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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>pg_rewind</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="app-pgresetwal.html" title="pg_resetwal" /><link rel="next" href="pgtestfsync.html" title="pg_test_fsync" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center"><span class="application">pg_rewind</span></th></tr><tr><td width="10%" align="left"><a accesskey="p" href="app-pgresetwal.html" title="pg_resetwal">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="reference-server.html" title="PostgreSQL Server Applications">Up</a></td><th width="60%" align="center">PostgreSQL Server Applications</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgtestfsync.html" title="pg_test_fsync">Next</a></td></tr></table><hr /></div><div class="refentry" id="APP-PGREWIND"><div class="titlepage"></div><a id="id-1.9.5.9.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle"><span class="application">pg_rewind</span></span></h2><p>pg_rewind — synchronize a <span class="productname">PostgreSQL</span> data directory with another data directory that was forked from it</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="cmdsynopsis"><p id="id-1.9.5.9.4.1"><code class="command">pg_rewind</code> [<em class="replaceable"><code>option</code></em>...] { <code class="option">-D</code> | <code class="option">--target-pgdata</code> }<em class="replaceable"><code> directory</code></em> { <code class="option">--source-pgdata=<em class="replaceable"><code>directory</code></em></code> | <code class="option">--source-server=<em class="replaceable"><code>connstr</code></em></code> } </p></div></div><div class="refsect1" id="id-1.9.5.9.5"><h2>Description</h2><p>
<span class="application">pg_rewind</span> 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 primary server back online
after failover as a standby that follows the new primary.
</p><p>
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 <span class="application">rsync</span>,
<span class="application">pg_rewind</span> 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.
</p><p>
<span class="application">pg_rewind</span> 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 <code class="filename">pg_wal</code> 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 <code class="filename">pg_wal</code> directory, or run
<span class="application">pg_rewind</span> with the <code class="literal">-c</code> option to
automatically retrieve them from the WAL archive. The use of
<span class="application">pg_rewind</span> 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.
</p><p>
After running <span class="application">pg_rewind</span>, 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 <span class="application">pg_rewind</span> was run, and
therefore could not be copied by the <span class="application">pg_rewind</span>
session, it must be made available when the target server is started.
This can be done by creating a <code class="filename">recovery.signal</code> file
in the target data directory and by configuring a suitable
<a class="xref" href="runtime-config-wal.html#GUC-RESTORE-COMMAND">restore_command</a> in
<code class="filename">postgresql.conf</code>.
</p><p>
<span class="application">pg_rewind</span> requires that the target server either has
the <a class="xref" href="runtime-config-wal.html#GUC-WAL-LOG-HINTS">wal_log_hints</a> option enabled
in <code class="filename">postgresql.conf</code> or data checksums enabled when
the cluster was initialized with <span class="application">initdb</span>. Neither of these
are currently on by default. <a class="xref" href="runtime-config-wal.html#GUC-FULL-PAGE-WRITES">full_page_writes</a>
must also be set to <code class="literal">on</code>, but is enabled by default.
</p><div class="warning"><h3 class="title">Warning</h3><p>
If <span class="application">pg_rewind</span> 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.
</p><p>
As <span class="application">pg_rewind</span> 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.
</p><p>
<span class="application">pg_rewind</span> 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
<span class="application">pg_rewind</span>. 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.
</p></div></div><div class="refsect1" id="id-1.9.5.9.6"><h2>Options</h2><p>
<span class="application">pg_rewind</span> accepts the following command-line
arguments:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="option">-D <em class="replaceable"><code>directory</code></em></code><br /></span><span class="term"><code class="option">--target-pgdata=<em class="replaceable"><code>directory</code></em></code></span></dt><dd><p>
This option specifies the target data directory that is synchronized
with the source. The target server must be shut down cleanly before
running <span class="application">pg_rewind</span>
</p></dd><dt><span class="term"><code class="option">--source-pgdata=<em class="replaceable"><code>directory</code></em></code></span></dt><dd><p>
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.
</p></dd><dt><span class="term"><code class="option">--source-server=<em class="replaceable"><code>connstr</code></em></code></span></dt><dd><p>
Specifies a libpq connection string to connect to the source
<span class="productname">PostgreSQL</span> 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 <span class="application">pg_rewind</span> on the source server
(see Notes section for details) or a superuser role. This option
requires the source server to be running and accepting connections.
</p></dd><dt><span class="term"><code class="option">-R</code><br /></span><span class="term"><code class="option">--write-recovery-conf</code></span></dt><dd><p>
Create <code class="filename">standby.signal</code> and append connection
settings to <code class="filename">postgresql.auto.conf</code> in the output
directory. <code class="literal">--source-server</code> is mandatory with
this option.
</p></dd><dt><span class="term"><code class="option">-n</code><br /></span><span class="term"><code class="option">--dry-run</code></span></dt><dd><p>
Do everything except actually modifying the target directory.
</p></dd><dt><span class="term"><code class="option">-N</code><br /></span><span class="term"><code class="option">--no-sync</code></span></dt><dd><p>
By default, <code class="command">pg_rewind</code> will wait for all files
to be written safely to disk. This option causes
<code class="command">pg_rewind</code> to return without waiting, which is
faster, but means that a subsequent operating system crash can leave
the data directory corrupt. Generally, this option is useful for
testing but should not be used on a production
installation.
</p></dd><dt><span class="term"><code class="option">-P</code><br /></span><span class="term"><code class="option">--progress</code></span></dt><dd><p>
Enables progress reporting. Turning this on will deliver an approximate
progress report while copying data from the source cluster.
</p></dd><dt><span class="term"><code class="option">-c</code><br /></span><span class="term"><code class="option">--restore-target-wal</code></span></dt><dd><p>
Use <code class="varname">restore_command</code> defined in the target cluster
configuration to retrieve WAL files from the WAL archive if these
files are no longer available in the <code class="filename">pg_wal</code>
directory.
</p></dd><dt><span class="term"><code class="option">--config-file=<em class="replaceable"><code>filename</code></em></code></span></dt><dd><p>
Use the specified main server configuration file for the target
cluster. This affects <span class="application">pg_rewind</span> when
it uses internally the <span class="application">postgres</span> command
for the rewind operation on this cluster (when retrieving
<code class="varname">restore_command</code> with the option
<code class="option">-c/--restore-target-wal</code> and when forcing a
completion of crash recovery).
</p></dd><dt><span class="term"><code class="option">--debug</code></span></dt><dd><p>
Print verbose debugging output that is mostly useful for developers
debugging <span class="application">pg_rewind</span>.
</p></dd><dt><span class="term"><code class="option">--no-ensure-shutdown</code></span></dt><dd><p>
<span class="application">pg_rewind</span> requires that the target server
is cleanly shut down before rewinding. By default, if the target server
is not shut down cleanly, <span class="application">pg_rewind</span> starts
the target server in single-user mode to complete crash recovery first,
and stops it.
By passing this option, <span class="application">pg_rewind</span> 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.
</p></dd><dt><span class="term"><code class="option">-V</code><br /></span><span class="term"><code class="option">--version</code></span></dt><dd><p>Display version information, then exit.</p></dd><dt><span class="term"><code class="option">-?</code><br /></span><span class="term"><code class="option">--help</code></span></dt><dd><p>Show help, then exit.</p></dd></dl></div><p>
</p></div><div class="refsect1" id="id-1.9.5.9.7"><h2>Environment</h2><p>
When <code class="option">--source-server</code> option is used,
<span class="application">pg_rewind</span> also uses the environment variables
supported by <span class="application">libpq</span> (see <a class="xref" href="libpq-envars.html" title="34.15. Environment Variables">Section 34.15</a>).
</p><p>
The environment variable <code class="envar">PG_COLOR</code> specifies whether to use
color in diagnostic messages. Possible values are
<code class="literal">always</code>, <code class="literal">auto</code> and
<code class="literal">never</code>.
</p></div><div class="refsect1" id="id-1.9.5.9.8"><h2>Notes</h2><p>
When executing <span class="application">pg_rewind</span> using an online
cluster as source, a role having sufficient permissions to execute the
functions used by <span class="application">pg_rewind</span> on the source
cluster can be used instead of a superuser. Here is how to create such
a role, named <code class="literal">rewind_user</code> here:
</p><pre class="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;
</pre><p>
</p><p>
When executing <span class="application">pg_rewind</span> using an online
cluster as source which has been recently promoted, it is necessary
to execute a <code class="command">CHECKPOINT</code> after promotion such that its
control file reflects up-to-date timeline information, which is used by
<span class="application">pg_rewind</span> to check if the target cluster
can be rewound using the designated source cluster.
</p><div class="refsect2" id="id-1.9.5.9.8.4"><h3>How It Works</h3><p>
The basic idea is to copy all file system-level changes from the source
cluster to the target cluster:
</p><div class="procedure"><ol class="procedure" type="1"><li class="step"><p>
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 <span class="application">pg_rewind</span> with
the <code class="option">-c</code> option to search for the missing files in
the WAL archive.
</p></li><li class="step"><p>
Copy all those changed blocks from the source cluster to
the target cluster, either using direct file system access
(<code class="option">--source-pgdata</code>) or SQL (<code class="option">--source-server</code>).
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.
</p></li><li class="step"><p>
Copy all other files, including new relation files, WAL segments,
<code class="filename">pg_xact</code>, and configuration files from the source
cluster to the target cluster. Similarly to base backups, the contents
of the directories <code class="filename">pg_dynshmem/</code>,
<code class="filename">pg_notify/</code>, <code class="filename">pg_replslot/</code>,
<code class="filename">pg_serial/</code>, <code class="filename">pg_snapshots/</code>,
<code class="filename">pg_stat_tmp/</code>, and <code class="filename">pg_subtrans/</code>
are omitted from the data copied from the source cluster. The files
<code class="filename">backup_label</code>,
<code class="filename">tablespace_map</code>,
<code class="filename">pg_internal.init</code>,
<code class="filename">postmaster.opts</code>,
<code class="filename">postmaster.pid</code> and
<code class="filename">.DS_Store</code> as well as any file or directory
beginning with <code class="filename">pgsql_tmp</code>, are omitted.
</p></li><li class="step"><p>
Create a <code class="filename">backup_label</code> file to begin WAL replay at
the checkpoint created at failover and configure the
<code class="filename">pg_control</code> file with a minimum consistency LSN
defined as the result of <code class="literal">pg_current_wal_insert_lsn()</code>
when rewinding from a live source or the last checkpoint LSN when
rewinding from a stopped source.
</p></li><li class="step"><p>
When starting the target, <span class="productname">PostgreSQL</span> replays
all the required WAL, resulting in a data directory in a consistent
state.
</p></li></ol></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="app-pgresetwal.html" title="pg_resetwal">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="reference-server.html" title="PostgreSQL Server Applications">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pgtestfsync.html" title="pg_test_fsync">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span class="application">pg_resetwal</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> <span class="application">pg_test_fsync</span></td></tr></table></div></body></html>
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