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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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>26.2. File System Level Backup</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="backup-dump.html" title="26.1. SQL Dump" /><link rel="next" href="continuous-archiving.html" title="26.3. Continuous Archiving and Point-in-Time Recovery (PITR)" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">26.2. File System Level Backup</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="backup-dump.html" title="26.1. SQL Dump">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="backup.html" title="Chapter 26. Backup and Restore">Up</a></td><th width="60%" align="center">Chapter 26. Backup and Restore</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="continuous-archiving.html" title="26.3. Continuous Archiving and Point-in-Time Recovery (PITR)">Next</a></td></tr></table><hr /></div><div class="sect1" id="BACKUP-FILE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">26.2. File System Level Backup <a href="#BACKUP-FILE" class="id_link">#</a></h2></div></div></div><p>3   An alternative backup strategy is to directly copy the files that4   <span class="productname">PostgreSQL</span> uses to store the data in the database;5   <a class="xref" href="creating-cluster.html" title="19.2. Creating a Database Cluster">Section 19.2</a> explains where these files6   are located.  You can use whatever method you prefer7   for doing file system backups; for example:8 9</p><pre class="programlisting">10tar -cf backup.tar /usr/local/pgsql/data11</pre><p>12  </p><p>13   There are two restrictions, however, which make this method14   impractical, or at least inferior to the <span class="application">pg_dump</span>15   method:16 17   </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>18      The database server <span class="emphasis"><em>must</em></span> be shut down in order to19      get a usable backup. Half-way measures such as disallowing all20      connections will <span class="emphasis"><em>not</em></span> work21      (in part because <code class="command">tar</code> and similar tools do not take22      an atomic snapshot of the state of the file system,23      but also because of internal buffering within the server).24      Information about stopping the server can be found in25      <a class="xref" href="server-shutdown.html" title="19.5. Shutting Down the Server">Section 19.5</a>.  Needless to say, you26      also need to shut down the server before restoring the data.27     </p></li><li class="listitem"><p>28      If you have dug into the details of the file system layout of the29      database, you might be tempted to try to back up or restore only certain30      individual tables or databases from their respective files or31      directories. This will <span class="emphasis"><em>not</em></span> work because the32      information contained in these files is not usable without33      the commit log files,34      <code class="filename">pg_xact/*</code>, which contain the commit status of35      all transactions. A table file is only usable with this36      information. Of course it is also impossible to restore only a37      table and the associated <code class="filename">pg_xact</code> data38      because that would render all other tables in the database39      cluster useless.  So file system backups only work for complete40      backup and restoration of an entire database cluster.41     </p></li></ol></div><p>42  </p><p>43   An alternative file-system backup approach is to make a44   <span class="quote">“<span class="quote">consistent snapshot</span>”</span> of the data directory, if the45   file system supports that functionality (and you are willing to46   trust that it is implemented correctly).  The typical procedure is47   to make a <span class="quote">“<span class="quote">frozen snapshot</span>”</span> of the volume containing the48   database, then copy the whole data directory (not just parts, see49   above) from the snapshot to a backup device, then release the frozen50   snapshot.  This will work even while the database server is running.51   However, a backup created in this way saves52   the database files in a state as if the database server was not53   properly shut down; therefore, when you start the database server54   on the backed-up data, it will think the previous server instance55   crashed and will replay the WAL log.  This is not a problem; just56   be aware of it (and be sure to include the WAL files in your backup).57   You can perform a <code class="command">CHECKPOINT</code> before taking the58   snapshot to reduce recovery time.59  </p><p>60   If your database is spread across multiple file systems, there might not61   be any way to obtain exactly-simultaneous frozen snapshots of all62   the volumes.  For example, if your data files and WAL log are on different63   disks, or if tablespaces are on different file systems, it might64   not be possible to use snapshot backup because the snapshots65   <span class="emphasis"><em>must</em></span> be simultaneous.66   Read your file system documentation very carefully before trusting67   the consistent-snapshot technique in such situations.68  </p><p>69   If simultaneous snapshots are not possible, one option is to shut down70   the database server long enough to establish all the frozen snapshots.71   Another option is to perform a continuous archiving base backup (<a class="xref" href="continuous-archiving.html#BACKUP-BASE-BACKUP" title="26.3.2. Making a Base Backup">Section 26.3.2</a>) because such backups are immune to file72   system changes during the backup.  This requires enabling continuous73   archiving just during the backup process; restore is done using74   continuous archive recovery (<a class="xref" href="continuous-archiving.html#BACKUP-PITR-RECOVERY" title="26.3.4. Recovering Using a Continuous Archive Backup">Section 26.3.4</a>).75  </p><p>76   Another option is to use <span class="application">rsync</span> to perform a file77   system backup.  This is done by first running <span class="application">rsync</span>78   while the database server is running, then shutting down the database79   server long enough to do an <code class="command">rsync --checksum</code>.80   (<code class="option">--checksum</code> is necessary because <code class="command">rsync</code> only81   has file modification-time granularity of one second.)  The82   second <span class="application">rsync</span> will be quicker than the first,83   because it has relatively little data to transfer, and the end result84   will be consistent because the server was down.  This method85   allows a file system backup to be performed with minimal downtime.86  </p><p>87   Note that a file system backup will typically be larger88   than an SQL dump. (<span class="application">pg_dump</span> does not need to dump89   the contents of indexes for example, just the commands to recreate90   them.)  However, taking a file system backup might be faster.91  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="backup-dump.html" title="26.1. SQL Dump">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="backup.html" title="Chapter 26. Backup and Restore">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="continuous-archiving.html" title="26.3. Continuous Archiving and Point-in-Time Recovery (PITR)">Next</a></td></tr><tr><td width="40%" align="left" valign="top">26.1. <acronym class="acronym">SQL</acronym> Dump </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"> 26.3. Continuous Archiving and Point-in-Time Recovery (PITR)</td></tr></table></div></body></html>