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2036 lines
87 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.119 2008/06/23 22:15:17 momjian Exp $ -->
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<chapter id="backup">
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<title>Backup and Restore</title>
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<indexterm zone="backup"><primary>backup</></>
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<para>
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As with everything that contains valuable data, <productname>PostgreSQL</>
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databases should be backed up regularly. While the procedure is
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essentially simple, it is important to have a clear understanding of
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the underlying techniques and assumptions.
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</para>
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<para>
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There are three fundamentally different approaches to backing up
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<productname>PostgreSQL</> data:
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<itemizedlist>
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<listitem><para><acronym>SQL</> dump</para></listitem>
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<listitem><para>File system level backup</para></listitem>
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<listitem><para>Continuous archiving</para></listitem>
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</itemizedlist>
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Each has its own strengths and weaknesses.
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Each is discussed in turn below.
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</para>
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<sect1 id="backup-dump">
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<title><acronym>SQL</> Dump</title>
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<para>
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The idea behind this dump method is to generate a text file with SQL
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commands that, when fed back to the server, will recreate the
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database in the same state as it was at the time of the dump.
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<productname>PostgreSQL</> provides the utility program
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<xref linkend="app-pgdump"> for this purpose. The basic usage of this
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command is:
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<synopsis>
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pg_dump <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">outfile</replaceable>
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</synopsis>
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As you see, <application>pg_dump</> writes its results to the
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standard output. We will see below how this can be useful.
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</para>
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<para>
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<application>pg_dump</> is a regular <productname>PostgreSQL</>
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client application (albeit a particularly clever one). This means
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that you can do this backup procedure from any remote host that has
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access to the database. But remember that <application>pg_dump</>
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does not operate with special permissions. In particular, it must
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have read access to all tables that you want to back up, so in
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practice you almost always have to run it as a database superuser.
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</para>
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<para>
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To specify which database server <application>pg_dump</> should
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contact, use the command line options <option>-h
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<replaceable>host</></> and <option>-p <replaceable>port</></>. The
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default host is the local host or whatever your
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<envar>PGHOST</envar> environment variable specifies. Similarly,
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the default port is indicated by the <envar>PGPORT</envar>
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environment variable or, failing that, by the compiled-in default.
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(Conveniently, the server will normally have the same compiled-in
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default.)
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</para>
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<para>
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Like any other <productname>PostgreSQL</> client application,
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<application>pg_dump</> will by default connect with the database
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user name that is equal to the current operating system user name. To override
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this, either specify the <option>-U</option> option or set the
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environment variable <envar>PGUSER</envar>. Remember that
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<application>pg_dump</> connections are subject to the normal
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client authentication mechanisms (which are described in <xref
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linkend="client-authentication">).
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</para>
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<para>
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Dumps created by <application>pg_dump</> are internally consistent,
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that is, the dump represents a snapshot of the database as of the time
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<application>pg_dump</> begins running. <application>pg_dump</> does not
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block other operations on the database while it is working.
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(Exceptions are those operations that need to operate with an
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exclusive lock, such as most forms of <command>ALTER TABLE</command>.)
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</para>
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<important>
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<para>
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If your database schema relies on OIDs (for instance as foreign
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keys) you must instruct <application>pg_dump</> to dump the OIDs
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as well. To do this, use the <option>-o</option> command line
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option.
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</para>
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</important>
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<sect2 id="backup-dump-restore">
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<title>Restoring the dump</title>
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<para>
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The text files created by <application>pg_dump</> are intended to
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be read in by the <application>psql</application> program. The
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general command form to restore a dump is
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<synopsis>
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psql <replaceable class="parameter">dbname</replaceable> < <replaceable class="parameter">infile</replaceable>
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</synopsis>
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where <replaceable class="parameter">infile</replaceable> is what
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you used as <replaceable class="parameter">outfile</replaceable>
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for the <application>pg_dump</> command. The database <replaceable
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class="parameter">dbname</replaceable> will not be created by this
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command, so you must create it yourself from <literal>template0</>
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before executing <application>psql</> (e.g., with
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<literal>createdb -T template0 <replaceable
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class="parameter">dbname</></literal>). <application>psql</>
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supports options similar to <application>pg_dump</>'s for specifying
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the database server to connect to and the user name to use. See
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the <xref linkend="app-psql"> reference page for more information.
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</para>
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<para>
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Before restoring a SQL dump, all the users who own objects or were
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granted permissions on objects in the dumped database must already
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exist. If they do not, then the restore will fail to recreate the
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objects with the original ownership and/or permissions.
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(Sometimes this is what you want, but usually it is not.)
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</para>
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<para>
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By default, the <application>psql</> script will continue to
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execute after an SQL error is encountered. You might wish to use the
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following command at the top of the script to alter that
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behaviour and have <application>psql</application> exit with an
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exit status of 3 if an SQL error occurs:
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<programlisting>
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\set ON_ERROR_STOP
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</programlisting>
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Either way, you will have an only partially restored database.
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Alternatively, you can specify that the whole dump should be
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restored as a single transaction, so the restore is either fully
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completed or fully rolled back. This mode can be specified by
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passing the <option>-1</> or <option>--single-transaction</>
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command-line options to <application>psql</>. When using this
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mode, be aware that even the smallest of errors can rollback a
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restore that has already run for many hours. However, that might
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still be preferable to manually cleaning up a complex database
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after a partially restored dump.
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</para>
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<para>
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The ability of <application>pg_dump</> and <application>psql</> to
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write to or read from pipes makes it possible to dump a database
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directly from one server to another, for example:
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<programlisting>
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pg_dump -h <replaceable>host1</> <replaceable>dbname</> | psql -h <replaceable>host2</> <replaceable>dbname</>
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</programlisting>
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</para>
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<important>
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<para>
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The dumps produced by <application>pg_dump</> are relative to
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<literal>template0</>. This means that any languages, procedures,
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etc. added via <literal>template1</> will also be dumped by
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<application>pg_dump</>. As a result, when restoring, if you are
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using a customized <literal>template1</>, you must create the
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empty database from <literal>template0</>, as in the example
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above.
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</para>
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</important>
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<para>
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After restoring a backup, it is wise to run <xref
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linkend="sql-analyze" endterm="sql-analyze-title"> on each
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database so the query optimizer has useful statistics. An easy way
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to do this is to run <command>vacuumdb -a -z</>; this is
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equivalent to running <command>VACUUM ANALYZE</> on each database
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manually. For more advice on how to load large amounts of data
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into <productname>PostgreSQL</> efficiently, refer to <xref
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linkend="populate">.
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</para>
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</sect2>
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<sect2 id="backup-dump-all">
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<title>Using <application>pg_dumpall</></title>
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<para>
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<application>pg_dump</> dumps only a single database at a time,
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and it does not dump information about roles or tablespaces
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(because those are cluster-wide rather than per-database).
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To support convenient dumping of the entire contents of a database
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cluster, the <xref linkend="app-pg-dumpall"> program is provided.
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<application>pg_dumpall</> backs up each database in a given
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cluster, and also preserves cluster-wide data such as role and
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tablespace definitions. The basic usage of this command is:
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<synopsis>
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pg_dumpall > <replaceable>outfile</>
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</synopsis>
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The resulting dump can be restored with <application>psql</>:
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<synopsis>
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psql -f <replaceable class="parameter">infile</replaceable> postgres
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</synopsis>
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(Actually, you can specify any existing database name to start from,
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but if you are reloading into an empty cluster then <literal>postgres</>
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should usually be used.) It is always necessary to have
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database superuser access when restoring a <application>pg_dumpall</>
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dump, as that is required to restore the role and tablespace information.
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If you use tablespaces, be careful that the tablespace paths in the
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dump are appropriate for the new installation.
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</para>
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<para>
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<application>pg_dumpall</> works by emitting commands to re-create
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roles, tablespaces, and empty databases, then invoking
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<application>pg_dump</> for each database. This means that while
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each database will be internally consistent, the snapshots of
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different databases might not be exactly in-sync.
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</para>
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</sect2>
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<sect2 id="backup-dump-large">
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<title>Handling large databases</title>
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<para>
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Since <productname>PostgreSQL</productname> allows tables larger
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than the maximum file size on your system, it can be problematic
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to dump such a table to a file, since the resulting file will likely
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be larger than the maximum size allowed by your system. Since
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<application>pg_dump</> can write to the standard output, you can
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use standard Unix tools to work around this possible problem.
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There are several ways to do it:
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</para>
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<formalpara>
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<title>Use compressed dumps.</title>
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<para>
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You can use your favorite compression program, for example
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<application>gzip</application>:
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<programlisting>
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pg_dump <replaceable class="parameter">dbname</replaceable> | gzip > <replaceable class="parameter">filename</replaceable>.gz
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</programlisting>
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Reload with:
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<programlisting>
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gunzip -c <replaceable class="parameter">filename</replaceable>.gz | psql <replaceable class="parameter">dbname</replaceable>
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</programlisting>
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or:
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<programlisting>
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cat <replaceable class="parameter">filename</replaceable>.gz | gunzip | psql <replaceable class="parameter">dbname</replaceable>
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</programlisting>
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</para>
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</formalpara>
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<formalpara>
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<title>Use <command>split</>.</title>
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<para>
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The <command>split</command> command
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allows you to split the output into pieces that are
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acceptable in size to the underlying file system. For example, to
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make chunks of 1 megabyte:
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<programlisting>
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pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 1m - <replaceable class="parameter">filename</replaceable>
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</programlisting>
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Reload with:
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<programlisting>
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cat <replaceable class="parameter">filename</replaceable>* | psql <replaceable class="parameter">dbname</replaceable>
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</programlisting>
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</para>
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</formalpara>
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<formalpara>
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<title>Use <application>pg_dump</>'s custom dump format.</title>
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<para>
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If <productname>PostgreSQL</productname> was built on a system with the
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<application>zlib</> compression library installed, the custom dump
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format will compress data as it writes it to the output file. This will
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produce dump file sizes similar to using <command>gzip</command>, but it
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has the added advantage that tables can be restored selectively. The
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following command dumps a database using the custom dump format:
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<programlisting>
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pg_dump -Fc <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">filename</replaceable>
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</programlisting>
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A custom-format dump is not a script for <application>psql</>, but
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instead must be restored with <application>pg_restore</>, for example:
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<programlisting>
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pg_restore -d <replaceable class="parameter">dbname</replaceable> <replaceable class="parameter">filename</replaceable>
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</programlisting>
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See the <xref linkend="app-pgdump"> and <xref
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linkend="app-pgrestore"> reference pages for details.
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</para>
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</formalpara>
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<para>
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For very large databases, you might need to combine <command>split</>
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with one of the other two approaches.
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</para>
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</sect2>
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</sect1>
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<sect1 id="backup-file">
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<title>File System Level Backup</title>
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<para>
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An alternative backup strategy is to directly copy the files that
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<productname>PostgreSQL</> uses to store the data in the database. In
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<xref linkend="creating-cluster"> it is explained where these files
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are located, but you have probably found them already if you are
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interested in this method. You can use whatever method you prefer
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for doing usual file system backups, for example:
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<programlisting>
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tar -cf backup.tar /usr/local/pgsql/data
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</programlisting>
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</para>
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<para>
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There are two restrictions, however, which make this method
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impractical, or at least inferior to the <application>pg_dump</>
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method:
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<orderedlist>
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<listitem>
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<para>
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The database server <emphasis>must</> be shut down in order to
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get a usable backup. Half-way measures such as disallowing all
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connections will <emphasis>not</emphasis> work
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(in part because <command>tar</command> and similar tools do not take
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an atomic snapshot of the state of the file system,
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but also because of internal buffering within the server).
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Information about stopping the server can be found in
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<xref linkend="server-shutdown">. Needless to say that you
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also need to shut down the server before restoring the data.
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</para>
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</listitem>
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<listitem>
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<para>
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If you have dug into the details of the file system layout of the
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database, you might be tempted to try to back up or restore only certain
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individual tables or databases from their respective files or
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directories. This will <emphasis>not</> work because the
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information contained in these files contains only half the
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truth. The other half is in the commit log files
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<filename>pg_clog/*</filename>, which contain the commit status of
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all transactions. A table file is only usable with this
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information. Of course it is also impossible to restore only a
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table and the associated <filename>pg_clog</filename> data
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because that would render all other tables in the database
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cluster useless. So file system backups only work for complete
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backup and restoration of an entire database cluster.
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</para>
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</listitem>
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</orderedlist>
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</para>
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<para>
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An alternative file-system backup approach is to make a
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<quote>consistent snapshot</quote> of the data directory, if the
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file system supports that functionality (and you are willing to
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trust that it is implemented correctly). The typical procedure is
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to make a <quote>frozen snapshot</> of the volume containing the
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database, then copy the whole data directory (not just parts, see
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above) from the snapshot to a backup device, then release the frozen
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snapshot. This will work even while the database server is running.
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However, a backup created in this way saves
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the database files in a state where the database server was not
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properly shut down; therefore, when you start the database server
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on the backed-up data, it will think the previous server instance had
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crashed and replay the WAL log. This is not a problem, just be aware of
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it (and be sure to include the WAL files in your backup).
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</para>
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<para>
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If your database is spread across multiple file systems, there might not
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be any way to obtain exactly-simultaneous frozen snapshots of all
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the volumes. For example, if your data files and WAL log are on different
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disks, or if tablespaces are on different file systems, it might
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not be possible to use snapshot backup because the snapshots
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<emphasis>must</> be simultaneous.
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Read your file system documentation very carefully before trusting
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to the consistent-snapshot technique in such situations.
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</para>
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<para>
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If simultaneous snapshots are not possible, one option is to shut down
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the database server long enough to establish all the frozen snapshots.
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Another option is perform a continuous archiving base backup (<xref
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linkend="backup-base-backup">) because such backups are immune to file
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system changes during the backup. This requires enabling continuous
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archiving just during the backup process; restore is done using
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continuous archive recovery (<xref linkend="backup-pitr-recovery">).
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</para>
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<para>
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Another option is to use <application>rsync</> to perform a file
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system backup. This is done by first running <application>rsync</>
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while the database server is running, then shutting down the database
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server just long enough to do a second <application>rsync</>. The
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second <application>rsync</> will be much quicker than the first,
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because it has relatively little data to transfer, and the end result
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will be consistent because the server was down. This method
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allows a file system backup to be performed with minimal downtime.
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</para>
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<para>
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Note that a file system backup will not necessarily be
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smaller than an SQL dump. On the contrary, it will most likely be
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larger. (<application>pg_dump</application> does not need to dump
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the contents of indexes for example, just the commands to recreate
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them.) However, taking a file system backup might be faster.
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</para>
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</sect1>
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<sect1 id="continuous-archiving">
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<title>Continuous Archiving and Point-In-Time Recovery (PITR)</title>
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<indexterm zone="backup">
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<primary>continuous archiving</primary>
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</indexterm>
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<indexterm zone="backup">
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<primary>point-in-time recovery</primary>
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</indexterm>
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<indexterm zone="backup">
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<primary>PITR</primary>
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</indexterm>
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<para>
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At all times, <productname>PostgreSQL</> maintains a
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<firstterm>write ahead log</> (WAL) in the <filename>pg_xlog/</>
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subdirectory of the cluster's data directory. The log describes
|
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every change made to the database's data files. This log exists
|
|
primarily for crash-safety purposes: if the system crashes, the
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database can be restored to consistency by <quote>replaying</> the
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log entries made since the last checkpoint. However, the existence
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of the log makes it possible to use a third strategy for backing up
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databases: we can combine a file-system-level backup with backup of
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the WAL files. If recovery is needed, we restore the backup and
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then replay from the backed-up WAL files to bring the backup up to
|
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current time. This approach is more complex to administer than
|
|
either of the previous approaches, but it has some significant
|
|
benefits:
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<itemizedlist>
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<listitem>
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<para>
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We do not need a perfectly consistent backup as the starting point.
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Any internal inconsistency in the backup will be corrected by log
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replay (this is not significantly different from what happens during
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crash recovery). So we don't need file system snapshot capability,
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just <application>tar</> or a similar archiving tool.
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</para>
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</listitem>
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<listitem>
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<para>
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Since we can string together an indefinitely long sequence of WAL files
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for replay, continuous backup can be achieved simply by continuing to archive
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the WAL files. This is particularly valuable for large databases, where
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it might not be convenient to take a full backup frequently.
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</para>
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</listitem>
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<listitem>
|
|
<para>
|
|
There is nothing that says we have 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
|
|
possible to restore the database to its state at any time since your base
|
|
backup was taken.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<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 <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>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<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 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 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
|
|
first base backup. Accordingly, we first discuss the mechanics of
|
|
archiving WAL files.
|
|
</para>
|
|
|
|
<sect2 id="backup-archiving-wal">
|
|
<title>Setting up WAL archiving</title>
|
|
|
|
<para>
|
|
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 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
|
|
interest and can be recycled.
|
|
</para>
|
|
|
|
<para>
|
|
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 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
|
|
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 <literal>cp</>, or it could invoke a complex shell
|
|
script — it's all up to you.
|
|
</para>
|
|
|
|
<para>
|
|
To enable WAL archiving, set the <xref
|
|
linkend="guc-archive-mode"> configuration parameter 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</>,
|
|
any <literal>%p</> is replaced by the path name of the file to
|
|
archive, while any <literal>%f</> is replaced by the file name only.
|
|
(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:
|
|
<programlisting>
|
|
archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'
|
|
</programlisting>
|
|
which will copy archivable WAL segments to the directory
|
|
<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>
|
|
The archive command will be executed under the ownership of the same
|
|
user that the <productname>PostgreSQL</> server is running as. Since
|
|
the series of WAL files being archived contains effectively everything
|
|
in your database, you will want to be sure that the archived data is
|
|
protected from prying eyes; for example, archive into a directory that
|
|
does not have group or world read access.
|
|
</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 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>
|
|
The archive command should generally be designed to refuse to overwrite
|
|
any pre-existing archive file. This is an important safety feature to
|
|
preserve the integrity of your archive in case of administrator error
|
|
(such as sending the output of two different servers to the same archive
|
|
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:
|
|
<programlisting>
|
|
archive_command = 'test ! -f .../%f && cp %p .../%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
|
|
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
|
|
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 reasonably quickly. The <filename>pg_xlog/</> directory will
|
|
continue to fill with WAL segment files until the situation is resolved.
|
|
(If the filesystem containing <filename>pg_xlog/</> fills up,
|
|
<productname>PostgreSQL</> will do a PANIC shutdown. No prior
|
|
transactions will be lost, but the database will be unavailable until
|
|
you free some space.)
|
|
</para>
|
|
|
|
<para>
|
|
The speed of the archiving command is not important, so 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
|
|
data that would be lost in the event of a disaster. It will also mean that
|
|
the <filename>pg_xlog/</> directory will contain large numbers of
|
|
not-yet-archived segment files, which could eventually exceed available
|
|
disk space. You are advised to monitor the archiving process to ensure that
|
|
it is working as you intend.
|
|
</para>
|
|
|
|
<para>
|
|
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 relative path (<literal>%p</>) but it is necessary to
|
|
remember 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,
|
|
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 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 ended 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>archive_mode</> is <literal>off</> some SQL commands
|
|
are optimized to avoid WAL logging, as described in <xref
|
|
linkend="populate-pitr">. If archiving 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>archive_mode</> 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">
|
|
<title>Making a Base Backup</title>
|
|
|
|
<para>
|
|
The procedure for making a base backup is relatively simple:
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>
|
|
Ensure that WAL archiving is enabled and working.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Connect to the database as a superuser, and issue the command:
|
|
<programlisting>
|
|
SELECT pg_start_backup('label');
|
|
</programlisting>
|
|
where <literal>label</> is any string you want to use to uniquely
|
|
identify this backup operation. (One good practice is to use the
|
|
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.
|
|
</para>
|
|
|
|
<para>
|
|
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>
|
|
<function>pg_start_backup</> can take a long time to finish.
|
|
This is because it performs a checkpoint, and the I/O
|
|
required for a 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">). Usually
|
|
this is what you want because it minimizes the impact on query
|
|
processing. If you just want to start the backup as soon as
|
|
possible, execute a <command>CHECKPOINT</> command
|
|
(which performs a checkpoint as quickly as possible) and then
|
|
immediately execute <function>pg_start_backup</>. Then there
|
|
will be very little for <function>pg_start_backup</>'s checkpoint
|
|
to do, and it won't take long.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Perform the backup, using any convenient file-system-backup tool
|
|
such as <application>tar</> or <application>cpio</>. 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:
|
|
<programlisting>
|
|
SELECT pg_stop_backup();
|
|
</programlisting>
|
|
This terminates the backup mode and performs an automatic switch to
|
|
the next WAL segment. The reason for the switch is to arrange that
|
|
the last WAL segment file written during the backup interval is
|
|
immediately ready to archive.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Once the WAL segment files used 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.
|
|
<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>
|
|
Some backup tools that you might wish to use emit warnings or errors
|
|
if the files they are trying to copy change while the copy proceeds.
|
|
This situation is normal, and not an error, when taking a base backup
|
|
of an active database; so 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 very 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. (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
|
|
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
|
|
your tablespaces).
|
|
</para>
|
|
|
|
<para>
|
|
You can, 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
|
|
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
|
|
reasons.
|
|
</para>
|
|
|
|
<para>
|
|
To make use of the backup, you will need to keep around 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.
|
|
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
|
|
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 can be deleted. However, you should
|
|
consider keeping several backup sets to be absolutely certain that
|
|
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 and WAL segments 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.
|
|
</para>
|
|
|
|
<para>
|
|
Since you have to keep around all the archived WAL files back to your
|
|
last base backup, the interval between base backups should usually be
|
|
chosen based on how much storage you want to expend on archived WAL
|
|
files. You should also consider how long you are prepared to spend
|
|
recovering, if recovery should be necessary — the system will have to
|
|
replay all those WAL segments, and that could take awhile if it has
|
|
been a long time since the last base backup.
|
|
</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</>.
|
|
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
|
|
exactly which backup session the dump file came from.
|
|
</para>
|
|
|
|
<para>
|
|
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 continuous archiving procedure above.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="backup-pitr-recovery">
|
|
<title>Recovering using a Continuous Archive Backup</title>
|
|
|
|
<para>
|
|
Okay, the worst has happened and you need to recover from your backup.
|
|
Here is the procedure:
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>
|
|
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
|
|
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 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
|
|
directory and under the root directories of any tablespaces you are using.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Restore the database files from your base backup. Be careful that they
|
|
are restored with the right ownership (the database system user, not
|
|
<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 recreate it,
|
|
being careful to ensure that you re-establish it as a symbolic link
|
|
if you had it set up that way before.
|
|
Be sure to recreate the subdirectory
|
|
<filename>pg_xlog/archive_status/</> as well.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
If you had 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
|
|
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 might
|
|
also want to temporarily modify <filename>pg_hba.conf</> to prevent
|
|
ordinary users from connecting until you are sure the recovery has worked.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
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
|
|
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.
|
|
</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
</para>
|
|
|
|
<para>
|
|
The key part of all this is to set up a recovery command 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
|
|
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
|
|
WAL file segments. Like the <varname>archive_command</>, this 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 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:
|
|
<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
|
|
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 files that are not present
|
|
in the archive; it must return nonzero when so asked. This is not an
|
|
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
|
|
preference to files in <filename>pg_xlog/</>. The system will not
|
|
overwrite the existing contents of <filename>pg_xlog/</> when retrieving
|
|
archived files.
|
|
</para>
|
|
|
|
<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). So 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, 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 still going on. (To
|
|
recover to such a time, you must go back to your previous base backup
|
|
and roll forward from there.)
|
|
</para>
|
|
</note>
|
|
|
|
<para>
|
|
If recovery finds a corruption in the WAL data then recovery will
|
|
complete 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>
|
|
|
|
|
|
<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 path name to copy
|
|
it to on the server.
|
|
(The path name is relative to the current working directory,
|
|
i.e., the cluster's data directory.)
|
|
Any <literal>%r</> is replaced by the name of the file containing the
|
|
last valid restart point. That is the earliest file that must be kept
|
|
to allow a restore to be restartable, so this information can be used
|
|
to truncate the archive to just the minimum required to support
|
|
restart of the current restore. <literal>%r</> would only be used in a
|
|
warm-standby configuration (see <xref linkend="warm-standby">).
|
|
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>
|
|
|
|
<varlistentry id="log-restartpoints"
|
|
xreflabel="log_restartpoints">
|
|
<term><varname>log_restartpoints</varname>
|
|
(<type>boolean</type>)
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Specifies whether to log each restart point as it occurs. This
|
|
can be helpful to track the progress of a long recovery.
|
|
Default is <literal>false</>.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
|
|
</sect3>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="backup-timelines">
|
|
<title>Timelines</title>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>timelines</primary>
|
|
</indexterm>
|
|
|
|
<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, 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 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 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</>. Whenever an archive recovery is completed,
|
|
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, and 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
|
|
situation where you aren't quite sure what point-in-time to recover to,
|
|
and so have to do several point-in-time recoveries by trial and error
|
|
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.
|
|
</para>
|
|
|
|
<para>
|
|
Each 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
|
|
contains multiple timelines. Therefore, they are archived into the WAL
|
|
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
|
|
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
|
|
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
|
|
timelines that branched off earlier than the base backup.
|
|
</para>
|
|
</sect2>
|
|
|
|
<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
|
|
could be negated.)
|
|
</para>
|
|
|
|
<para>
|
|
To prepare for standalone hot backups, set <varname>archive_mode</> to
|
|
<literal>on</>, and set up an <varname>archive_command</> that performs
|
|
archiving only when a <quote>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();"
|
|
sleep 20
|
|
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>
|
|
</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 continuous archiving
|
|
technique. These will probably be fixed in future releases:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Operations on hash indexes are not presently WAL-logged, so
|
|
replay will not update these indexes. The recommended workaround
|
|
is to manually <xref linkend="sql-reindex" endterm="sql-reindex-title">
|
|
each such index after completing a recovery operation.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
If a <xref linkend="sql-createdatabase" endterm="sql-createdatabase-title">
|
|
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>
|
|
<xref linkend="sql-createtablespace" endterm="sql-createtablespace-title">
|
|
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 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>
|
|
|
|
<sect1 id="warm-standby">
|
|
<title>Warm Standby Servers for High Availability</title>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>warm standby</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>PITR standby</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>standby server</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>log shipping</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>witness server</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>STONITH</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>high availability</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
Continuous archiving can be used to create a <firstterm>high
|
|
availability</> (HA) cluster configuration with one or more
|
|
<firstterm>standby servers</> ready to take over operations if the
|
|
primary server fails. This capability is widely referred to as
|
|
<firstterm>warm standby</> or <firstterm>log shipping</>.
|
|
</para>
|
|
|
|
<para>
|
|
The primary and standby server work together to provide this capability,
|
|
though the servers are only loosely coupled. The primary server operates
|
|
in continuous archiving mode, while each standby server operates in
|
|
continuous recovery mode, reading the WAL files from the primary. No
|
|
changes to the database tables are required to enable this capability,
|
|
so it offers low administration overhead in comparison with some other
|
|
replication approaches. This configuration also has relatively low
|
|
performance impact on the primary server.
|
|
</para>
|
|
|
|
<para>
|
|
Directly moving WAL records from one database server to another
|
|
is typically described as log shipping. <productname>PostgreSQL</>
|
|
implements file-based log shipping, which means that WAL records are
|
|
transferred one file (WAL segment) at a time. WAL files (16MB) can be
|
|
shipped easily and cheaply over any distance, whether it be to an
|
|
adjacent system, another system on the same site or another system on
|
|
the far side of the globe. The bandwidth required for this technique
|
|
varies according to the transaction rate of the primary server.
|
|
Record-based log shipping is also possible with custom-developed
|
|
procedures, as discussed in <xref linkend="warm-standby-record">.
|
|
</para>
|
|
|
|
<para>
|
|
It should be noted that the log shipping is asynchronous, i.e. the WAL
|
|
records are shipped after transaction commit. As a result there is a
|
|
window for data loss should the primary server suffer a catastrophic
|
|
failure: transactions not yet shipped will be lost. The length of the
|
|
window of data loss can be limited by use of the
|
|
<varname>archive_timeout</varname> parameter, which can be set as low
|
|
as a few seconds if required. However such low settings will
|
|
substantially increase the bandwidth requirements for file shipping.
|
|
If you need a window of less than a minute or so, it's probably better
|
|
to look into record-based log shipping.
|
|
</para>
|
|
|
|
<para>
|
|
The standby server is not available for access, since it is continually
|
|
performing recovery processing. Recovery performance is sufficiently
|
|
good that the standby will typically be only moments away from full
|
|
availability once it has been activated. As a result, we refer to this
|
|
capability as a warm standby configuration that offers high
|
|
availability. Restoring a server from an archived base backup and
|
|
rollforward will take considerably longer, so that technique only
|
|
offers a solution for disaster recovery, not high availability.
|
|
</para>
|
|
|
|
<sect2 id="warm-standby-planning">
|
|
<title>Planning</title>
|
|
|
|
<para>
|
|
It is usually wise to create the primary and standby servers
|
|
so that they are as similar as possible, at least from the
|
|
perspective of the database server. In particular, the path names
|
|
associated with tablespaces will be passed across as-is, so both
|
|
primary and standby servers must have the same mount paths for
|
|
tablespaces if that feature is used. Keep in mind that if
|
|
<xref linkend="sql-createtablespace" endterm="sql-createtablespace-title">
|
|
is executed on the primary, any new mount point needed for it must
|
|
be created on both the primary and all standby servers before the command
|
|
is executed. Hardware need not be exactly the same, but experience shows
|
|
that maintaining two identical systems is easier than maintaining two
|
|
dissimilar ones over the lifetime of the application and system.
|
|
In any case the hardware architecture must be the same — shipping
|
|
from, say, a 32-bit to a 64-bit system will not work.
|
|
</para>
|
|
|
|
<para>
|
|
In general, log shipping between servers running different major
|
|
<productname>PostgreSQL</> release
|
|
levels will not be possible. It is the policy of the PostgreSQL Global
|
|
Development Group not to make changes to disk formats during minor release
|
|
upgrades, so it is likely that running different minor release levels
|
|
on primary and standby servers will work successfully. However, no
|
|
formal support for that is offered and you are advised to keep primary
|
|
and standby servers at the same release level as much as possible.
|
|
When updating to a new minor release, the safest policy is to update
|
|
the standby servers first — a new minor release is more likely
|
|
to be able to read WAL files from a previous minor release than vice
|
|
versa.
|
|
</para>
|
|
|
|
<para>
|
|
There is no special mode required to enable a standby server. The
|
|
operations that occur on both primary and standby servers are entirely
|
|
normal continuous archiving and recovery tasks. The only point of
|
|
contact between the two database servers is the archive of WAL files
|
|
that both share: primary writing to the archive, standby reading from
|
|
the archive. Care must be taken to ensure that WAL archives for separate
|
|
primary servers do not become mixed together or confused. The archive
|
|
need not be large, if it is only required for the standby operation.
|
|
</para>
|
|
|
|
<para>
|
|
The magic that makes the two loosely coupled servers work together is
|
|
simply a <varname>restore_command</> used on the standby that,
|
|
when asked for the next WAL file, waits for it to become available from
|
|
the primary. The <varname>restore_command</> is specified in the
|
|
<filename>recovery.conf</> file on the standby server. Normal recovery
|
|
processing would request a file from the WAL archive, reporting failure
|
|
if the file was unavailable. For standby processing it is normal for
|
|
the next WAL file to be unavailable, so we must be patient and wait for
|
|
it to appear. For files ending in <literal>.backup</> or
|
|
<literal>.history</> there is no need to wait, and a non-zero return
|
|
code must be returned. A waiting <varname>restore_command</> can be
|
|
written as a custom script that loops after polling for the existence of
|
|
the next WAL file. There must also be some way to trigger failover, which
|
|
should interrupt the <varname>restore_command</>, break the loop and
|
|
return a file-not-found error to the standby server. This ends recovery
|
|
and the standby will then come up as a normal server.
|
|
</para>
|
|
|
|
<para>
|
|
Pseudocode for a suitable <varname>restore_command</> is:
|
|
<programlisting>
|
|
triggered = false;
|
|
while (!NextWALFileReady() && !triggered)
|
|
{
|
|
sleep(100000L); /* wait for ~0.1 sec */
|
|
if (CheckForExternalTrigger())
|
|
triggered = true;
|
|
}
|
|
if (!triggered)
|
|
CopyWALFileForRecovery();
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
A working example of a waiting <varname>restore_command</> is provided
|
|
as a <filename>contrib</> module named <application>pg_standby</>. It
|
|
should be used as a reference on how to correctly implement the logic
|
|
described above. It can also be extended as needed to support specific
|
|
configurations or environments.
|
|
</para>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> does not provide the system
|
|
software required to identify a failure on the primary and notify
|
|
the standby system and then the standby database server. Many such
|
|
tools exist and are well integrated with other aspects required for
|
|
successful failover, such as IP address migration.
|
|
</para>
|
|
|
|
<para>
|
|
The means for triggering failover is an important part of planning and
|
|
design. The <varname>restore_command</> is executed in full once
|
|
for each WAL file. The process running the <varname>restore_command</>
|
|
is therefore created and dies for each file, so there is no daemon
|
|
or server process and so we cannot use signals and a signal
|
|
handler. A more permanent notification is required to trigger the
|
|
failover. It is possible to use a simple timeout facility,
|
|
especially if used in conjunction with a known
|
|
<varname>archive_timeout</> setting on the primary. This is
|
|
somewhat error prone since a network problem or busy primary server might
|
|
be sufficient to initiate failover. A notification mechanism such
|
|
as the explicit creation of a trigger file is less error prone, if
|
|
this can be arranged.
|
|
</para>
|
|
|
|
<para>
|
|
The size of the WAL archive can be minimized by using the <literal>%r</>
|
|
option of the <varname>restore_command</>. This option specifies the
|
|
last archive file name that needs to be kept to allow the recovery to
|
|
restart correctly. This can be used to truncate the archive once
|
|
files are no longer required, if the archive is writable from the
|
|
standby server.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="warm-standby-config">
|
|
<title>Implementation</title>
|
|
|
|
<para>
|
|
The short procedure for configuring a standby server is as follows. For
|
|
full details of each step, refer to previous sections as noted.
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>
|
|
Set up primary and standby systems as near identically as
|
|
possible, including two identical copies of
|
|
<productname>PostgreSQL</> at the same release level.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Set up continuous archiving from the primary to a WAL archive located
|
|
in a directory on the standby server. Ensure that
|
|
<xref linkend="guc-archive-mode">,
|
|
<xref linkend="guc-archive-command"> and
|
|
<xref linkend="guc-archive-timeout">
|
|
are set appropriately on the primary
|
|
(see <xref linkend="backup-archiving-wal">).
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Make a base backup of the primary server (see <xref
|
|
linkend="backup-base-backup">), and load this data onto the standby.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Begin recovery on the standby server from the local WAL
|
|
archive, using a <filename>recovery.conf</> that specifies a
|
|
<varname>restore_command</> that waits as described
|
|
previously (see <xref linkend="backup-pitr-recovery">).
|
|
</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
</para>
|
|
|
|
<para>
|
|
Recovery treats the WAL archive as read-only, so once a WAL file has
|
|
been copied to the standby system it can be copied to tape at the same
|
|
time as it is being read by the standby database server.
|
|
Thus, running a standby server for high availability can be performed at
|
|
the same time as files are stored for longer term disaster recovery
|
|
purposes.
|
|
</para>
|
|
|
|
<para>
|
|
For testing purposes, it is possible to run both primary and standby
|
|
servers on the same system. This does not provide any worthwhile
|
|
improvement in server robustness, nor would it be described as HA.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="warm-standby-failover">
|
|
<title>Failover</title>
|
|
|
|
<para>
|
|
If the primary server fails then the standby server should begin
|
|
failover procedures.
|
|
</para>
|
|
|
|
<para>
|
|
If the standby server fails then no failover need take place. If the
|
|
standby server can be restarted, even some time later, then the recovery
|
|
process can also be immediately restarted, taking advantage of
|
|
restartable recovery. If the standby server cannot be restarted, then a
|
|
full new standby server instance should be created.
|
|
</para>
|
|
|
|
<para>
|
|
If the primary server fails and then immediately restarts, you must have
|
|
a mechanism for informing it that it is no longer the primary. This is
|
|
sometimes known as STONITH (Shoot the Other Node In The Head), which is
|
|
necessary to avoid situations where both systems think they are the
|
|
primary, which will lead to confusion and ultimately data loss.
|
|
</para>
|
|
|
|
<para>
|
|
Many failover systems use just two systems, the primary and the standby,
|
|
connected by some kind of heartbeat mechanism to continually verify the
|
|
connectivity between the two and the viability of the primary. It is
|
|
also possible to use a third system (called a witness server) to prevent
|
|
some cases of inappropriate failover, but the additional complexity
|
|
might not be worthwhile unless it is set up with sufficient care and
|
|
rigorous testing.
|
|
</para>
|
|
|
|
<para>
|
|
Once failover to the standby occurs, we have only a
|
|
single server in operation. This is known as a degenerate state.
|
|
The former standby is now the primary, but the former primary is down
|
|
and might stay down. To return to normal operation we must
|
|
fully recreate a standby server,
|
|
either on the former primary system when it comes up, or on a third,
|
|
possibly new, system. Once complete the primary and standby can be
|
|
considered to have switched roles. Some people choose to use a third
|
|
server to provide backup to the new primary until the new standby
|
|
server is recreated,
|
|
though clearly this complicates the system configuration and
|
|
operational processes.
|
|
</para>
|
|
|
|
<para>
|
|
So, switching from primary to standby server can be fast but requires
|
|
some time to re-prepare the failover cluster. Regular switching from
|
|
primary to standby is useful, since it allows regular downtime on
|
|
each system for maintenance. This also serves as a test of the
|
|
failover mechanism to ensure that it will really work when you need it.
|
|
Written administration procedures are advised.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="warm-standby-record">
|
|
<title>Record-based Log Shipping</title>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> directly supports file-based
|
|
log shipping as described above. It is also possible to implement
|
|
record-based log shipping, though this requires custom development.
|
|
</para>
|
|
|
|
<para>
|
|
An external program can call the <function>pg_xlogfile_name_offset()</>
|
|
function (see <xref linkend="functions-admin">)
|
|
to find out the file name and the exact byte offset within it of
|
|
the current end of WAL. It can then access the WAL file directly
|
|
and copy the data from the last known end of WAL through the current end
|
|
over to the standby server(s). With this approach, the window for data
|
|
loss is the polling cycle time of the copying program, which can be very
|
|
small, but there is no wasted bandwidth from forcing partially-used
|
|
segment files to be archived. Note that the standby servers'
|
|
<varname>restore_command</> scripts still deal in whole WAL files,
|
|
so the incrementally copied data is not ordinarily made available to
|
|
the standby servers. It is of use only when the primary dies —
|
|
then the last partial WAL file is fed to the standby before allowing
|
|
it to come up. So correct implementation of this process requires
|
|
cooperation of the <varname>restore_command</> script with the data
|
|
copying program.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="backup-incremental-updated">
|
|
<title>Incrementally Updated Backups</title>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>incrementally updated backups</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="backup">
|
|
<primary>change accumulation</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
In a warm standby configuration, it is possible to offload the expense of
|
|
taking periodic base backups from the primary server; instead base backups
|
|
can be made by backing
|
|
up a standby server's files. This concept is generally known as
|
|
incrementally updated backups, log change accumulation, or more simply,
|
|
change accumulation.
|
|
</para>
|
|
|
|
<para>
|
|
If we take a backup of the standby server's data directory while it is processing
|
|
logs shipped from the primary, we will be able to reload that data and
|
|
restart the standby's recovery process from the last restart point.
|
|
We no longer need to keep WAL files from before the restart point.
|
|
If we need to recover, it will be faster to recover from the incrementally
|
|
updated backup than from the original base backup.
|
|
</para>
|
|
|
|
<para>
|
|
Since the standby server is not <quote>live</>, it is not possible to
|
|
use <function>pg_start_backup()</> and <function>pg_stop_backup()</>
|
|
to manage the backup process; it will be up to you to determine how
|
|
far back you need to keep WAL segment files to have a recoverable
|
|
backup. You can do this by running <application>pg_controldata</>
|
|
on the standby server to inspect the control file and determine the
|
|
current checkpoint WAL location, or by using the
|
|
<varname>log_restartpoints</> option to print values to the server log.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="migration">
|
|
<title>Migration Between Releases</title>
|
|
|
|
<indexterm zone="migration">
|
|
<primary>upgrading</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="migration">
|
|
<primary>version</primary>
|
|
<secondary>compatibility</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
This section discusses how to migrate your database data from one
|
|
<productname>PostgreSQL</> release to a newer one.
|
|
The software installation procedure <foreignphrase>per se</> is not the
|
|
subject of this section; those details are in <xref linkend="installation">.
|
|
</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 8.1.1, 8.2.3, and 8.3 are
|
|
not compatible, whereas 8.2.3 and 8.2.4 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</productname>, so no great harm can be done by
|
|
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 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:
|
|
|
|
<programlisting>
|
|
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 begin to run
|
|
<application>pg_dumpall</>, otherwise you will lose that data. See <xref
|
|
linkend="client-authentication"> for information on how to prohibit
|
|
access.
|
|
</para>
|
|
|
|
<para>
|
|
It is also possible to use replication methods, such as
|
|
<productname>Slony</>, to create a slave server with the updated version of
|
|
<productname>PostgreSQL</>. The slave 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 slave the master and shut down the older
|
|
database instance. Such a switch-over results in only several seconds
|
|
of downtime for an upgrade.
|
|
</para>
|
|
|
|
<para>
|
|
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, and restore the data. For example:
|
|
|
|
<programlisting>
|
|
pg_dumpall > backup
|
|
pg_ctl stop
|
|
mv /usr/local/pgsql /usr/local/pgsql.old
|
|
cd ~/postgresql-&version;
|
|
gmake install
|
|
initdb -D /usr/local/pgsql/data
|
|
postgres -D /usr/local/pgsql/data
|
|
psql -f backup postgres
|
|
</programlisting>
|
|
|
|
See <xref linkend="runtime"> about ways to start and stop the
|
|
server and other details. The installation instructions will advise
|
|
you of strategic places to perform these steps.
|
|
</para>
|
|
|
|
<note>
|
|
<para>
|
|
When you <quote>move the old installation out of the way</quote>
|
|
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
|
|
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, 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>
|
|
|
|
<para>
|
|
In practice you probably want to test your client applications on the
|
|
new version before switching over completely. This is another reason
|
|
for setting 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 involved changes in the backend function API, which is written
|
|
in the C programming language. Such changes effect code that
|
|
references backend functions deep inside the server.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
|
|
</sect1>
|
|
</chapter>
|