under
the database cluster directory (e.g., <filename>/usr/local/pgsql/data</>).
If you are using tablespaces that do not reside underneath this directory,
be careful to include them as well (and be sure that your backup dump
- archives symbolic links as links, otherwise the restore will mess up
+ archives symbolic links as links, otherwise the restore will corrupt
your tablespaces).
</para>
<para>
- You may, however, omit from the backup dump the files within the
- <filename>pg_xlog/</> subdirectory of the cluster directory. This
- slight complication is worthwhile because it reduces the risk
+ You can, however, omit from the backup dump the files within the
+ cluster's <filename>pg_xlog/</> subdirectory. This
+ slight adjustment is worthwhile because it reduces the risk
of mistakes when restoring. This is easy to arrange if
<filename>pg_xlog/</> is a symbolic link pointing to someplace outside
the cluster directory, which is a common setup anyway for performance
</para>
<para>
- To make use of this backup, you will need to keep around all the WAL
+ To make use of the backup, you will need to keep all the WAL
segment files generated during and after the file system backup.
To aid you in doing this, the <function>pg_stop_backup</> function
creates a <firstterm>backup history file</> that is immediately
stored into the WAL archive area. This file is named after the first
- WAL segment file that you need to have to make use of the backup.
+ WAL segment file that you need for the file system backup.
For example, if the starting WAL file is
<literal>0000000100001234000055CD</> the backup history file will be
named something like
<literal>0000000100001234000055CD.007C9330.backup</>. (The second
- number in the file name stands for an exact position within the WAL
+ part of the file name stands for an exact position within the WAL
file, and can ordinarily be ignored.) Once you have safely archived
the file system backup and the WAL segment files used during the
backup (as specified in the backup history file), all archived WAL
segments with names numerically less are no longer needed to recover
- the file system backup and may be deleted. However, you should
+ the file system backup and can be deleted. However, you should
consider keeping several backup sets to be absolutely certain that
- you are can recover your data. Keep in mind that only completed WAL
- segment files are archived, so there will be delay between running
- <function>pg_stop_backup</> and the archiving of all WAL segment
- files needed to make the file system backup consistent.
+ you can recover your data.
</para>
+
<para>
The backup history file is just a small text file. It contains the
label string you gave to <function>pg_start_backup</>, as well as
- the starting and ending times of the backup. If you used the label
- to identify where the associated dump file is kept, then the
- archived history file is enough to tell you which dump file to
- restore, should you need to do so.
+ the starting and ending times and WAL segments of the backup.
+ If you used the label to identify the associated dump file,
+ then the archived history file is enough to tell you which dump file to
+ restore.
</para>
<para>
<para>
It's also worth noting that the <function>pg_start_backup</> function
makes a file named <filename>backup_label</> in the database cluster
- directory, which is then removed again by <function>pg_stop_backup</>.
+ directory, which is removed by <function>pg_stop_backup</>.
This file will of course be archived as a part of your backup dump file.
The backup label file includes the label string you gave to
<function>pg_start_backup</>, as well as the time at which
<function>pg_start_backup</> was run, and the name of the starting WAL
- file. In case of confusion it will
- therefore be possible to look inside a backup dump file and determine
+ file. In case of confusion it is
+ therefore possible to look inside a backup dump file and determine
exactly which backup session the dump file came from.
</para>
<para>
- It is also possible to make a backup dump while the postmaster is
+ It is also possible to make a backup dump while the server is
stopped. In this case, you obviously cannot use
<function>pg_start_backup</> or <function>pg_stop_backup</>, and
you will therefore be left to your own devices to keep track of which
backup dump is which and how far back the associated WAL files go.
- It is generally better to follow the on-line backup procedure above.
+ It is generally better to follow the continuous archiving procedure above.
</para>
</sect2>
<sect2 id="backup-pitr-recovery">
- <title>Recovering with an On-line Backup</title>
+ <title>Recovering using a Continuous Archive Backup</title>
<para>
Okay, the worst has happened and you need to recover from your backup.
<orderedlist>
<listitem>
<para>
- Stop the postmaster, if it's running.
+ Stop the server, if it's running.
</para>
</listitem>
<listitem>
<para>
If you have the space to do so,
- copy the whole cluster data directory and any tablespaces to a temporary
+ copy the whole cluster data directory and any tablespaces to a temporary
location in case you need them later. Note that this precaution will
require that you have enough free space on your system to hold two
- copies of your existing database. If you do not have enough space,
- you need at the least to copy the contents of the <filename>pg_xlog</>
- subdirectory of the cluster data directory, as it may contain logs which
+ copies of your existing database. If you do not have enough space,
+ you should at least save the contents of the cluster's <filename>pg_xlog</>
+ subdirectory, as it might contain logs which
were not archived before the system went down.
</para>
</listitem>
<listitem>
<para>
- Clean out all existing files and subdirectories under the cluster data
+ Remove all existing files and subdirectories under the cluster data
directory and under the root directories of any tablespaces you are using.
</para>
</listitem>
<listitem>
<para>
- Restore the database files from your backup dump. Be careful that they
+ Restore the database files from your file system backup. Be sure that they
are restored with the right ownership (the database system user, not
- root!) and with the right permissions. If you are using tablespaces,
- you may want to verify that the symbolic links in <filename>pg_tblspc/</>
+ <literal>root</>!) and with the right permissions. If you are using
+ tablespaces,
+ you should verify that the symbolic links in <filename>pg_tblspc/</>
were correctly restored.
</para>
</listitem>
<listitem>
<para>
Remove any files present in <filename>pg_xlog/</>; these came from the
- backup dump and are therefore probably obsolete rather than current.
- If you didn't archive <filename>pg_xlog/</> at all, then re-create it,
- and be sure to re-create the subdirectory
- <filename>pg_xlog/archive_status/</> as well.
+ file system backup and are therefore probably obsolete rather than current.
+ If you didn't archive <filename>pg_xlog/</> at all, then recreate
+ it with proper permissions,
+ being careful to ensure that you re-establish it as a symbolic link
+ if you had it set up that way before.
</para>
</listitem>
<listitem>
<para>
- If you had unarchived WAL segment files that you saved in step 2,
+ If you have unarchived WAL segment files that you saved in step 2,
copy them into <filename>pg_xlog/</>. (It is best to copy them,
- not move them, so that you still have the unmodified files if a
+ not move them, so you still have the unmodified files if a
problem occurs and you have to start over.)
</para>
</listitem>
<listitem>
<para>
Create a recovery command file <filename>recovery.conf</> in the cluster
- data directory (see <xref linkend="recovery-config-settings">). You may
- also want to temporarily modify <filename>pg_hba.conf</> to prevent
- ordinary users from connecting until you are sure the recovery has worked.
+ data directory (see <xref linkend="recovery-config">). You might
+ also want to temporarily modify <filename>pg_hba.conf</> to prevent
+ ordinary users from connecting until you are sure the recovery was successful.
</para>
</listitem>
<listitem>
<para>
- Start the postmaster. The postmaster will go into recovery mode and
- proceed to read through the archived WAL files it needs. Upon completion
- of the recovery process, the postmaster will rename
+ Start the server. The server will go into recovery mode and
+ proceed to read through the archived WAL files it needs. Should the
+ recovery be terminated because of an external error, the server can
+ simply be restarted and it will continue recovery. Upon completion
+ of the recovery process, the server will rename
<filename>recovery.conf</> to <filename>recovery.done</> (to prevent
- accidentally re-entering recovery mode in case of a crash later) and then
+ accidentally re-entering recovery mode later) and then
commence normal database operations.
</para>
</listitem>
<listitem>
<para>
Inspect the contents of the database to ensure you have recovered to
- where you want to be. If not, return to step 1. If all is well,
- let in your users by restoring <filename>pg_hba.conf</> to normal.
+ the desired state. If not, return to step 1. If all is well,
+ allow your users to connect by restoring <filename>pg_hba.conf</> to normal.
</para>
</listitem>
</orderedlist>
</para>
<para>
- The key part of all this is to set up a recovery command file that
+ The key part of all this is to set up a recovery configuration file that
describes how you want to recover and how far the recovery should
run. You can use <filename>recovery.conf.sample</> (normally
- installed in the installation <filename>share/</> directory) as a
+ located in the installation's <filename>share/</> directory) as a
prototype. The one thing that you absolutely must specify in
<filename>recovery.conf</> is the <varname>restore_command</>,
- which tells <productname>PostgreSQL</> how to get back archived
+ which tells <productname>PostgreSQL</> how to retrieve archived
WAL file segments. Like the <varname>archive_command</>, this is
- a shell command string. It may contain <literal>%f</>, which is
+ a shell command string. It can contain <literal>%f</>, which is
replaced by the name of the desired log file, and <literal>%p</>,
- which is replaced by the absolute path to copy the log file to.
+ which is replaced by the path name to copy the log file to.
+ (The path name is relative to the current working directory,
+ i.e., the cluster's data directory.)
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is
- something like
+ something like:
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f %p'
</programlisting>
which will copy previously archived WAL segments from the directory
- <filename>/mnt/server/archivedir</>. You could of course use something
+ <filename>/mnt/server/archivedir</>. Of course, you can use something
much more complicated, perhaps even a shell script that requests the
operator to mount an appropriate tape.
</para>
<para>
It is important that the command return nonzero exit status on failure.
- The command <emphasis>will</> be asked for log files that are not present
+ The command <emphasis>will</> be called requesting files that are not present
in the archive; it must return nonzero when so asked. This is not an
- error condition. Be aware also that the base name of the <literal>%p</>
- path will be different from <literal>%f</>; do not expect them to be
- interchangeable.
+ error condition. Not all of the requested files will be WAL segment
+ files; you should also expect requests for files with a suffix of
+ <literal>.backup</> or <literal>.history</>. Also be aware that
+ the base name of the <literal>%p</> path will be different from
+ <literal>%f</>; do not expect them to be interchangeable.
</para>
<para>
WAL segments that cannot be found in the archive will be sought in
<filename>pg_xlog/</>; this allows use of recent un-archived segments.
- However segments that are available from the archive will be used in
+ However, segments that are available from the archive will be used in
preference to files in <filename>pg_xlog/</>. The system will not
overwrite the existing contents of <filename>pg_xlog/</> when retrieving
archived files.
<para>
Normally, recovery will proceed through all available WAL segments,
thereby restoring the database to the current point in time (or as
- close as we can get given the available WAL segments). But if you want
- to recover to some previous point in time (say, right before the junior
- DBA dropped your main transaction table), just specify the required
- stopping point in <filename>recovery.conf</>. You can specify the stop
- point, known as the <quote>recovery target</>, either by date/time or
- by completion of a specific transaction ID. As of this writing only
- the date/time option is very usable, since there are no tools to help
- you identify with any accuracy which transaction ID to use.
+ close as possible given the available WAL segments). Therefore, a normal
+ recovery will end with a <quote>file not found</> message, the exact text
+ of the error message depending upon your choice of
+ <varname>restore_command</>. You may also see an error message
+ at the start of recovery for a file named something like
+ <filename>00000001.history</>. This is also normal and does not
+ indicate a problem in simple recovery situations; see
+ <xref linkend="backup-timelines"> for discussion.
+ </para>
+
+ <para>
+ If you want to recover to some previous point in time (say, right before
+ the junior DBA dropped your main transaction table), just specify the
+ required stopping point in <filename>recovery.conf</>. You can specify
+ the stop point, known as the <quote>recovery target</>, either by
+ date/time or by completion of a specific transaction ID. As of this
+ writing only the date/time option is very usable, since there are no tools
+ to help you identify with any accuracy which transaction ID to use.
</para>
<note>
<para>
- The stop point must be after the ending time of the base backup (the
- time of <function>pg_stop_backup</>). You cannot use a base backup
- to recover to a time when that backup was still going on. (To
+ The stop point must be after the ending time of the base backup, i.e.,
+ the end time of <function>pg_stop_backup</>. You cannot use a base backup
+ to recover to a time when that backup was in progress. (To
recover to such a time, you must go back to your previous base backup
and roll forward from there.)
</para>
- </note>
+ </note>
- <sect3 id="recovery-config-settings" xreflabel="Recovery Settings">
- <title>Recovery Settings</title>
-
- <para>
- These settings can only be made in the <filename>recovery.conf</>
- file, and apply only for the duration of the recovery. They must be
- reset for any subsequent recovery you wish to perform. They cannot be
- changed once recovery has begun.
- </para>
-
- <variablelist>
-
- <varlistentry id="restore-command" xreflabel="restore_command">
- <term><varname>restore_command</varname> (<type>string</type>)</term>
- <listitem>
- <para>
- The shell command to execute to retrieve an archived segment of
- the WAL file series. This parameter is required.
- Any <literal>%f</> in the string is
- replaced by the name of the file to retrieve from the archive,
- and any <literal>%p</> is replaced by the absolute path to copy
- it to on the server.
- Write <literal>%%</> to embed an actual <literal>%</> character
- in the command.
- </para>
- <para>
- It is important for the command to return a zero exit status if and
- only if it succeeds. The command <emphasis>will</> be asked for file
- names that are not present in the archive; it must return nonzero
- when so asked. Examples:
-<programlisting>
-restore_command = 'cp /mnt/server/archivedir/%f "%p"'
-restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
-</programlisting>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry id="recovery-target-time" xreflabel="recovery_target_time">
- <term><varname>recovery_target_time</varname>
- (<type>timestamp</type>)
- </term>
- <listitem>
- <para>
- This parameter specifies the time stamp up to which recovery
- will proceed.
- At most one of <varname>recovery_target_time</> and
- <xref linkend="recovery-target-xid"> can be specified.
- The default is to recover to the end of the WAL log.
- The precise stopping point is also influenced by
- <xref linkend="recovery-target-inclusive">.
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry id="recovery-target-xid" xreflabel="recovery_target_xid">
- <term><varname>recovery_target_xid</varname> (<type>string</type>)</term>
- <listitem>
- <para>
- This parameter specifies the transaction ID up to which recovery
- will proceed. Keep in mind
- that while transaction IDs are assigned sequentially at transaction
- start, transactions can complete in a different numeric order.
- The transactions that will be recovered are those that committed
- before (and optionally including) the specified one.
- At most one of <varname>recovery_target_xid</> and
- <xref linkend="recovery-target-time"> can be specified.
- The default is to recover to the end of the WAL log.
- The precise stopping point is also influenced by
- <xref linkend="recovery-target-inclusive">.
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry id="recovery-target-inclusive"
- xreflabel="recovery_target_inclusive">
- <term><varname>recovery_target_inclusive</varname>
- (<type>boolean</type>)
- </term>
- <listitem>
- <para>
- Specifies whether we stop just after the specified recovery target
- (<literal>true</literal>), or just before the recovery target
- (<literal>false</literal>).
- Applies to both <xref linkend="recovery-target-time">
- and <xref linkend="recovery-target-xid">, whichever one is
- specified for this recovery. This indicates whether transactions
- having exactly the target commit time or ID, respectively, will
- be included in the recovery. Default is <literal>true</>.
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry id="recovery-target-timeline"
- xreflabel="recovery_target_timeline">
- <term><varname>recovery_target_timeline</varname>
- (<type>string</type>)
- </term>
- <listitem>
- <para>
- Specifies recovering into a particular timeline. The default is
- to recover along the same timeline that was current when the
- base backup was taken. You would only need to set this parameter
- in complex re-recovery situations, where you need to return to
- a state that itself was reached after a point-in-time recovery.
- See <xref linkend="backup-timelines"> for discussion.
- </para>
- </listitem>
- </varlistentry>
-
- </variablelist>
-
- </sect3>
+ <para>
+ If recovery finds corrupted WAL data, recovery will
+ halt at that point and the server will not start. In such a case the
+ recovery process could be re-run from the beginning, specifying a
+ <quote>recovery target</> before the point of corruption so that recovery
+ can complete normally.
+ If recovery fails for an external reason, such as a system crash or
+ if the WAL archive has become inaccessible, then the recovery can simply
+ be restarted and it will restart almost from where it failed.
+ Recovery restart works much like checkpointing in normal operation:
+ the server periodically forces all its state to disk, and then updates
+ the <filename>pg_control</> file to indicate that the already-processed
+ WAL data need not be scanned again.
+ </para>
</sect2>
<para>
The ability to restore the database to a previous point in time creates
some complexities that are akin to science-fiction stories about time
- travel and parallel universes. In the original history of the database,
- perhaps you dropped a critical table at 5:15PM on Tuesday evening.
+ travel and parallel universes. For example, in the original history of the database,
+ suppose you dropped a critical table at 5:15PM on Tuesday evening, but
+ didn't realize your mistake until Wednesday noon.
Unfazed, you get out your backup, restore to the point-in-time 5:14PM
Tuesday evening, and are up and running. In <emphasis>this</> history of
- the database universe, you never dropped the table at all. But suppose
- you later realize this wasn't such a great idea after all, and would like
- to return to some later point in the original history. You won't be able
+ the database universe, you never dropped the table. But suppose
+ you later realize this wasn't such a great idea, and would like
+ to return to sometime Wednesday morning in the original history.
+ You won't be able
to if, while your database was up-and-running, it overwrote some of the
- sequence of WAL segment files that led up to the time you now wish you
- could get back to. So you really want to distinguish the series of
+ WAL segment files that led up to the time you now wish you
+ could get back to. Thus, to avoid this, you need to distinguish the series of
WAL records generated after you've done a point-in-time recovery from
those that were generated in the original database history.
</para>
<para>
- To deal with these problems, <productname>PostgreSQL</> has a notion
- of <firstterm>timelines</>. Each time you recover to a point-in-time
- earlier than the end of the WAL sequence, a new timeline is created
- to identify the series of WAL records generated after that recovery.
- (If recovery proceeds all the way to the end of WAL, however, we do not
- start a new timeline: we just extend the existing one.) The timeline
- ID number is part of WAL segment file names, and so a new timeline does
+ To deal with this problem, <productname>PostgreSQL</> has a notion
+ of <firstterm>timelines</>. Whenever an archive recovery completes,
+ a new timeline is created to identify the series of WAL records
+ generated after that recovery. The timeline
+ ID number is part of WAL segment file names so a new timeline does
not overwrite the WAL data generated by previous timelines. It is
in fact possible to archive many different timelines. While that might
seem like a useless feature, it's often a lifesaver. Consider the
until you find the best place to branch off from the old history. Without
timelines this process would soon generate an unmanageable mess. With
timelines, you can recover to <emphasis>any</> prior state, including
- states in timeline branches that you later abandoned.
+ states in timeline branches that you abandoned earlier.
</para>
<para>
- E
ach time a new timeline is created, <productname>PostgreSQL</> creates
+ E
very time a new timeline is created, <productname>PostgreSQL</> creates
a <quote>timeline history</> file that shows which timeline it branched
off from and when. These history files are necessary to allow the system
to pick the right WAL segment files when recovering from an archive that
archive area just like WAL segment files. The history files are just
small text files, so it's cheap and appropriate to keep them around
indefinitely (unlike the segment files which are large). You can, if
- you like, add comments to a history file to make your own notes about
- how and why this particular timeline came to be. Such comments will be
+ you like, add comments to a history file to record your own notes about
+ how and why this particular timeline was created. Such comments will be
especially valuable when you have a thicket of different timelines as
a result of experimentation.
</para>
<para>
The default behavior of recovery is to recover along the same timeline
- that was current when the base backup was taken. If you want to recover
+ that was current when the base backup was taken. If you wish to recover
into some child timeline (that is, you want to return to some state that
was itself generated after a recovery attempt), you need to specify the
target timeline ID in <filename>recovery.conf</>. You cannot recover into
</para>
</sect2>
- <sect2 id="backup-online-caveats">
+ <sect2 id="backup-tips">
+ <title>Tips and Examples</title>
+
+ <para>
+ Some tips for configuring continuous archiving are given here.
+ </para>
+
+ <sect3 id="backup-standalone">
+ <title>Standalone hot backups</title>
+
+ <para>
+ It is possible to use <productname>PostgreSQL</>'s backup facilities to
+ produce standalone hot backups. These are backups that cannot be used
+ for point-in-time recovery, yet are typically much faster to backup and
+ restore than <application>pg_dump</> dumps. (They are also much larger
+ than <application>pg_dump</> dumps, so in some cases the speed advantage
+ might be negated.)
+ </para>
+
+ <para>
+ To prepare for standalone hot backups, set <varname>wal_level</> to
+ <literal>archive</> (or <literal>hot_standby</>), <varname>archive_mode</> to
+ <literal>on</>, and set up an <varname>archive_command</> that performs
+ archiving only when a <emphasis>switch file</> exists. For example:
+<programlisting>
+archive_command = 'test ! -f /var/lib/pgsql/backup_in_progress || cp -i %p /var/lib/pgsql/archive/%f < /dev/null'
+</programlisting>
+ This command will perform archiving when
+ <filename>/var/lib/pgsql/backup_in_progress</> exists, and otherwise
+ silently return zero exit status (allowing <productname>PostgreSQL</>
+ to recycle the unwanted WAL file).
+ </para>
+
+ <para>
+ With this preparation, a backup can be taken using a script like the
+ following:
+<programlisting>
+touch /var/lib/pgsql/backup_in_progress
+psql -c "select pg_start_backup('hot_backup');"
+tar -cf /var/lib/pgsql/backup.tar /var/lib/pgsql/data/
+psql -c "select pg_stop_backup();"
+rm /var/lib/pgsql/backup_in_progress
+tar -rf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/
+</programlisting>
+ The switch file <filename>/var/lib/pgsql/backup_in_progress</> is
+ created first, enabling archiving of completed WAL files to occur.
+ After the backup the switch file is removed. Archived WAL files are
+ then added to the backup so that both base backup and all required
+ WAL files are part of the same <application>tar</> file.
+ Please remember to add error handling to your backup scripts.
+ </para>
+
+ <para>
+ If archive storage size is a concern, use <application>pg_compresslog</>,
+ <ulink url="http://pglesslog.projects.postgresql.org"></ulink>, to
+ remove unnecessary <xref linkend="guc-full-page-writes"> and trailing
+ space from the WAL files. You can then use
+ <application>gzip</application> to further compress the output of
+ <application>pg_compresslog</>:
+<programlisting>
+archive_command = 'pg_compresslog %p - | gzip > /var/lib/pgsql/archive/%f'
+</programlisting>
+ You will then need to use <application>gunzip</> and
+ <application>pg_decompresslog</> during recovery:
+<programlisting>
+restore_command = 'gunzip < /mnt/server/archivedir/%f | pg_decompresslog - %p'
+</programlisting>
+ </para>
+ </sect3>
+
+ <sect3 id="backup-scripts">
+ <title><varname>archive_command</varname> scripts</title>
+
+ <para>
+ Many people choose to use scripts to define their
+ <varname>archive_command</varname>, so that their
+ <filename>postgresql.conf</> entry looks very simple:
+<programlisting>
+archive_command = 'local_backup_script.sh'
+</programlisting>
+ Using a separate script file is advisable any time you want to use
+ more than a single command in the archiving process.
+ This allows all complexity to be managed within the script, which
+ can be written in a popular scripting language such as
+ <application>bash</> or <application>perl</>.
+ Any messages written to <literal>stderr</> from the script will appear
+ in the database server log, allowing complex configurations to be
+ diagnosed easily if they fail.
+ </para>
+
+ <para>
+ Examples of requirements that might be solved within a script include:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Copying data to secure off-site data storage
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Batching WAL files so that they are transferred every three hours,
+ rather than one at a time
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Interfacing with other backup and recovery software
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Interfacing with monitoring software to report errors
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect3>
+ </sect2>
+
+ <sect2 id="continuous-archiving-caveats">
<title>Caveats</title>
<para>
- At this writing, there are several limitations of the on-line backup
+ At this writing, there are several limitations of the continuous archiving
technique. These will probably be fixed in future releases:
<itemizedlist>
<listitem>
<para>
- Operations on non-B-tree indexes (hash, R-tree, and GiST indexes) are
- not presently WAL-logged, so replay will not update these index types.
- The recommended workaround is to manually <command>REINDEX</> each
- such index after completing a recovery operation.
+ Operations on hash indexes are not presently WAL-logged, so
+ replay will not update these indexes. This will mean that any new inserts
+ will be ignored by the index, updated rows will apparently disappear and
+ deleted rows will still retain pointers. In other words, if you modify a
+ table with a hash index on it then you will get incorrect query results
+ on a standby server. When recovery completes it is recommended that you
+ manually <xref linkend="sql-reindex">
+ each such index after completing a recovery operation.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ If a <xref linkend="sql-createdatabase">
+ command is executed while a base backup is being taken, and then
+ the template database that the <command>CREATE DATABASE</> copied
+ is modified while the base backup is still in progress, it is
+ possible that recovery will cause those modifications to be
+ propagated into the created database as well. This is of course
+ undesirable. To avoid this risk, it is best not to modify any
+ template databases while taking a base backup.
</para>
</listitem>
<listitem>
<para>
- <command>CREATE TABLESPACE</> commands are WAL-logged with the literal
- absolute path, and will therefore be replayed as tablespace creations
- with the same absolute path. This might be undesirable if the log is
- being replayed on a different machine. It can be dangerous even if
- the log is being replayed on the same machine, but into a new data
- directory: the replay will still overwrite the contents of the original
- tablespace. To avoid potential gotchas of this sort, the best practice
- is to take a new base backup after creating or dropping tablespaces.
+ <xref linkend="sql-createtablespace">
+ commands are WAL-logged with the literal absolute path, and will
+ therefore be replayed as tablespace creations with the same
+ absolute path. This might be undesirable if the log is being
+ replayed on a different machine. It can be dangerous even if the
+ log is being replayed on the same machine, but into a new data
+ directory: the replay will still overwrite the contents of the
+ original tablespace. To avoid potential gotchas of this sort,
+ the best practice is to take a new base backup after creating or
+ dropping tablespaces.
</para>
</listitem>
</itemizedlist>
</para>
<para>
- It should also be noted that the present <acronym>WAL</acronym>
- format is extremely bulky since it includes many disk page
- snapshots. This is appropriate for crash recovery purposes,
- since we may need to fix partially-written disk pages. It is not
- necessary to store so many page copies for PITR operations, however.
- An area for future development is to compress archived WAL data by
- removing unnecessary page copies. In the meantime, administrators
- may wish to reduce the number of page snapshots included in WAL by
- increasing the checkpoint interval parameters as much as feasible.
+ It should also be noted that the default <acronym>WAL</acronym>
+ format is fairly bulky since it includes many disk page snapshots.
+ These page snapshots are designed to support crash recovery, since
+ we might need to fix partially-written disk pages. Depending on
+ your system hardware and software, the risk of partial writes might
+ be small enough to ignore, in which case you can significantly
+ reduce the total volume of archived logs by turning off page
+ snapshots using the <xref linkend="guc-full-page-writes">
+ parameter. (Read the notes and warnings in <xref linkend="wal">
+ before you do so.) Turning off page snapshots does not prevent
+ use of the logs for PITR operations. An area for future
+ development is to compress archived WAL data by removing
+ unnecessary page copies even when <varname>full_page_writes</> is
+ on. In the meantime, administrators might wish to reduce the number
+ of page snapshots included in WAL by increasing the checkpoint
+ interval parameters as much as feasible.
</para>
</sect2>
</sect1>
</para>
<para>
- As a general rule, the internal data storage format is subject to
- change between major releases of <productname>PostgreSQL</> (where
- the number after the first dot changes). This does not apply to
- different minor releases under the same major release (where the
- number after the second dot changes); these always have compatible
- storage formats. For example, releases 7.0.1, 7.1.2, and 7.2 are
- not compatible, whereas 7.1.1 and 7.1.2 are. When you update
- between compatible versions, you can simply replace the executables
- and reuse the data directory on disk. Otherwise you need to back
- up your data and restore it on the new server. This has to be done
- using <application>pg_dump</>; file system level backup methods
- obviously won't work. There are checks in place that prevent you
- from using a data directory with an incompatible version of
+ <productname>PostgreSQL</> major versions are represented by the
+ first two digit groups of the version number, e.g., 8.4.
+ <productname>PostgreSQL</> minor versions are represented by the
+ third group of version digits, e.g., 8.4.2 is the second minor
+ release of 8.4. Minor releases never change the internal storage
+ format and are always compatible with earlier and later minor
+ releases of the same major version number, e.g., 8.4.2 is compatible
+ with 8.4, 8.4.1 and 8.4.6. To update between compatible versions,
+ you simply replace the executables while the server is down and
+ restart the server. The data directory remains unchanged —
+ minor upgrades are that simple.
+ </para>
+
+ <para>
+ For <emphasis>major</> releases of <productname>PostgreSQL</>, the
+ internal data storage format is subject to change, thus complicating
+ upgrades. The traditional method for moving data to a new major version
+ is to dump and reload the database. Other, less-well-tested possibilities
+ are available, as discussed below.
+ </para>
+
+ <para>
+ New major versions also typically introduce some user-visible
+ incompatibilities, so application programming changes may be required.
+ Cautious users will want to test their client applications on the new
+ version before switching over fully; therefore, it's often a good idea to
+ set up concurrent installations of old and new versions. When
+ testing a <productname>PostgreSQL</> major upgrade, consider the
+ following categories of possible changes:
+ </para>
+
+ <variablelist>
+
+ <varlistentry>
+ <term>Administration</term>
+ <listitem>
+ <para>
+ The capabilities available for administrators to monitor and control
+ the server often change and improve in each major release.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>SQL</term>
+ <listitem>
+ <para>
+ Typically this includes new SQL command capabilities and not changes
+ in behavior, unless specifically mentioned in the release notes.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Library API</term>
+ <listitem>
+ <para>
+ Typically libraries like <application>libpq</> only add new
+ functionality, again unless mentioned in the release notes.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>System Catalogs</term>
+ <listitem>
+ <para>
+ System catalog changes usually only affect database management tools.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Server C-language API</term>
+ <listitem>
+ <para>
+ This involves changes in the backend function API, which is written
+ in the C programming language. Such changes affect code that
+ references backend functions deep inside the server.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+
+ <sect2 id="migration-methods-pgdump">
+ <title>Migrating data via <application>pg_dump</></title>
+
+ <para>
+ To dump data from one major version of <productname>PostgreSQL</> and
+ reload it in another, you must use <application>pg_dump</>; file system
+ level backup methods will not work. (There are checks in place that prevent
+ you from using a data directory with an incompatible version of
<productname>PostgreSQL</productname>, so no great harm can be done by
- trying to start the wrong server version on a data directory.
+ trying to start the wrong server version on a data directory.)
</para>
<para>
It is recommended that you use the <application>pg_dump</> and
<application>pg_dumpall</> programs from the newer version of
- <productname>PostgreSQL</>, to take advantage of any enhancements
- that may have been made in these programs. Current releases of the
+ <productname>PostgreSQL</>, to take advantage of enhancements
+ that might have been made in these programs. Current releases of the
dump programs can read data from any server version back to 7.0.
</para>
<para>
The least downtime can be achieved by installing the new server in
a different directory and running both the old and the new servers
- in parallel, on different ports. Then you can use something like
+ in parallel, on different ports. Then you can use something like:
<programlisting>
-pg_dumpall -p 5432 | psql -d template1 -p 6543
+pg_dumpall -p 5432 | psql -d postgres -p 6543
</programlisting>
- to transfer your data. Or use an intermediate file if you want.
- Then you can shut down the old server and start the new server at
- the port the old one was running at. You should make sure that the
- old database is not updated after you run <application>pg_dumpall</>,
- otherwise you will obviously lose that data. See <xref
- linkend="client-authentication"> for information on how to prohibit
+ to transfer your data. Or you can use an intermediate file if you wish.
+ Then you can shut down the old server and start the new server using
+ the port the old one was running on. You should make sure that the
+ old database is not updated after you begin to run
+ <application>pg_dumpall</>, otherwise you will lose those updates. See
+ <xref linkend="client-authentication"> for information on how to prohibit
access.
</para>
<para>
- In practice you probably want to test your client
- applications on the new setup before switching over completely.
- This is another reason for setting up concurrent installations
- of old and new versions.
- </para>
-
- <para>
- If you cannot or do not want to run two servers in parallel you can
+ If you cannot or do not want to run two servers in parallel, you can
do the backup step before installing the new version, bring down
- the server, move the old version out of the way, install the new
- version, start the new server, restore the data. For example:
+ the old server, move the old version out of the way, install the new
+ version, start the new server, and restore the data. For example:
<programlisting>
pg_dumpall > backup
pg_ctl stop
mv /usr/local/pgsql /usr/local/pgsql.old
+# Rename any tablespace directories as well
cd ~/postgresql-&version;
gmake install
initdb -D /usr/local/pgsql/data
-postmaster -D /usr/local/pgsql/data
-psql -f backup template1
+postgres -D /usr/local/pgsql/data
+psql -f backup postgres
</programlisting>
See <xref linkend="runtime"> about ways to start and stop the
<note>
<para>
When you <quote>move the old installation out of the way</quote>
- it may no longer be perfectly usable. Some of the executable programs
+ it might no longer be perfectly usable. Some of the executable programs
contain absolute paths to various installed programs and data files.
- This is usually not a big problem but if you plan on using two
+ This is usually not a big problem, but if you plan on using two
installations in parallel for a while you should assign them
different installation directories at build time. (This problem
- is rectified in <productname>PostgreSQL</> 8.0 and later, but you
- need to be wary of moving older installations.)
+ is rectified in <productname>PostgreSQL</> version 8.0 and later, so long
+ as you move all subdirectories containing installed files together;
+ for example if <filename>/usr/local/postgres/bin/</> goes to
+ <filename>/usr/local/postgres.old/bin/</>, then
+ <filename>/usr/local/postgres/share/</> must go to
+ <filename>/usr/local/postgres.old/share/</>. In pre-8.0 releases
+ moving an installation like this will not work.)
</para>
</note>
+ </sect2>
+
+ <sect2 id="migration-methods-other">
+ <title>Other data migration methods</title>
+
+ <para>
+ The <filename>contrib</> program
+ <link linkend="pgupgrade"><application>pg_upgrade</application></link>
+ allows an installation to be migrated in-place from one major
+ <productname>PostgreSQL</> version to the next. Keep in mind that this
+ method does not provide any scope for running old and new versions
+ concurrently. Also, <application>pg_upgrade</application> is much less
+ battle-tested than <application>pg_dump</application>, so having an
+ up-to-date backup is strongly recommended in case something goes wrong.
+ </para>
+
+ <para>
+ It is also possible to use certain replication methods, such as
+ <productname>Slony</>, to create a standby server with the updated version of
+ <productname>PostgreSQL</>. The standby can be on the same computer or
+ a different computer. Once it has synced up with the master server
+ (running the older version of <productname>PostgreSQL</>), you can
+ switch masters and make the standby the master and shut down the older
+ database instance. Such a switch-over results in only several seconds
+ of downtime for an upgrade.
+ </para>
+
+ </sect2>
</sect1>
</chapter>
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projects / postgresql / blobdiff