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<!--
doc/src/sgml/ref/pg_rewind.sgml
PostgreSQL documentation
-->
<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 the first one</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</> 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>
The result is equivalent to replacing the target data directory with the
source one. All files are copied, including configuration files. The
advantage of <application>pg_rewind</> over taking a new base backup, or
tools like <application>rsync</>, is that <application>pg_rewind</> does
not require reading through all unchanged files in the cluster. That makes
it a lot faster when the database is large and only a small portion of it
differs between the clusters.
</para>
<para>
<application>pg_rewind</> 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_xlog</> directory
reaching all the way back to the point of divergence. In the typical
failover scenario where the target cluster was shut down soon after the
divergence, that is not a problem, but if the target cluster had run for a
long time after the divergence, the old WAL files might not be present
anymore. In that case, they can be manually copied from the WAL archive to
the <filename>pg_xlog</> directory. Fetching missing files from a WAL
archive automatically is currently not supported.
</para>
<para>
When the target server is started up for the first time after running
<application>pg_rewind</>, it will go into recovery mode and replay all
WAL generated in the source server after the point of divergence.
If some of the WAL was no longer available in the source server when
<application>pg_rewind</> was run, and therefore could not be copied by
<application>pg_rewind</> session, it needs to be made available when the
target server is started up. That can be done by creating a
<filename>recovery.conf</> file in the target data directory with a
suitable <varname>restore_command</>.
</para>
<para>
<application>pg_rewind</> requires that the target server either has
the <xref linkend="guc-wal-log-hints"> option is enabled
in <filename>postgresql.conf</> or that data checksums were enabled when
the cluster was initialized with <application>initdb</>. Neither of these
are currently on by default.
<xref linkend="guc-full-page-writes"> must also be enabled. That is the
default.
</para>
</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 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 path to the data directory of the source server, to
synchronize the target with. When <option>--source-pgdata</> is
used, the source server must 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</> server to synchronize the target with.
The connection must be a normal (non-replication) connection
with superuser access. The server must be up and running,
and must not be in recovery mode.
</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>-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 over from the source cluster.
</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</>.
</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 is used,
<application>pg_rewind</application> also uses the environment variables
supported by <application>libpq</> (see <xref linkend="libpq-envars">).
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<refsect2>
<title>How it works</title>
<para>
The basic idea is to copy everything from the new cluster to the old
cluster, except for the blocks that we know to be the same.
</para>
<procedure>
<step>
<para>
Scan the WAL log of the old cluster, starting from the last checkpoint
before the point where the new cluster's timeline history forked off
from the old cluster. For each WAL record, make a note of the data
blocks that were touched. This yields a list of all the data blocks
that were changed in the old cluster, after the new cluster forked off.
</para>
</step>
<step>
<para>
Copy all those changed blocks from the new cluster to the old cluster.
</para>
</step>
<step>
<para>
Copy all other files such as <filename>clog</filename> and configuration files from the new cluster
to the old cluster, everything except the relation files.
</para>
</step>
<step>
<para>
Apply the WAL from the new cluster, starting from the checkpoint
created at failover. (Strictly speaking, <application>pg_rewind</>
doesn't apply the WAL, it just creates a backup label file indicating
that when <productname>PostgreSQL</> is started, it will start replay
from that checkpoint and apply all the required WAL.)
</para>
</step>
</procedure>
</refsect2>
</refsect1>
</refentry>
|
