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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 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</> 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. Only changed blocks from relation files are copied;
all other files are copied in full, 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 unchanged blocks in the cluster. This makes
it a lot faster when the database is large and only a small
fraction of blocks differ 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_wal</> 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, the old WAL
files might no longer be present. In that case, they can be manually
copied from the WAL archive to the <filename>pg_wal</> directory, or
fetched on startup by configuring <filename>recovery.conf</>. The use of
<application>pg_rewind</> is not limited to failover, e.g. a standby
server can be promoted, run some write transactions, and then rewinded
to become a standby again.
</para>
<para>
When the target server is started 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 the
<application>pg_rewind</> session, it must be made available when the
target server is started. This 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 enabled
in <filename>postgresql.conf</> or data checksums 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 set to <literal>on</>, but is enabled by 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 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</> server to synchronize the target with.
The connection must be a normal (non-replication) connection
with superuser access. This option requires the source
server to be running and not 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 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 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.
</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</>) or SQL (<option>--source-server</>).
</para>
</step>
<step>
<para>
Copy all other files such as <filename>pg_xact</filename> and
configuration files from the source cluster to the target cluster
(everything except the relation files).
</para>
</step>
<step>
<para>
Apply the WAL from the source 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 that
makes <productname>PostgreSQL</> start by replaying all WAL from
that checkpoint forward.)
</para>
</step>
</procedure>
</refsect2>
</refsect1>
</refentry>
|
