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| <!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 16.3 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: Failures While Rewinding</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><div class="refsect2" id="id-1.9.5.9.8.3"><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 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> <span class="application">pg_test_fsync</span></td></tr></table></div></body></html> |
