postgres/doc/src/sgml/ref/pg_rewind.sgml

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doc/src/sgml/ref/pg_rewind.sgml
PostgreSQL documentation
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<refentry id="app-pgrewind">
 <indexterm zone="app-pgrewind">
  <primary>pg_rewind</primary>
 </indexterm>

 <refmeta>
  <refentrytitle><application>pg_rewind</application></refentrytitle>
  <manvolnum>1</manvolnum>
  <refmiscinfo>Application</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>pg_rewind</refname>
  <refpurpose>synchronize a <productname>PostgreSQL</productname> data directory with another data directory that was forked from it</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
  <cmdsynopsis>
   <command>pg_rewind</command>
   <arg rep="repeat"><replaceable>option</replaceable></arg>
   <group choice="plain">
    <group choice="req">
     <arg choice="plain"><option>-D </option></arg>
     <arg choice="plain"><option>--target-pgdata</option></arg>
    </group>
    <replaceable> directory</replaceable>
    <group choice="req">
     <arg choice="plain"><option>--source-pgdata=<replaceable>directory</replaceable></option></arg>
     <arg choice="plain"><option>--source-server=<replaceable>connstr</replaceable></option></arg>
    </group>
   </group>
  </cmdsynopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <application>pg_rewind</application> is a tool for synchronizing a PostgreSQL cluster
   with another copy of the same cluster, after the clusters' timelines have
   diverged. A typical scenario is to bring an old master server back online
   after failover as a standby that follows the new master.
  </para>

  <para>
   After a successful rewind, the state of the target data directory is
   analogous to a base backup of the source data directory. Unlike taking
   a new base backup or using a tool like <application>rsync</application>,
   <application>pg_rewind</application> does not require comparing or copying
   unchanged relation blocks in the cluster. Only changed blocks from existing
   relation files are copied; all other files, including new relation files,
   configuration files, and WAL segments, are copied in full. As such the
   rewind operation is significantly faster than other approaches when the
   database is large and only a small fraction of blocks differ between the
   clusters.
  </para>

  <para>
   <application>pg_rewind</application> examines the timeline histories of the source
   and target clusters to determine the point where they diverged, and
   expects to find WAL in the target cluster's <filename>pg_wal</filename> directory
   reaching all the way back to the point of divergence. The point of divergence
   can be found either on the target timeline, the source timeline, or their common
   ancestor. In the typical failover scenario where the target cluster was
   shut down soon after the divergence, this is not a problem, but if the
   target cluster ran for a long time after the divergence, its old WAL
   files might no longer be present. In this case, you can manually copy them
   from the WAL archive to the <filename>pg_wal</filename> directory, or run
   <application>pg_rewind</application> with the <literal>-c</literal> option to
   automatically retrieve them from the WAL archive. The use of
   <application>pg_rewind</application> is not limited to failover, e.g.,  a standby
   server can be promoted, run some write transactions, and then rewound
   to become a standby again.
  </para>

  <para>
   After running <application>pg_rewind</application>, WAL replay needs to
   complete for the data directory to be in a consistent state. When the
   target server is started again it will enter archive recovery and replay
   all WAL generated in the source server from the last checkpoint before
   the point of divergence. If some of the WAL was no longer available in the
   source server when <application>pg_rewind</application> was run, and
   therefore could not be copied by the <application>pg_rewind</application>
   session, it must be made available when the target server is started.
   This can be done by creating a <filename>recovery.signal</filename> file
   in the target data directory and by configuring a suitable
   <xref linkend="guc-restore-command"/> in
   <filename>postgresql.conf</filename>.
  </para>

  <para>
   <application>pg_rewind</application> requires that the target server either has
   the <xref linkend="guc-wal-log-hints"/> option enabled
   in <filename>postgresql.conf</filename> or data checksums enabled when
   the cluster was initialized with <application>initdb</application>.  Neither of these
   are currently on by default.  <xref linkend="guc-full-page-writes"/>
   must also be set to <literal>on</literal>, but is enabled by default.
  </para>

  <warning>
   <para>
    If <application>pg_rewind</application> fails while processing, then
    the data folder of the target is likely not in a state that can be
    recovered.  In such a case, taking a new fresh backup is recommended.
   </para>

   <para>
    As <application>pg_rewind</application> copies configuration files
    entirely from the source, it may be required to correct the configuration
    used for recovery before restarting the target server, especially if
    the target is reintroduced as a standby of the source. If you restart
    the server after the rewind operation has finished but without configuring
    recovery, the target may again diverge from the primary.
   </para>

   <para>
    <application>pg_rewind</application> will fail immediately if it finds
    files it cannot write directly to.  This can happen for example when
    the source and the target server use the same file mapping for read-only
    SSL keys and certificates.  If such files are present on the target server
    it is recommended to remove them before running
    <application>pg_rewind</application>.  After doing the rewind, some of
    those files may have been copied from the source, in which case it may
    be necessary to remove the data copied and restore back the set of links
    used before the rewind.
   </para>
  </warning>
 </refsect1>

 <refsect1>
  <title>Options</title>

   <para>
    <application>pg_rewind</application> accepts the following command-line
    arguments:

    <variablelist>
     <varlistentry>
      <term><option>-D <replaceable class="parameter">directory</replaceable></option></term>
      <term><option>--target-pgdata=<replaceable class="parameter">directory</replaceable></option></term>
      <listitem>
       <para>
        This option specifies the target data directory that is synchronized
        with the source. The target server must be shut down cleanly before
        running <application>pg_rewind</application>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--source-pgdata=<replaceable class="parameter">directory</replaceable></option></term>
      <listitem>
       <para>
        Specifies the file system path to the data directory of the source
        server to synchronize the target with. This option requires the
        source server to be cleanly shut down.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--source-server=<replaceable class="parameter">connstr</replaceable></option></term>
      <listitem>
       <para>
        Specifies a libpq connection string to connect to the source
        <productname>PostgreSQL</productname> server to synchronize the target
        with. The connection must be a normal (non-replication) connection
        with a role having sufficient permissions to execute the functions
        used by <application>pg_rewind</application> on the source server
        (see Notes section for details) or a superuser role.  This option
        requires the source server to be running and not in recovery mode.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-R</option></term>
      <term><option>--write-recovery-conf</option></term>
      <listitem>
       <para>
        Create <filename>standby.signal</filename> and append connection
        settings to <filename>postgresql.auto.conf</filename> in the output
        directory.  <literal>--source-server</literal> is mandatory with
        this option.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-n</option></term>
      <term><option>--dry-run</option></term>
      <listitem>
       <para>
        Do everything except actually modifying the target directory.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-N</option></term>
      <term><option>--no-sync</option></term>
      <listitem>
       <para>
        By default, <command>pg_rewind</command> will wait for all files
        to be written safely to disk.  This option causes
        <command>pg_rewind</command> to return without waiting, which is
        faster, but means that a subsequent operating system crash can leave
        the synchronized data directory corrupt.  Generally, this option is
        useful for testing but should not be used on a production
        installation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-P</option></term>
      <term><option>--progress</option></term>
      <listitem>
       <para>
        Enables progress reporting. Turning this on will deliver an approximate
        progress report while copying data from the source cluster.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-c</option></term>
      <term><option>--restore-target-wal</option></term>
      <listitem>
       <para>
        Use <varname>restore_command</varname> defined in the target cluster
        configuration to retrieve WAL files from the WAL archive if these
        files are no longer available in the <filename>pg_wal</filename>
        directory.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--debug</option></term>
      <listitem>
       <para>
        Print verbose debugging output that is mostly useful for developers
        debugging <application>pg_rewind</application>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--no-ensure-shutdown</option></term>
      <listitem>
       <para>
        <application>pg_rewind</application> requires that the target server
        is cleanly shut down before rewinding. By default, if the target server
        is not shut down cleanly, <application>pg_rewind</application> starts
        the target server in single-user mode to complete crash recovery first,
        and stops it.
        By passing this option, <application>pg_rewind</application> skips
        this and errors out immediately if the server is not cleanly shut
        down. Users are expected to handle the situation themselves in that
        case.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-V</option></term>
      <term><option>--version</option></term>
      <listitem><para>Display version information, then exit.</para></listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-?</option></term>
      <term><option>--help</option></term>
      <listitem><para>Show help, then exit.</para></listitem>
     </varlistentry>

    </variablelist>
   </para>
 </refsect1>

 <refsect1>
  <title>Environment</title>

  <para>
   When <option>--source-server</option> option is used,
   <application>pg_rewind</application> also uses the environment variables
   supported by <application>libpq</application> (see <xref linkend="libpq-envars"/>).
  </para>

  <para>
   The environment variable <envar>PG_COLOR</envar> specifies whether to use
   color in diagnostic messages. Possible values are
   <literal>always</literal>, <literal>auto</literal> and
   <literal>never</literal>.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   When executing <application>pg_rewind</application> using an online
   cluster as source, a role having sufficient permissions to execute the
   functions used by <application>pg_rewind</application> on the source
   cluster can be used instead of a superuser.  Here is how to create such
   a role, named <literal>rewind_user</literal> here:
<programlisting>
CREATE USER rewind_user LOGIN;
GRANT EXECUTE ON function pg_catalog.pg_ls_dir(text, boolean, boolean) TO rewind_user;
GRANT EXECUTE ON function pg_catalog.pg_stat_file(text, boolean) TO rewind_user;
GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text) TO rewind_user;
GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text, bigint, bigint, boolean) TO rewind_user;
</programlisting>
  </para>

  <para>
   When executing <application>pg_rewind</application> using an online
   cluster as source which has been recently promoted, it is necessary
   to execute a <command>CHECKPOINT</command> after promotion such that its
   control file reflects up-to-date timeline information, which is used by
   <application>pg_rewind</application> to check if the target cluster
   can be rewound using the designated source cluster.
  </para>

  <refsect2>
   <title>How It Works</title>

   <para>
    The basic idea is to copy all file system-level changes from the source
    cluster to the target cluster:
   </para>

   <procedure>
    <step>
     <para>
      Scan the WAL log of the target cluster, starting from the last
      checkpoint before the point where the source cluster's timeline
      history forked off from the target cluster. For each WAL record,
      record each data block that was touched. This yields a list of all
      the data blocks that were changed in the target cluster, after the
      source cluster forked off. If some of the WAL files are no longer
      available, try re-running <application>pg_rewind</application> with
      the <option>-c</option> option to search for the missing files in
      the WAL archive.
     </para>
    </step>
    <step>
     <para>
      Copy all those changed blocks from the source cluster to
      the target cluster, either using direct file system access
      (<option>--source-pgdata</option>) or SQL (<option>--source-server</option>).
      Relation files are now in a state equivalent to the moment of the last
      completed checkpoint prior to the point at which the WAL timelines of the
      source and target diverged plus the current state on the source of any
      blocks changed on the target after that divergence.
     </para>
    </step>
    <step>
     <para>
      Copy all other files, including new relation files, WAL segments,
      <filename>pg_xact</filename>, and configuration files from the source
      cluster to the target cluster. Similarly to base backups, the contents
      of the directories <filename>pg_dynshmem/</filename>,
      <filename>pg_notify/</filename>, <filename>pg_replslot/</filename>,
      <filename>pg_serial/</filename>, <filename>pg_snapshots/</filename>,
      <filename>pg_stat_tmp/</filename>, and <filename>pg_subtrans/</filename>
      are omitted from the data copied from the source cluster. The files
      <filename>backup_label</filename>,
      <filename>tablespace_map</filename>,
      <filename>pg_internal.init</filename>,
      <filename>postmaster.opts</filename>, and
      <filename>postmaster.pid</filename>, as well as any file or directory
      beginning with <filename>pgsql_tmp</filename>, are omitted.
     </para>
    </step>
    <step>
     <para>
      Create a <filename>backup_label</filename> file to begin WAL replay at
      the checkpoint created at failover and configure the
      <filename>pg_control</filename> file with a minimum consistency LSN
      defined as the result of <literal>pg_current_wal_insert_lsn()</literal>
      when rewinding from a live source or the last checkpoint LSN when
      rewinding from a stopped source.
     </para>
    </step>
    <step>
     <para>
      When starting the target, <productname>PostgreSQL</productname> replays
      all the required WAL, resulting in a data directory in a consistent
      state.
     </para>
    </step>
   </procedure>
  </refsect2>
 </refsect1>

</refentry>