Editorial improvements to backup and warm-standby documentation.

[postgresql] / doc / src / sgml / backup.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.95 2006/12/01 03:29:15 tgl Exp $ -->

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

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

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

 <para>
  There are three fundamentally different approaches to backing up
  <productname>PostgreSQL</> data:
  <itemizedlist>
   <listitem><para><acronym>SQL</> 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.
 </para>

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

  <para>
   The idea behind this dump method is to generate a text file with SQL
   commands that, when fed back to the server, will recreate the
   database in the same state as it was at the time of the dump.
   <productname>PostgreSQL</> provides the utility program
   <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">outfile</replaceable>
</synopsis>
   As you see, <application>pg_dump</> writes its results to the
   standard output. We will see below how this can be useful.
  </para>

  <para>
   <application>pg_dump</> is a regular <productname>PostgreSQL</>
   client application (albeit a particularly clever one). This means
   that you can do this backup procedure from any remote host that has
   access to the database. But remember that <application>pg_dump</>
   does not operate with special permissions. In particular, it must
   have read access to all tables that you want to back up, so in
   practice you almost always have to run it as a database superuser.
  </para>

  <para>
   To specify which database server <application>pg_dump</> should
   contact, use the command line options <option>-h
   <replaceable>host</></> and <option>-p <replaceable>port</></>. 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>
   As any other <productname>PostgreSQL</> client application,
   <application>pg_dump</> will by default connect with the database
   user name that is equal to the current operating system user name. To override
   this, either specify the <option>-U</option> option or set the
   environment variable <envar>PGUSER</envar>. Remember that
   <application>pg_dump</> connections are subject to the normal
   client authentication mechanisms (which are described in <xref
   linkend="client-authentication">).
  </para>

  <para>
   Dumps created by <application>pg_dump</> are internally consistent,
   that is, updates to the database while <application>pg_dump</> is
   running will not be in the dump. <application>pg_dump</> does not
   block other operations on the database while it is working.
   (Exceptions are those operations that need to operate with an
   exclusive lock, such as <command>VACUUM FULL</command>.)
  </para>

  <important>
   <para>
    If your database schema relies on OIDs (for instance as foreign
    keys) you must instruct <application>pg_dump</> to dump the OIDs
    as well. To do this, use the <option>-o</option> command line
    option.
   </para>
  </important>

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

   <para>
    The text files created by <application>pg_dump</> 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">infile</replaceable>
</synopsis>
    where <replaceable class="parameter">infile</replaceable> is what
    you used as <replaceable class="parameter">outfile</replaceable>
    for the <application>pg_dump</> command. The database <replaceable
    class="parameter">dbname</replaceable> will not be created by this
    command, so you must create it yourself from <literal>template0</>
    before executing <application>psql</> (e.g., with
    <literal>createdb -T template0 <replaceable
    class="parameter">dbname</></literal>).  <application>psql</>
    supports similar options to <application>pg_dump</> for specifying
    the database server to connect to and the user name to use. See
    the <xref linkend="app-psql"> reference page for more information.
   </para>

   <para>
    Before restoring a 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, then the restore will fail to recreate the
    objects with the original ownership and/or permissions.
    (Sometimes this is what you want, but usually it is not.)
   </para>

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

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

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

   <para>
    After restoring a backup, it is wise to run <xref
    linkend="sql-analyze" endterm="sql-analyze-title"> on each
    database so the query optimizer has useful statistics. An easy way
    to do this is to run <command>vacuumdb -a -z</>; this is
    equivalent to running <command>VACUUM ANALYZE</> on each database
    manually.  For more advice on how to load large amounts of data
    into <productname>PostgreSQL</> efficiently, refer to <xref
    linkend="populate">.
   </para>
  </sect2>

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

   <para>
    <application>pg_dump</> dumps only a single database at a time,
    and it does not dump information about roles or tablespaces
    (because those are cluster-wide rather than per-database).
    To support convenient dumping of the entire contents of a database
    cluster, the <xref linkend="app-pg-dumpall"> program is provided.
    <application>pg_dumpall</> backs up each database in a given
    cluster, and also preserves cluster-wide data such as role and
    tablespace definitions. The basic usage of this command is:
<synopsis>
pg_dumpall &gt; <replaceable>outfile</>
</synopsis>
    The resulting dump can be restored with <application>psql</>:
<synopsis>
psql -f <replaceable class="parameter">infile</replaceable> postgres
</synopsis>
    (Actually, you can specify any existing database name to start from,
    but if you are reloading in an empty cluster then <literal>postgres</>
    should generally be used.)  It is always necessary to have
    database superuser access when restoring a <application>pg_dumpall</>
    dump, as that is required to restore the role and tablespace information.
    If you use tablespaces, be careful that the tablespace paths in the
    dump are appropriate for the new installation.
   </para>
  </sect2>

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

   <para>
    Since <productname>PostgreSQL</productname> allows tables larger
    than the maximum file size on your system, it can be problematic
    to dump such a table to a file, since the resulting file will likely
    be larger than the maximum size allowed by your system. Since
    <application>pg_dump</> can write to the standard output, you can
    use standard Unix tools to work around this possible problem.
   </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>
createdb <replaceable class="parameter">dbname</replaceable>
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</>.</title>
    <para>
     The <command>split</command> command
     allows you to split the output into pieces that are
     acceptable in size to the underlying file system. For example, to
     make chunks of 1 megabyte:

<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 1m - <replaceable class="parameter">filename</replaceable>
</programlisting>

     Reload with

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

   <formalpara>
    <title>Use the custom dump format.</title>
    <para>
     If <productname>PostgreSQL</productname> was built on a system with the
     <application>zlib</> 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</>, but
     instead must be restored with <application>pg_restore</>.
     See the <xref linkend="app-pgdump"> and <xref
     linkend="app-pgrestore"> reference pages for details.
    </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</> uses to store the data in the database. In
   <xref linkend="creating-cluster"> it is explained where these files
   are located, but you have probably found them already if you are
   interested in this method. You can use whatever method you prefer
   for doing usual file system backups, for example

<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</>
   method:

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

    <listitem>
     <para>
      If you have dug into the details of the file system layout of the
      database, you may be tempted to try to back up or restore only certain
      individual tables or databases from their respective files or
      directories. This will <emphasis>not</> work because the
      information contained in these files contains only half the
      truth. The other half is in the commit log files
      <filename>pg_clog/*</filename>, which contain the commit status of
      all transactions. A table file is only usable with this
      information. Of course it is also impossible to restore only a
      table and the associated <filename>pg_clog</filename> data
      because that would render all other tables in the database
      cluster useless.  So file system backups only work for complete
      restoration of an entire database cluster.
     </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</> 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 where the database server was not
   properly shut down; therefore, when you start the database server
   on the backed-up data, it will think the server had crashed
   and replay the WAL log.  This is not a problem, just be aware of
   it (and be sure to include the WAL files in your backup).
  </para>

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

  <para>
   Another option is to use <application>rsync</> to perform a file
   system backup.  This is done by first running <application>rsync</>
   while the database server is running, then shutting down the database
   server just long enough to do a second <application>rsync</>.  The
   second <application>rsync</> will be much 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 not necessarily be
   smaller than an SQL dump. On the contrary, it will most likely be
   larger. (<application>pg_dump</application> does not need to dump
   the contents of indexes for example, just the commands to recreate
   them.)
  </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</> maintains a
   <firstterm>write ahead log</> (WAL) in the <filename>pg_xlog/</>
   subdirectory of the cluster's data directory. The log describes
   every change made to the database's data files.  This log exists
   primarily for crash-safety purposes: if the system crashes, the
   database can be restored to consistency by <quote>replaying</> the
   log entries made since the last checkpoint.  However, the existence
   of the log makes it possible to use a third strategy for backing up
   databases: we can combine a file-system-level backup with backup of
   the WAL files.  If recovery is needed, we restore the backup and
   then replay from the backed-up WAL files to bring the backup up to
   current time.  This approach is more complex to administer than
   either of the previous approaches, but it has some significant
   benefits:
  <itemizedlist>
   <listitem>
    <para>
     We do not need a perfectly consistent backup as the starting point.
     Any internal inconsistency in the backup will be corrected by log
     replay (this is not significantly different from what happens during
     crash recovery).  So we don't need file system snapshot capability,
     just <application>tar</> or a similar archiving tool.
    </para>
   </listitem>
   <listitem>
    <para>
     Since we can string together an indefinitely long sequence of WAL files
     for replay, continuous backup can be achieved simply by continuing to archive
     the WAL files.  This is particularly valuable for large databases, where
     it may not be convenient to take a full backup frequently.
    </para>
   </listitem>
   <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 may 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 "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 setup 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 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 want 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 &mdash; it's all up to you.
   </para>

   <para>
    The shell command to use is specified by the <xref
    linkend="guc-archive-command"> configuration parameter, which in practice
    will always be placed in the <filename>postgresql.conf</filename> file.
    In this string,
    any <literal>%p</> is replaced by the 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 working directory of the server,
    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 &lt;/dev/null'
</programlisting>
    which will copy archivable WAL segments to the directory
    <filename>/mnt/server/archivedir</>.  (This is an example, not a 
    recommendation, and may not work on all platforms.)
   </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 WAL segment file has been
    successfully archived, and will remove or recycle it.
    However, a nonzero status tells
    <productname>PostgreSQL</> that the file was not archived; it will try
    again periodically until it succeeds.
   </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 &amp;&amp; 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 relatively quickly. The <filename>pg_xlog/</> directory will
    continue to fill with WAL segment files until the situation is resolved.
   </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 may be up to 64 characters long and may 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 may 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</> &mdash; 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 immediately.  Other utility
    functions related to WAL management are listed in <xref
    linkend="functions-admin-backup-table">.
   </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>
   </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 needs to be archived to complete the backup.  
     Archival of these files will happen automatically, since you have
     already configured <varname>archive_command</>. In many cases, this
     happens fairly quickly, but you are advised to monitor your archival
     system to ensure this has taken place so that you can be certain you
     have a complete backup.  
    </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</> consider it an error if a file
    is changed while <application>tar</> is copying it.  There does not seem
    to be any very convenient way to distinguish this error from other types
    of errors, other than manual inspection of <application>tar</>'s messages.
    GNU <application>tar</> is therefore not the best tool for making base
    backups.
   </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 may 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 may, however, omit from the backup dump the files within the
    <filename>pg_xlog/</> subdirectory of the cluster directory.  This
    slight complication is worthwhile because it reduces the risk
    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
    number in the file name stands for an exact position within the WAL
    file, and can ordinarily be ignored.) Once you have safely archived
    the file system backup and the WAL segment files used during the
    backup (as specified in the backup history file), all archived WAL
    segments with names numerically less are no longer needed to recover
    the file system backup and may be deleted. However, you should
    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 &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>

   <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 may 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 backup dump.  Be careful that they
     are restored with the right ownership (the database system user, not
     root!) and with the right permissions.  If you are using tablespaces,
     you 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,
     and 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 may 
     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 setup 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 may 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 working directory of the server,
    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 log files that are not present
    in the archive; it must return nonzero when so asked.  This is not an
    error condition.  Be aware also that the base name of the <literal>%p</>
    path will be different from <literal>%f</>; do not expect them to be
    interchangeable.
   </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).  But if you want
    to recover to some previous point in time (say, right before the junior
    DBA dropped your main transaction table), just specify the required
    stopping point in <filename>recovery.conf</>.  You can specify the stop
    point, known as the <quote>recovery target</>, either by date/time or
    by completion of a specific transaction ID.  As of this writing only
    the date/time option is very usable, since there are no tools to help
    you identify with any accuracy which transaction ID to use.
   </para>

   <note>
     <para>
      The stop point must be after the ending time of the base backup (the
      time of <function>pg_stop_backup</>).  You cannot use a base backup
      to recover to a time when that backup was still going on.  (To
      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 working directory of the server,
        i.e., the cluster's data directory.)
        Write <literal>%%</> to embed an actual <literal>%</> character
        in the command. 
       </para>
       <para>
        It is important for the command to return a zero exit status if and
        only if it succeeds.  The command <emphasis>will</> be asked for file
        names that are not present in the archive; it must return nonzero
        when so asked.  Examples:
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f "%p"'
restore_command = 'copy /mnt/server/archivedir/%f "%p"'  # Windows
</programlisting>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-time" xreflabel="recovery_target_time">
      <term><varname>recovery_target_time</varname> 
           (<type>timestamp</type>)
      </term>
      <listitem>
       <para>
        This parameter specifies the time stamp up to which recovery
        will proceed.
        At most one of <varname>recovery_target_time</> and
        <xref linkend="recovery-target-xid"> can be specified.
        The default is to recover to the end of the WAL log.
        The precise stopping point is also influenced by 
        <xref linkend="recovery-target-inclusive">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-xid" xreflabel="recovery_target_xid">
      <term><varname>recovery_target_xid</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        This parameter specifies the transaction ID up to which recovery
        will proceed. Keep in mind 
        that while transaction IDs are assigned sequentially at transaction 
        start, transactions can complete in a different numeric order.
        The transactions that will be recovered are those that committed
        before (and optionally including) the specified one.
        At most one of <varname>recovery_target_xid</> and
        <xref linkend="recovery-target-time"> can be specified.
        The default is to recover to the end of the WAL log.
        The precise stopping point is also influenced by 
        <xref linkend="recovery-target-inclusive">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-inclusive" 
                   xreflabel="recovery_target_inclusive">
      <term><varname>recovery_target_inclusive</varname> 
        (<type>boolean</type>)
      </term>
      <listitem>
       <para>
        Specifies whether we stop just after the specified recovery target
        (<literal>true</literal>), or just before the recovery target 
        (<literal>false</literal>).
        Applies to both <xref linkend="recovery-target-time">
        and <xref linkend="recovery-target-xid">, whichever one is
        specified for this recovery.  This indicates whether transactions
        having exactly the target commit time or ID, respectively, will
        be included in the recovery.  Default is <literal>true</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-timeline" 
                   xreflabel="recovery_target_timeline">
      <term><varname>recovery_target_timeline</varname> 
        (<type>string</type>)
      </term>
      <listitem>
       <para>
        Specifies recovering into a particular timeline.  The default is
        to recover along the same timeline that was current when the
        base backup was taken.  You would only need to set this parameter
        in complex re-recovery situations, where you need to return to
        a state that itself was reached after a point-in-time recovery.
        See <xref linkend="backup-timelines"> for discussion.
       </para>
      </listitem>
     </varlistentry>

   </variablelist>

   </sect3>

  </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.
    Unfazed, you get out your backup, restore to the point-in-time 5:14PM
    Tuesday evening, and are up and running.  In <emphasis>this</> history of
    the database universe, you never dropped the table at all.  But suppose
    you later realize this wasn't such a great idea after all, and would like
    to return to some later point in the original history.  You won't be able
    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</>.  Each time you recover to a point-in-time
    earlier than the end of the WAL sequence, a new timeline is created
    to identify the series of WAL records generated after that recovery.
    (If recovery proceeds all the way to the end of WAL, however, we do not
    start a new timeline: we just extend the existing one.)  The timeline
    ID number is part of WAL segment file names, and so a new timeline does
    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="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 may need to fix partially-written disk pages.  Depending on
    your system hardware and software, the risk of partial writes may
    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 may 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 or "log" 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 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
   really offers a solution for disaster recovery, not HA.
  </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 &mdash; 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 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 &mdash; 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.
   </para>

   <para>
    The magic that makes the two loosely coupled servers work together
    is simply a <varname>restore_command</> used on the standby that waits for
    the next WAL file 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 file to be
    unavailable, so we must be patient and wait for it to appear. 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>
    <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>
  </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-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 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 can 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 avoid
    some problems of inappropriate failover, but the additional complexity
    may 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 may 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 encouraged, since it allows regular downtime on
    each system for maintenance. This also acts 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 &mdash;
    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 files while it is following
    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.
   </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 7.2.1, 7.3.2, and 7.4 are
   not compatible, whereas 7.2.1 and 7.2.2 are. When you update
   between compatible versions, you can simply replace the executables
   and reuse the data directory on disk. Otherwise you need to back
   up your data and restore it on the new server.  This has to be done
   using <application>pg_dump</>; file system level backup methods
   obviously won't work. There are checks in place that prevent you
   from using a data directory with an incompatible version of
   <productname>PostgreSQL</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 may 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 run <application>pg_dumpall</>,
   otherwise you will obviously lose that data. See <xref
   linkend="client-authentication"> for information on how to prohibit
   access.
  </para>

  <para>
   In practice you probably want to test your client
   applications on the new setup before switching over completely.
   This is another reason for setting up concurrent installations
   of old and new versions.
  </para>

  <para>
   If you cannot or do not want to run two servers in parallel you can
   do the backup step before installing the new version, bring down
   the server, move the old version out of the way, install the new
   version, start the new server, restore the data. For example:

<programlisting>
pg_dumpall &gt; 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 may 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, but you
    need to be wary of moving older installations.)
   </para>
  </note>
 </sect1>
</chapter>