doc: clarify pg_dump --no-comments meaning as SQL comments

[pgsql.git] / doc / src / sgml / backup.sgml

<!-- doc/src/sgml/backup.sgml -->

<chapter id="backup">
 <title>Backup and Restore</title>

 <indexterm zone="backup"><primary>backup</primary></indexterm>

 <para>
  As with everything that contains valuable data, <productname>PostgreSQL</productname>
  databases should be backed up regularly. While the procedure is
  essentially simple, it is important to have a clear understanding of
  the underlying techniques and assumptions.
 </para>

 <para>
  There are three fundamentally different approaches to backing up
  <productname>PostgreSQL</productname> data:
  <itemizedlist>
   <listitem><para><acronym>SQL</acronym> dump</para></listitem>
   <listitem><para>File system level backup</para></listitem>
   <listitem><para>Continuous archiving</para></listitem>
  </itemizedlist>
  Each has its own strengths and weaknesses; each is discussed in turn
  in the following sections.
 </para>

 <sect1 id="backup-dump">
  <title><acronym>SQL</acronym> Dump</title>

  <para>
   The idea behind this dump method is to generate a file with SQL
   commands that, when fed back to the server, will recreate the
   database in the same state as it was at the time of the dump.
   <productname>PostgreSQL</productname> provides the utility program
   <xref linkend="app-pgdump"/> for this purpose. The basic usage of this
   command is:
<synopsis>
pg_dump <replaceable class="parameter">dbname</replaceable> &gt; <replaceable class="parameter">dumpfile</replaceable>
</synopsis>
   As you see, <application>pg_dump</application> writes its result to the
   standard output. We will see below how this can be useful.
   While the above command creates a text file, <application>pg_dump</application>
   can create files in other formats that allow for parallelism and more
   fine-grained control of object restoration.
  </para>

  <para>
   <application>pg_dump</application> is a regular <productname>PostgreSQL</productname>
   client application (albeit a particularly clever one). This means
   that you can perform this backup procedure from any remote host that has
   access to the database. But remember that <application>pg_dump</application>
   does not operate with special permissions. In particular, it must
   have read access to all tables that you want to back up, so in order
   to back up the entire database you almost always have to run it as a
   database superuser.  (If you do not have sufficient privileges to back up
   the entire database, you can still back up portions of the database to which
   you do have access using options such as
   <option>-n <replaceable>schema</replaceable></option>
   or <option>-t <replaceable>table</replaceable></option>.)
  </para>

  <para>
   To specify which database server <application>pg_dump</application> should
   contact, use the command line options <option>-h
   <replaceable>host</replaceable></option> and <option>-p <replaceable>port</replaceable></option>. The
   default host is the local host or whatever your
   <envar>PGHOST</envar> environment variable specifies. Similarly,
   the default port is indicated by the <envar>PGPORT</envar>
   environment variable or, failing that, by the compiled-in default.
   (Conveniently, the server will normally have the same compiled-in
   default.)
  </para>

  <para>
   Like any other <productname>PostgreSQL</productname> client application,
   <application>pg_dump</application> will by default connect with the database
   user name that is equal to the current operating system user name. To override
   this, either specify the <option>-U</option> option or set the
   environment variable <envar>PGUSER</envar>. Remember that
   <application>pg_dump</application> connections are subject to the normal
   client authentication mechanisms (which are described in <xref
   linkend="client-authentication"/>).
  </para>

  <para>
   An important advantage of <application>pg_dump</application> over the other backup
   methods described later is that <application>pg_dump</application>'s output can
   generally be re-loaded into newer versions of <productname>PostgreSQL</productname>,
   whereas file-level backups and continuous archiving are both extremely
   server-version-specific.  <application>pg_dump</application> is also the only method
   that will work when transferring a database to a different machine
   architecture, such as going from a 32-bit to a 64-bit server.
  </para>

  <para>
   Dumps created by <application>pg_dump</application> are internally consistent,
   meaning, the dump represents a snapshot of the database at the time
   <application>pg_dump</application> began running. <application>pg_dump</application> does not
   block other operations on the database while it is working.
   (Exceptions are those operations that need to operate with an
   exclusive lock, such as most forms of <command>ALTER TABLE</command>.)
  </para>

  <sect2 id="backup-dump-restore">
   <title>Restoring the Dump</title>

   <para>
    Text files created by <application>pg_dump</application> are intended to
    be read in by the <application>psql</application> program. The
    general command form to restore a dump is
<synopsis>
psql <replaceable class="parameter">dbname</replaceable> &lt; <replaceable class="parameter">dumpfile</replaceable>
</synopsis>
    where <replaceable class="parameter">dumpfile</replaceable> is the
    file output by the <application>pg_dump</application> command. The database <replaceable
    class="parameter">dbname</replaceable> will not be created by this
    command, so you must create it yourself from <literal>template0</literal>
    before executing <application>psql</application> (e.g., with
    <literal>createdb -T template0 <replaceable
    class="parameter">dbname</replaceable></literal>).  <application>psql</application>
    supports options similar to <application>pg_dump</application> for specifying
    the database server to connect to and the user name to use. See
    the <xref linkend="app-psql"/> reference page for more information.
    Non-text file dumps are restored using the <xref
    linkend="app-pgrestore"/> utility.
   </para>

   <para>
    Before restoring an SQL dump, all the users who own objects or were
    granted permissions on objects in the dumped database must already
    exist. If they do not, the restore will fail to recreate the
    objects with the original ownership and/or permissions.
    (Sometimes this is what you want, but usually it is not.)
   </para>

   <para>
    By default, the <application>psql</application> script will continue to
    execute after an SQL error is encountered. You might wish to run
    <application>psql</application> with
    the <literal>ON_ERROR_STOP</literal> variable set to alter that
    behavior and have <application>psql</application> exit with an
    exit status of 3 if an SQL error occurs:
<programlisting>
psql --set ON_ERROR_STOP=on <replaceable>dbname</replaceable> &lt; <replaceable>dumpfile</replaceable>
</programlisting>
    Either way, you will only have a partially restored database.
    Alternatively, you can specify that the whole dump should be
    restored as a single transaction, so the restore is either fully
    completed or fully rolled back. This mode can be specified by
    passing the <option>-1</option> or <option>--single-transaction</option>
    command-line options to <application>psql</application>. When using this
    mode, be aware that even a minor error can rollback a
    restore that has already run for many hours. However, that might
    still be preferable to manually cleaning up a complex database
    after a partially restored dump.
   </para>

   <para>
    The ability of <application>pg_dump</application> and <application>psql</application> to
    write to or read from pipes makes it possible to dump a database
    directly from one server to another, for example:
<programlisting>
pg_dump -h <replaceable>host1</replaceable> <replaceable>dbname</replaceable> | psql -h <replaceable>host2</replaceable> <replaceable>dbname</replaceable>
</programlisting>
   </para>

   <important>
    <para>
     The dumps produced by <application>pg_dump</application> are relative to
     <literal>template0</literal>. This means that any languages, procedures,
     etc. added via <literal>template1</literal> will also be dumped by
     <application>pg_dump</application>. As a result, when restoring, if you are
     using a customized <literal>template1</literal>, you must create the
     empty database from <literal>template0</literal>, as in the example
     above.
    </para>
   </important>

   <para>
    After restoring a backup, it is wise to run <link
    linkend="sql-analyze"><command>ANALYZE</command></link> on each
    database so the query optimizer has useful statistics;
    see <xref linkend="vacuum-for-statistics"/>
    and <xref linkend="autovacuum"/> for more information.
    For more advice on how to load large amounts of data
    into <productname>PostgreSQL</productname> efficiently, refer to <xref
    linkend="populate"/>.
   </para>
  </sect2>

  <sect2 id="backup-dump-all">
   <title>Using <application>pg_dumpall</application></title>

   <para>
    <application>pg_dump</application> dumps only a single database at a time,
    and it does not dump information about roles or tablespaces
    (because those are cluster-wide rather than per-database).
    To support convenient dumping of the entire contents of a database
    cluster, the <xref linkend="app-pg-dumpall"/> program is provided.
    <application>pg_dumpall</application> backs up each database in a given
    cluster, and also preserves cluster-wide data such as role and
    tablespace definitions. The basic usage of this command is:
<synopsis>
pg_dumpall &gt; <replaceable>dumpfile</replaceable>
</synopsis>
    The resulting dump can be restored with <application>psql</application>:
<synopsis>
psql -f <replaceable class="parameter">dumpfile</replaceable> postgres
</synopsis>
    (Actually, you can specify any existing database name to start from,
    but if you are loading into an empty cluster then <literal>postgres</literal>
    should usually be used.)  It is always necessary to have
    database superuser access when restoring a <application>pg_dumpall</application>
    dump, as that is required to restore the role and tablespace information.
    If you use tablespaces, make sure that the tablespace paths in the
    dump are appropriate for the new installation.
   </para>

   <para>
    <application>pg_dumpall</application> works by emitting commands to re-create
    roles, tablespaces, and empty databases, then invoking
    <application>pg_dump</application> for each database.  This means that while
    each database will be internally consistent, the snapshots of
    different databases are not synchronized.
   </para>

   <para>
    Cluster-wide data can be dumped alone using the
    <application>pg_dumpall</application> <option>--globals-only</option> option.
    This is necessary to fully backup the cluster if running the
    <application>pg_dump</application> command on individual databases.
   </para>
  </sect2>

  <sect2 id="backup-dump-large">
   <title>Handling Large Databases</title>

   <para>
    Some operating systems have maximum file size limits that cause
    problems when creating large <application>pg_dump</application> output files.
    Fortunately, <application>pg_dump</application> can write to the standard
    output, so you can use standard Unix tools to work around this
    potential problem.  There are several possible methods:
   </para>

   <formalpara>
    <title>Use compressed dumps.</title>
    <para>
     You can use your favorite compression program, for example
     <application>gzip</application>:

<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | gzip &gt; <replaceable class="parameter">filename</replaceable>.gz
</programlisting>

     Reload with:

<programlisting>
gunzip -c <replaceable class="parameter">filename</replaceable>.gz | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>

     or:

<programlisting>
cat <replaceable class="parameter">filename</replaceable>.gz | gunzip | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
    </para>
   </formalpara>

   <formalpara>
    <title>Use <command>split</command>.</title>
    <para>
     The <command>split</command> command
     allows you to split the output into smaller files that are
     acceptable in size to the underlying file system. For example, to
     make 2 gigabyte chunks:

<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 2G - <replaceable class="parameter">filename</replaceable>
</programlisting>

     Reload with:

<programlisting>
cat <replaceable class="parameter">filename</replaceable>* | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>

     If using GNU <application>split</application>, it is possible to
     use it and <application>gzip</application> together:

<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 2G --filter='gzip > $FILE.gz'
</programlisting>

     It can be restored using <command>zcat</command>.
    </para>
   </formalpara>

   <formalpara>
    <title>Use <application>pg_dump</application>'s custom dump format.</title>
    <para>
     If <productname>PostgreSQL</productname> was built on a system with the
     <application>zlib</application> compression library installed, the custom dump
     format will compress data as it writes it to the output file. This will
     produce dump file sizes similar to using <command>gzip</command>, but it
     has the added advantage that tables can be restored selectively. The
     following command dumps a database using the custom dump format:

<programlisting>
pg_dump -Fc <replaceable class="parameter">dbname</replaceable> &gt; <replaceable class="parameter">filename</replaceable>
</programlisting>

     A custom-format dump is not a script for <application>psql</application>, but
     instead must be restored with <application>pg_restore</application>, for example:

<programlisting>
pg_restore -d <replaceable class="parameter">dbname</replaceable> <replaceable class="parameter">filename</replaceable>
</programlisting>

     See the <xref linkend="app-pgdump"/> and <xref
     linkend="app-pgrestore"/> reference pages for details.
    </para>
   </formalpara>

   <para>
    For very large databases, you might need to combine <command>split</command>
    with one of the other two approaches.
   </para>

   <formalpara>
    <title>Use <application>pg_dump</application>'s parallel dump feature.</title>
    <para>
     To speed up the dump of a large database, you can use
     <application>pg_dump</application>'s parallel mode. This will dump
     multiple tables at the same time. You can control the degree of
     parallelism with the <command>-j</command> parameter. Parallel dumps
     are only supported for the "directory" archive format.

<programlisting>
pg_dump -j <replaceable class="parameter">num</replaceable> -F d -f <replaceable class="parameter">out.dir</replaceable> <replaceable class="parameter">dbname</replaceable>
</programlisting>

     You can use <command>pg_restore -j</command> to restore a dump in parallel.
     This will work for any archive of either the "custom" or the "directory"
     archive mode, whether or not it has been created with <command>pg_dump -j</command>.
    </para>
   </formalpara>
  </sect2>
 </sect1>

 <sect1 id="backup-file">
  <title>File System Level Backup</title>

  <para>
   An alternative backup strategy is to directly copy the files that
   <productname>PostgreSQL</productname> uses to store the data in the database;
   <xref linkend="creating-cluster"/> explains where these files
   are located.  You can use whatever method you prefer
   for doing file system backups; for example:

<programlisting>
tar -cf backup.tar /usr/local/pgsql/data
</programlisting>
  </para>

  <para>
   There are two restrictions, however, which make this method
   impractical, or at least inferior to the <application>pg_dump</application>
   method:

   <orderedlist>
    <listitem>
     <para>
      The database server <emphasis>must</emphasis> be shut down in order to
      get a usable backup. Half-way measures such as disallowing all
      connections will <emphasis>not</emphasis> work
      (in part because <command>tar</command> and similar tools do not take
      an atomic snapshot of the state of the file system,
      but also because of internal buffering within the server).
      Information about stopping the server can be found in
      <xref linkend="server-shutdown"/>.  Needless to say, you
      also need to shut down the server before restoring the data.
     </para>
    </listitem>

    <listitem>
     <para>
      If you have dug into the details of the file system layout of the
      database, you might be tempted to try to back up or restore only certain
      individual tables or databases from their respective files or
      directories. This will <emphasis>not</emphasis> work because the
      information contained in these files is not usable without
      the commit log files,
      <filename>pg_xact/*</filename>, which contain the commit status of
      all transactions. A table file is only usable with this
      information. Of course it is also impossible to restore only a
      table and the associated <filename>pg_xact</filename> data
      because that would render all other tables in the database
      cluster useless.  So file system backups only work for complete
      backup and restoration of an entire database cluster.
     </para>
    </listitem>
   </orderedlist>
  </para>

  <para>
   An alternative file-system backup approach is to make a
   <quote>consistent snapshot</quote> of the data directory, if the
   file system supports that functionality (and you are willing to
   trust that it is implemented correctly).  The typical procedure is
   to make a <quote>frozen snapshot</quote> of the volume containing the
   database, then copy the whole data directory (not just parts, see
   above) from the snapshot to a backup device, then release the frozen
   snapshot.  This will work even while the database server is running.
   However, a backup created in this way saves
   the database files in a state as if the database server was not
   properly shut down; therefore, when you start the database server
   on the backed-up data, it will think the previous server instance
   crashed and will replay the WAL log.  This is not a problem; just
   be aware of it (and be sure to include the WAL files in your backup).
   You can perform a <command>CHECKPOINT</command> before taking the
   snapshot to reduce recovery time.
  </para>

  <para>
   If your database is spread across multiple file systems, there might not
   be any way to obtain exactly-simultaneous frozen snapshots of all
   the volumes.  For example, if your data files and WAL log are on different
   disks, or if tablespaces are on different file systems, it might
   not be possible to use snapshot backup because the snapshots
   <emphasis>must</emphasis> be simultaneous.
   Read your file system documentation very carefully before trusting
   the consistent-snapshot technique in such situations.
  </para>

  <para>
   If simultaneous snapshots are not possible, one option is to shut down
   the database server long enough to establish all the frozen snapshots.
   Another option is to perform a continuous archiving base backup (<xref
   linkend="backup-base-backup"/>) because such backups are immune to file
   system changes during the backup.  This requires enabling continuous
   archiving just during the backup process; restore is done using
   continuous archive recovery (<xref linkend="backup-pitr-recovery"/>).
  </para>

  <para>
   Another option is to use <application>rsync</application> to perform a file
   system backup.  This is done by first running <application>rsync</application>
   while the database server is running, then shutting down the database
   server long enough to do an <command>rsync --checksum</command>.
   (<option>--checksum</option> is necessary because <command>rsync</command> only
   has file modification-time granularity of one second.)  The
   second <application>rsync</application> will be quicker than the first,
   because it has relatively little data to transfer, and the end result
   will be consistent because the server was down.  This method
   allows a file system backup to be performed with minimal downtime.
  </para>

  <para>
   Note that a file system backup will typically be larger
   than an SQL dump. (<application>pg_dump</application> does not need to dump
   the contents of indexes for example, just the commands to recreate
   them.)  However, taking a file system backup might be faster.
  </para>
 </sect1>

 <sect1 id="continuous-archiving">
  <title>Continuous Archiving and Point-in-Time Recovery (PITR)</title>

  <indexterm zone="backup">
   <primary>continuous archiving</primary>
  </indexterm>

  <indexterm zone="backup">
   <primary>point-in-time recovery</primary>
  </indexterm>

  <indexterm zone="backup">
   <primary>PITR</primary>
  </indexterm>

  <para>
   At all times, <productname>PostgreSQL</productname> maintains a
   <firstterm>write ahead log</firstterm> (WAL) in the <filename>pg_wal/</filename>
   subdirectory of the cluster's data directory. The log records
   every change made to the database's data files.  This log exists
   primarily for crash-safety purposes: if the system crashes, the
   database can be restored to consistency by <quote>replaying</quote> the
   log entries made since the last checkpoint.  However, the existence
   of the log makes it possible to use a third strategy for backing up
   databases: we can combine a file-system-level backup with backup of
   the WAL files.  If recovery is needed, we restore the file system backup and
   then replay from the backed-up WAL files to bring the system to a
   current state.  This approach is more complex to administer than
   either of the previous approaches, but it has some significant
   benefits:
  <itemizedlist>
   <listitem>
    <para>
     We do not need a perfectly consistent file system backup as the starting point.
     Any internal inconsistency in the backup will be corrected by log
     replay (this is not significantly different from what happens during
     crash recovery).  So we do not need a file system snapshot capability,
     just <application>tar</application> or a similar archiving tool.
    </para>
   </listitem>
   <listitem>
    <para>
     Since we can combine an indefinitely long sequence of WAL files
     for replay, continuous backup can be achieved simply by continuing to archive
     the WAL files.  This is particularly valuable for large databases, where
     it might not be convenient to take a full backup frequently.
    </para>
   </listitem>
   <listitem>
    <para>
     It is not necessary to replay the WAL entries all the
     way to the end.  We could stop the replay at any point and have a
     consistent snapshot of the database as it was at that time.  Thus,
     this technique supports <firstterm>point-in-time recovery</firstterm>: 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</firstterm> 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>

  <note>
   <para>
    <application>pg_dump</application> and
    <application>pg_dumpall</application> do not produce file-system-level
    backups and cannot be used as part of a continuous-archiving solution.
    Such dumps are <emphasis>logical</emphasis> and do not contain enough
    information to be used by WAL replay.
   </para>
  </note>

  <para>
   As with the plain file-system-backup technique, this method can only
   support restoration of an entire database cluster, not a subset.
   Also, it requires a lot of archival storage: the base backup 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</quote> 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</emphasis> 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</productname> system
    produces an indefinitely long sequence of WAL records.  The system
    physically divides this sequence into WAL <firstterm>segment
    files</firstterm>, which are normally 16MB apiece (although the segment size
    can be altered during <application>initdb</application>).  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</quote> them by renaming no-longer-needed segment files
    to higher segment numbers.  It's assumed that segment files whose
    contents precede the last checkpoint are 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</quote>: 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 flexibility,
    <productname>PostgreSQL</productname> tries not to make any assumptions about how
    the archiving will be done.  Instead, <productname>PostgreSQL</productname> lets
    the administrator specify a shell command or an archive library to be executed to copy a
    completed segment file to wherever it needs to go.  This could be as simple
    as a shell command that uses <literal>cp</literal>, or it could invoke a
    complex C function &mdash; it's all up to you.
   </para>

   <para>
    To enable WAL archiving, set the <xref linkend="guc-wal-level"/>
    configuration parameter to <literal>replica</literal> or higher,
    <xref linkend="guc-archive-mode"/> to <literal>on</literal>,
    specify the shell command to use in the <xref
    linkend="guc-archive-command"/> configuration parameter
    or specify the library to use in the <xref
    linkend="guc-archive-library"/> configuration parameter.  In practice
    these settings will always be placed in the
    <filename>postgresql.conf</filename> file.
   </para>

   <para>
    In <varname>archive_command</varname>,
    <literal>%p</literal> is replaced by the path name of the file to
    archive, while <literal>%f</literal> is replaced by only the file name.
    (The path name is relative to the current working directory,
    i.e., the cluster's data directory.)
    Use <literal>%%</literal> if you need to embed an actual <literal>%</literal>
    character in the command.  The simplest useful command is something
    like:
<programlisting>
archive_command = 'test ! -f /mnt/server/archivedir/%f &amp;&amp; cp %p /mnt/server/archivedir/%f'  # Unix
archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"'  # Windows
</programlisting>
    which will copy archivable WAL segments to the directory
    <filename>/mnt/server/archivedir</filename>.  (This is an example, not a
    recommendation, and might not work on all platforms.)  After the
    <literal>%p</literal> and <literal>%f</literal> parameters have been replaced,
    the actual command executed might look like this:
<programlisting>
test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065
</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</productname> 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 succeeds.  Upon getting a zero result,
    <productname>PostgreSQL</productname> will assume that the file has been
    successfully archived, and will remove or recycle it.  However, a nonzero
    status tells <productname>PostgreSQL</productname> that the file was not archived;
    it will try again periodically until it succeeds.
   </para>

   <para>
    Another way to archive is to use a custom archive module as the
    <varname>archive_library</varname>.  Since such modules are written in
    <literal>C</literal>, creating your own may require considerably more effort
    than writing a shell command.  However, archive modules can be more
    performant than archiving via shell, and they will have access to many
    useful server resources.  For more information about archive modules, see
    <xref linkend="archive-modules"/>.
   </para>

   <para>
    When the archive command is terminated by a signal (other than
    <systemitem>SIGTERM</systemitem> that is used as part of a server
    shutdown) or an error by the shell with an exit status greater than
    125 (such as command not found), or if the archive function emits an
    <literal>ERROR</literal> or <literal>FATAL</literal>, the archiver process
    aborts and gets restarted by the postmaster. In such cases, the failure is
    not reported in <xref linkend="pg-stat-archiver-view"/>.
   </para>

   <para>
    Archive commands and libraries 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 library to ensure
    that it does not overwrite an existing file.
   </para>

   <para>
    In rare cases, <productname>PostgreSQL</productname> may attempt to
    re-archive a WAL file that was previously archived.  For example, if the
    system crashes before the server makes a durable record of archival
    success, the server will attempt to archive the file again after
    restarting (provided archiving is still enabled).  When an archive command or library
    encounters a pre-existing file, it should return a zero status or <literal>true</literal>, respectively,
    if the WAL file has identical contents to the pre-existing archive and the
    pre-existing archive is fully persisted to storage.  If a pre-existing
    file contains different contents than the WAL file being archived, the
    archive command or library <emphasis>must</emphasis> return a nonzero status or
    <literal>false</literal>, respectively.
   </para>

   <para>
    The example command above for Unix avoids overwriting a pre-existing archive
    by including a separate
    <command>test</command> step.  On some Unix platforms, <command>cp</command> has
    switches such as <option>-i</option> that can be used to do the same thing
    less verbosely, but you should not rely on these without verifying that
    the right exit status is returned.  (In particular, GNU <command>cp</command>
    will return status zero when <option>-i</option> is used and the target file
    already exists, which is <emphasis>not</emphasis> the desired behavior.)
   </para>

   <para>
    While designing your archiving setup, consider what will happen if
    the archive command or library 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_wal/</filename> directory will
    continue to fill with WAL segment files until the situation is resolved.
    (If the file system containing <filename>pg_wal/</filename> fills up,
    <productname>PostgreSQL</productname> will do a PANIC shutdown.  No committed
    transactions will be lost, but the database will remain offline until
    you free some space.)
   </para>

   <para>
    The speed of the archive command or library is unimportant as long as it can keep up
    with the average rate at which your server generates WAL data.  Normal
    operation continues even if the archiving process falls a little behind.
    If archiving falls significantly behind, this will increase the amount of
    data that would be lost in the event of a disaster. It will also mean that
    the <filename>pg_wal/</filename> 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 or library, 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
    preserve the original relative path (<literal>%p</literal>) but it is necessary to
    preserve the file name (<literal>%f</literal>).
   </para>

   <para>
    Note that although WAL archiving will allow you to restore any
    modifications made to the data in your <productname>PostgreSQL</productname> database,
    it will not restore changes made to configuration files (that is,
    <filename>postgresql.conf</filename>, <filename>pg_hba.conf</filename> and
    <filename>pg_ident.conf</filename>), 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 or function is only invoked on completed WAL segments.  Hence,
    if your server generates only little WAL traffic (or has slack periods
    where it does so), there could be a long delay between the completion
    of a transaction and its safe recording in archive storage.  To put
    a limit on how old unarchived data can be, you can set
    <xref linkend="guc-archive-timeout"/> to force the server to switch
    to a new WAL segment file at least that often.  Note that archived
    files that are archived early due to a forced switch are still the same
    length as completely full files.  It is therefore unwise to set a very
    short <varname>archive_timeout</varname> &mdash; it will bloat your archive
    storage.  <varname>archive_timeout</varname> settings of a minute or so are
    usually reasonable.
   </para>

   <para>
    Also, you can force a segment switch manually with
    <function>pg_switch_wal</function> if you want to ensure that a
    just-finished transaction is archived as soon as possible.  Other utility
    functions related to WAL management are listed in <xref
    linkend="functions-admin-backup-table"/>.
   </para>

   <para>
    When <varname>wal_level</varname> is <literal>minimal</literal> some SQL commands
    are optimized to avoid WAL logging, as described in <xref
    linkend="populate-pitr"/>.  If archiving or streaming replication were
    turned on during execution of one of these statements, WAL would not
    contain enough information for archive recovery.  (Crash recovery is
    unaffected.)  For this reason, <varname>wal_level</varname> can only be changed at
    server start.  However, <varname>archive_command</varname> and <varname>archive_library</varname> can be changed with a
    configuration file reload.  If you are archiving via shell and wish to
    temporarily stop archiving,
    one way to do it is to set <varname>archive_command</varname> to the empty
    string (<literal>''</literal>).
    This will cause WAL files to accumulate in <filename>pg_wal/</filename> until a
    working <varname>archive_command</varname> is re-established.
   </para>
  </sect2>

  <sect2 id="backup-base-backup">
   <title>Making a Base Backup</title>

   <para>
    The easiest way to perform a base backup is to use the
    <xref linkend="app-pgbasebackup"/> tool. It can create
    a base backup either as regular files or as a tar archive. If more
    flexibility than <xref linkend="app-pgbasebackup"/> can provide is
    required, you can also make a base backup using the low level API
    (see <xref linkend="backup-lowlevel-base-backup"/>).
   </para>

   <para>
    It is not necessary to be concerned about the amount of time it takes
    to make a base backup. However, if you normally run the
    server with <varname>full_page_writes</varname> disabled, you might notice a drop
    in performance while the backup runs since <varname>full_page_writes</varname> is
    effectively forced on during backup mode.
   </para>

   <para>
    To make use of the backup, you will need to keep all the WAL
    segment files generated during and after the file system backup.
    To aid you in doing this, the base backup process
    creates a <firstterm>backup history file</firstterm> that is immediately
    stored into the WAL archive area. This file is named after the first
    WAL segment file that you need for the file system backup.
    For example, if the starting WAL file is
    <literal>0000000100001234000055CD</literal> the backup history file will be
    named something like
    <literal>0000000100001234000055CD.007C9330.backup</literal>. (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 <xref linkend="app-pgbasebackup"/>, as well as
    the starting and ending times and WAL segments of the backup.
    If you used the label to identify the associated dump file,
    then the archived history file is enough to tell you which dump file to
    restore.
   </para>

   <para>
    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 &mdash; 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>
  </sect2>

  <sect2 id="backup-incremental-backup">
   <title>Making an Incremental Backup</title>

   <para>
    You can use <xref linkend="app-pgbasebackup"/> to take an incremental
    backup by specifying the <literal>--incremental</literal> option. You must
    supply, as an argument to <literal>--incremental</literal>, the backup
    manifest to an earlier backup from the same server. In the resulting
    backup, non-relation files will be included in their entirety, but some
    relation files may be replaced by smaller incremental files which contain
    only the blocks which have been changed since the earlier backup and enough
    metadata to reconstruct the current version of the file.
   </para>

   <para>
    To figure out which blocks need to be backed up, the server uses WAL
    summaries, which are stored in the data directory, inside the directory
    <literal>pg_wal/summaries</literal>. If the required summary files are not
    present, an attempt to take an incremental backup will fail. The summaries
    present in this directory must cover all LSNs from the start LSN of the
    prior backup to the start LSN of the current backup. Since the server looks
    for WAL summaries just after establishing the start LSN of the current
    backup, the necessary summary files probably won't be instantly present
    on disk, but the server will wait for any missing files to show up.
    This also helps if the WAL summarization process has fallen behind.
    However, if the necessary files have already been removed, or if the WAL
    summarizer doesn't catch up quickly enough, the incremental backup will
    fail.
   </para>

   <para>
    When restoring an incremental backup, it will be necessary to have not
    only the incremental backup itself but also all earlier backups that
    are required to supply the blocks omitted from the incremental backup.
    See <xref linkend="app-pgcombinebackup"/> for further information about
    this requirement. Note that there are restrictions on the use of
    <literal>pg_combinebackup</literal> when the checksum status of the
    cluster has been changed; see
    <link linkend="app-pgcombinebackup-limitations">pg_combinebackup
    limitations</link>.
   </para>

   <para>
    Note that all of the requirements for making use of a full backup also
    apply to an incremental backup. For instance, you still need all of the
    WAL segment files generated during and after the file system backup, and
    any relevant WAL history files. And you still need to create a
    <literal>recovery.signal</literal> (or <literal>standby.signal</literal>)
    and perform recovery, as described in
    <xref linkend="backup-pitr-recovery" />. The requirement to have earlier
    backups available at restore time and to use
    <literal>pg_combinebackup</literal> is an additional requirement on top of
    everything else. Keep in mind that <application>PostgreSQL</application>
    has no built-in mechanism to figure out which backups are still needed as
    a basis for restoring later incremental backups. You must keep track of
    the relationships between your full and incremental backups on your own,
    and be certain not to remove earlier backups if they might be needed when
    restoring later incremental backups.
   </para>

   <para>
    Incremental backups typically only make sense for relatively large
    databases where a significant portion of the data does not change, or only
    changes slowly. For a small database, it's simpler to ignore the existence
    of incremental backups and simply take full backups, which are simpler
    to manage. For a large database all of which is heavily modified,
    incremental backups won't be much smaller than full backups.
   </para>

   <para>
    An incremental backup is only possible if replay would begin from a later
    checkpoint than for the previous backup upon which it depends.  If you
    take the incremental backup on the primary, this condition is always
    satisfied, because each backup triggers a new checkpoint.  On a standby,
    replay begins from the most recent restartpoint.  Therefore, an
    incremental backup of a standby server can fail if there has been very
    little activity since the previous backup, since no new restartpoint might
    have been created.
   </para>
  </sect2>

  <sect2 id="backup-lowlevel-base-backup">
   <title>Making a Base Backup Using the Low Level API</title>
   <para>
    Instead of taking a full or incremental base backup using
    <xref linkend="app-pgbasebackup"/>, you can take a base backup using the
    low-level API. This procedure contains a few more steps than
    the <application>pg_basebackup</application> method, but is relatively
    simple. It is very important that these steps are executed in
    sequence, and that the success of a step is verified before
    proceeding to the next step.
   </para>
    <para>
     Multiple backups are able to be run concurrently (both those
     started using this backup API and those started using
     <xref linkend="app-pgbasebackup"/>).
    </para>
    <para>
  <orderedlist>
   <listitem>
    <para>
     Ensure that WAL archiving is enabled and working.
    </para>
   </listitem>
   <listitem>
    <para>
     Connect to the server (it does not matter which database) as a user with
     rights to run <function>pg_backup_start</function> (superuser,
     or a user who has been granted <literal>EXECUTE</literal> on the
     function) and issue the command:
<programlisting>
SELECT pg_backup_start(label => 'label', fast => false);
</programlisting>
     where <literal>label</literal> is any string you want to use to uniquely
     identify this backup operation. The connection
     calling <function>pg_backup_start</function> must be maintained until the end of
     the backup, or the backup will be automatically aborted.
    </para>

    <para>
     Online backups are always started at the beginning of a checkpoint.
     By default, <function>pg_backup_start</function> will wait for the next
     regularly scheduled checkpoint to complete, which may take a long time (see the
     configuration parameters <xref linkend="guc-checkpoint-timeout"/> and
     <xref linkend="guc-checkpoint-completion-target"/>).  This is
     usually preferable as it minimizes the impact on the running system.  If you
     want to start the backup as soon as possible, pass <literal>true</literal> as
     the second parameter to <function>pg_backup_start</function> and it will
     request an immediate checkpoint, which will finish as fast as possible using
     as much I/O as possible.
    </para>

   </listitem>
   <listitem>
    <para>
     Perform the backup, using any convenient file-system-backup tool
     such as <application>tar</application> or <application>cpio</application> (not
     <application>pg_dump</application> or
     <application>pg_dumpall</application>).  It is neither
     necessary nor desirable to stop normal operation of the database
     while you do this. See
     <xref linkend="backup-lowlevel-base-backup-data"/> for things to
     consider during this backup.
    </para>
   </listitem>
   <listitem>
    <para>
     In the same connection as before, issue the command:
<programlisting>
SELECT * FROM pg_backup_stop(wait_for_archive => true);
</programlisting>
     This terminates backup mode. On a primary, it also performs an automatic
     switch to the next WAL segment.  On a standby, it is not possible to
     automatically switch WAL segments, so you may wish to run
     <function>pg_switch_wal</function> on the primary to perform a manual
     switch. The reason for the switch is to arrange for
     the last WAL segment file written during the backup interval to be
     ready to archive.
    </para>
    <para>
     <function>pg_backup_stop</function> will return one row with three
     values. The second of these fields should be written to a file named
     <filename>backup_label</filename> in the root directory of the backup. The
     third field should be written to a file named
     <filename>tablespace_map</filename> unless the field is empty. These files are
     vital to the backup working and must be written byte for byte without
     modification, which may require opening the file in binary mode.
    </para>
   </listitem>
   <listitem>
    <para>
     Once the WAL segment files active during the backup are archived, you are
     done.  The file identified by <function>pg_backup_stop</function>'s first return
     value is the last segment that is required to form a complete set of
     backup files.  On a primary, if <varname>archive_mode</varname> is enabled and the
     <literal>wait_for_archive</literal> parameter is <literal>true</literal>,
     <function>pg_backup_stop</function> does not return until the last segment has
     been archived.
     On a standby, <varname>archive_mode</varname> must be <literal>always</literal> in order
     for <function>pg_backup_stop</function> to wait.
     Archiving of these files happens automatically since you have
     already configured <varname>archive_command</varname> or <varname>archive_library</varname>.
     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 or library, 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_backup_stop</function>, set an appropriate
     <varname>statement_timeout</varname> value, but make note that if
     <function>pg_backup_stop</function> terminates because of this your backup
     may not be valid.
    </para>
    <para>
     If the backup process monitors and ensures that all WAL segment files
     required for the backup are successfully archived then the
     <literal>wait_for_archive</literal> parameter (which defaults to true) can be set
     to false to have
     <function>pg_backup_stop</function> return as soon as the stop backup record is
     written to the WAL.  By default, <function>pg_backup_stop</function> will wait
     until all WAL has been archived, which can take some time.  This option
     must be used with caution: if WAL archiving is not monitored correctly
     then the backup might not include all of the WAL files and will
     therefore be incomplete and not able to be restored.
    </para>
   </listitem>
  </orderedlist>
    </para>
   <sect3 id="backup-lowlevel-base-backup-data">
   <title>Backing Up the Data Directory</title>
   <para>
    Some file system backup tools emit warnings or errors
    if the files they are trying to copy change while the copy proceeds.
    When taking a base backup of an active database, this situation is normal
    and not an error.  However, you need to ensure that you can distinguish
    complaints of this sort from real errors.  For example, some versions
    of <application>rsync</application> return a separate exit code for
    <quote>vanished source files</quote>, and you can write a driver script to
    accept this exit code as a non-error case.  Also, some versions of
    GNU <application>tar</application> return an error code indistinguishable from
    a fatal error if a file was truncated while <application>tar</application> was
    copying it.  Fortunately, GNU <application>tar</application> versions 1.16 and
    later exit with 1 if a file was changed during the backup,
    and 2 for other errors.  With GNU <application>tar</application> version 1.23 and
    later, you can use the warning options <literal>--warning=no-file-changed
    --warning=no-file-removed</literal> to hide the related warning messages.
   </para>

   <para>
    Be certain that your backup includes all of the files under
    the database cluster directory (e.g., <filename>/usr/local/pgsql/data</filename>).
    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
    archives symbolic links as links, otherwise the restore will corrupt
    your tablespaces).
   </para>

   <para>
    You should, however, omit from the backup the files within the
    cluster's <filename>pg_wal/</filename> subdirectory.  This
    slight adjustment is worthwhile because it reduces the risk
    of mistakes when restoring.  This is easy to arrange if
    <filename>pg_wal/</filename> is a symbolic link pointing to someplace outside
    the cluster directory, which is a common setup anyway for performance
    reasons.  You might also want to exclude <filename>postmaster.pid</filename>
    and <filename>postmaster.opts</filename>, which record information
    about the running <application>postmaster</application>, not about the
    <application>postmaster</application> which will eventually use this backup.
    (These files can confuse <application>pg_ctl</application>.)
   </para>

   <para>
    It is often a good idea to also omit from the backup the files
    within the cluster's <filename>pg_replslot/</filename> directory, so that
    replication slots that exist on the primary do not become part of the
    backup.  Otherwise, the subsequent use of the backup to create a standby
    may result in indefinite retention of WAL files on the standby, and
    possibly bloat on the primary if hot standby feedback is enabled, because
    the clients that are using those replication slots will still be connecting
    to and updating the slots on the primary, not the standby.  Even if the
    backup is only intended for use in creating a new primary, copying the
    replication slots isn't expected to be particularly useful, since the
    contents of those slots will likely be badly out of date by the time
    the new primary comes on line.
   </para>

   <para>
    The contents of the directories <filename>pg_dynshmem/</filename>,
    <filename>pg_notify/</filename>, <filename>pg_serial/</filename>,
    <filename>pg_snapshots/</filename>, <filename>pg_stat_tmp/</filename>,
    and <filename>pg_subtrans/</filename> (but not the directories themselves) can be
    omitted from the backup as they will be initialized on postmaster startup.
   </para>

   <para>
    Any file or directory beginning with <filename>pgsql_tmp</filename> can be
    omitted from the backup.  These files are removed on postmaster start and
    the directories will be recreated as needed.
   </para>

   <para>
    <filename>pg_internal.init</filename> files can be omitted from the
    backup whenever a file of that name is found.  These files contain
    relation cache data that is always rebuilt when recovering.
   </para>

   <para>
    The backup label
    file includes the label string you gave to <function>pg_backup_start</function>,
    as well as the time at which <function>pg_backup_start</function> was run, and
    the name of the starting WAL file.  In case of confusion it is therefore
    possible to look inside a backup file and determine exactly which
    backup session the dump file came from.  The tablespace map file includes
    the symbolic link names as they exist in the directory
    <filename>pg_tblspc/</filename> and the full path of each symbolic link.
    These files are not merely for your information; their presence and
    contents are critical to the proper operation of the system's recovery
    process.
   </para>

   <para>
    It is also possible to make a backup while the server is
    stopped.  In this case, you obviously cannot use
    <function>pg_backup_start</function> or <function>pg_backup_stop</function>, and
    you will therefore be left to your own devices to keep track of which
    backup is which and how far back the associated WAL files go.
    It is generally better to follow the continuous archiving procedure above.
   </para>
   </sect3>
  </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 should at least save the contents of the cluster's <filename>pg_wal</filename>
     subdirectory, as it might contain WAL files which
     were not archived before the system went down.
    </para>
   </listitem>
   <listitem>
    <para>
     Remove all existing files and subdirectories under the cluster data
     directory and under the root directories of any tablespaces you are using.
    </para>
   </listitem>
   <listitem>
    <para>
     If you're restoring a full backup, you can restore the database files
     directly into the target directories.  Be sure that they
     are restored with the right ownership (the database system user, not
     <literal>root</literal>!) and with the right permissions.  If you are using
     tablespaces,
     you should verify that the symbolic links in <filename>pg_tblspc/</filename>
     were correctly restored.
    </para>
   </listitem>
   <listitem>
    <para>
     If you're restoring an incremental backup, you'll need to restore the
     incremental backup and all earlier backups upon which it directly or
     indirectly depends to the machine where you are performing the restore.
     These backups will need to be placed in separate directories, not the
     target directories where you want the running server to end up.
     Once this is done, use <xref linkend="app-pgcombinebackup"/> to pull
     data from the full backup and all of the subsequent incremental backups
     and write out a synthetic full backup to the target directories. As above,
     verify that permissions and tablespace links are correct.
    </para>
   </listitem>
   <listitem>
    <para>
     Remove any files present in <filename>pg_wal/</filename>; these came from the
     file system backup and are therefore probably obsolete rather than current.
     If you didn't archive <filename>pg_wal/</filename> at all, then recreate
     it with proper permissions,
     being careful to ensure that you re-establish it as a symbolic link
     if you had it set up that way before.
    </para>
   </listitem>
   <listitem>
    <para>
     If you have unarchived WAL segment files that you saved in step 2,
     copy them into <filename>pg_wal/</filename>.  (It is best to copy them,
     not move them, so you still have the unmodified files if a
     problem occurs and you have to start over.)
    </para>
   </listitem>
   <listitem>
    <para>
     Set recovery configuration settings in
     <filename>postgresql.conf</filename> (see <xref
     linkend="runtime-config-wal-archive-recovery"/>) and create a file
     <filename>recovery.signal</filename> in the cluster
     data directory. You might
     also want to temporarily modify <filename>pg_hba.conf</filename> to prevent
     ordinary users from connecting until you are sure the recovery was successful.
    </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 remove
     <filename>recovery.signal</filename> (to prevent
     accidentally re-entering recovery mode later) and then
     commence normal database operations.
    </para>
   </listitem>
   <listitem>
    <para>
     Inspect the contents of the database to ensure you have recovered to
     the desired state.  If not, return to step 1.  If all is well,
     allow your users to connect by restoring <filename>pg_hba.conf</filename> to normal.
    </para>
   </listitem>
  </orderedlist>
   </para>

   <para>
    The key part of all this is to set up a recovery configuration that
    describes how you want to recover and how far the recovery should
    run.  The one thing that you absolutely must specify is the <varname>restore_command</varname>,
    which tells <productname>PostgreSQL</productname> how to retrieve archived
    WAL file segments.  Like the <varname>archive_command</varname>, this is
    a shell command string.  It can contain <literal>%f</literal>, which is
    replaced by the name of the desired WAL file, and <literal>%p</literal>,
    which is replaced by the path name to copy the WAL file to.
    (The path name is relative to the current working directory,
    i.e., the cluster's data directory.)
    Write <literal>%%</literal> if you need to embed an actual <literal>%</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</filename>.  Of course, you can use something
    much more complicated, perhaps even a shell script that requests the
    operator to mount an appropriate tape.
   </para>

   <para>
    It is important that the command return nonzero exit status on failure.
    The command <emphasis>will</emphasis> be called requesting files that are not
    present in the archive; it must return nonzero when so asked.  This is not
    an error condition.  An exception is that if the command was terminated by
    a signal (other than <systemitem>SIGTERM</systemitem>, which is used as
    part of a database server shutdown) or an error by the shell (such as
    command not found), then recovery will abort and the server will not start
    up.
   </para>

   <para>
    Not all of the requested files will be WAL segment
    files; you should also expect requests for files with a suffix of
    <literal>.history</literal>. Also be aware that
    the base name of the <literal>%p</literal> path will be different from
    <literal>%f</literal>; do not expect them to be interchangeable.
   </para>

   <para>
    WAL segments that cannot be found in the archive will be sought in
    <filename>pg_wal/</filename>; 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_wal/</filename>.
   </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 possible given the available WAL segments).  Therefore, a normal
    recovery will end with a <quote>file not found</quote> message, the exact text
    of the error message depending upon your choice of
    <varname>restore_command</varname>.  You may also see an error message
    at the start of recovery for a file named something like
    <filename>00000001.history</filename>.  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 <link linkend="runtime-config-wal-recovery-target">stopping point</link>.  You can specify
    the stop point, known as the <quote>recovery target</quote>, either by
    date/time, named restore point or by completion of a specific transaction
    ID.  As of this writing only the date/time and named restore point options
    are 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_backup_stop</function>.  You cannot use a base backup
      to recover to a time when that backup was in progress.  (To
      recover to such a time, you must go back to your previous base backup
      and roll forward from there.)
     </para>
   </note>

   <para>
    If recovery finds corrupted WAL data, recovery will
    halt at that point and the server will not start. In such a case the
    recovery process could be re-run from the beginning, specifying a
    <quote>recovery target</quote> 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</filename> file to indicate that the already-processed
    WAL data need not be scanned again.
   </para>

  </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.  For example, in the original history of the database,
    suppose you dropped a critical table at 5:15PM on Tuesday evening, but
    didn't realize your mistake until Wednesday noon.
    Unfazed, you get out your backup, restore to the point-in-time 5:14PM
    Tuesday evening, and are up and running.  In <emphasis>this</emphasis> history of
    the database universe, you never dropped the table.  But suppose
    you later realize this wasn't such a great idea, and would like
    to return to sometime Wednesday morning in the original history.
    You won't be able
    to if, while your database was up-and-running, it overwrote some of the
    WAL segment files that led up to the time you now wish you
    could get back to.  Thus, to avoid this, you need to distinguish the series of
    WAL records generated after you've done a point-in-time recovery from
    those that were generated in the original database history.
   </para>

   <para>
    To deal with this problem, <productname>PostgreSQL</productname> has a notion
    of <firstterm>timelines</firstterm>.  Whenever an archive recovery completes,
    a new timeline is created to identify the series of WAL records
    generated after that recovery.  The timeline
    ID number is part of WAL segment file names so a new timeline does
    not overwrite the WAL data generated by previous timelines.
    For example, in the WAL file name
    <filename>0000000100001234000055CD</filename>, the leading
    <literal>00000001</literal> is the timeline ID in hexadecimal.  (Note that
    in other contexts, such as server log messages, timeline IDs are
    usually printed in decimal.)
   </para>

   <para>
    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</emphasis> prior state, including
    states in timeline branches that you abandoned earlier.
   </para>

   <para>
    Every time a new timeline is created, <productname>PostgreSQL</productname> creates
    a <quote>timeline history</quote> 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 record your own notes about
    how and why this particular timeline was created.  Such comments will be
    especially valuable when you have a thicket of different timelines as
    a result of experimentation.
   </para>

   <para>
    The default behavior of recovery is to recover to the latest timeline found
    in the archive. If you wish to recover to the timeline that was current
    when the base backup was taken or into a specific child timeline (that
    is, you want to return to some state that was itself generated after a
    recovery attempt), you need to specify <literal>current</literal> or the
    target timeline ID in <xref linkend="guc-recovery-target-timeline"/>. 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</productname>'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</application> dumps.  (They are also much larger
      than <application>pg_dump</application> dumps, so in some cases the speed advantage
      might be negated.)
     </para>

     <para>
      As with base backups, the easiest way to produce a standalone
      hot backup is to use the <xref linkend="app-pgbasebackup"/>
      tool. If you include the <literal>-X</literal> parameter when calling
      it, all the write-ahead log required to use the backup will be
      included in the backup automatically, and no special action is
      required to restore the backup.
     </para>
    </sect3>

    <sect3 id="compressed-archive-logs">
     <title>Compressed Archive Logs</title>

     <para>
      If archive storage size is a concern, you can use
      <application>gzip</application> to compress the archive files:
<programlisting>
archive_command = 'gzip &lt; %p &gt; /mnt/server/archivedir/%f.gz'
</programlisting>
      You will then need to use <application>gunzip</application> during recovery:
<programlisting>
restore_command = 'gunzip &lt; /mnt/server/archivedir/%f.gz &gt; %p'
</programlisting>
     </para>
    </sect3>

    <sect3 id="backup-scripts">
     <title><varname>archive_command</varname> Scripts</title>

     <para>
      Many people choose to use scripts to define their
      <varname>archive_command</varname>, so that their
      <filename>postgresql.conf</filename> entry looks very simple:
<programlisting>
archive_command = 'local_backup_script.sh "%p" "%f"'
</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</application> or <application>perl</application>.
     </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>

     <tip>
      <para>
       When using an <varname>archive_command</varname> script, it's desirable
       to enable <xref linkend="guc-logging-collector"/>.
       Any messages written to <systemitem>stderr</systemitem> from the script will then
       appear in the database server log, allowing complex configurations to
       be diagnosed easily if they fail.
      </para>
     </tip>
    </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>
     If a <link linkend="sql-createdatabase"><command>CREATE DATABASE</command></link>
     command is executed while a base backup is being taken, and then
     the template database that the <command>CREATE DATABASE</command> 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>
     <link linkend="sql-createtablespace"><command>CREATE TABLESPACE</command></link>
     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 WAL is being
     replayed on a different machine.  It can be dangerous even if the
     WAL 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 WAL files 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 WAL 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</varname> 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>

</chapter>