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diff --git a/doc/src/sgml/html/backup-file.html b/doc/src/sgml/html/backup-file.html new file mode 100644 index 0000000..cdc50c2 --- /dev/null +++ b/doc/src/sgml/html/backup-file.html @@ -0,0 +1,91 @@ +<?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>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 15.5 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</h2></div></div></div><p> + An alternative backup strategy is to directly copy the files that + <span class="productname">PostgreSQL</span> uses to store the data in the database; + <a class="xref" href="creating-cluster.html" title="19.2. Creating a Database Cluster">Section 19.2</a> explains where these files + are located. You can use whatever method you prefer + for doing file system backups; for example: + +</p><pre class="programlisting"> +tar -cf backup.tar /usr/local/pgsql/data +</pre><p> + </p><p> + There are two restrictions, however, which make this method + impractical, or at least inferior to the <span class="application">pg_dump</span> + method: + + </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> + The database server <span class="emphasis"><em>must</em></span> be shut down in order to + get a usable backup. Half-way measures such as disallowing all + connections will <span class="emphasis"><em>not</em></span> work + (in part because <code class="command">tar</code> and similar tools do not take + an atomic snapshot of the state of the file system, + but also because of internal buffering within the server). + Information about stopping the server can be found in + <a class="xref" href="server-shutdown.html" title="19.5. Shutting Down the Server">Section 19.5</a>. Needless to say, you + also need to shut down the server before restoring the data. + </p></li><li class="listitem"><p> + If you have dug into the details of the file system layout of the + database, you might be tempted to try to back up or restore only certain + individual tables or databases from their respective files or + directories. This will <span class="emphasis"><em>not</em></span> work because the + information contained in these files is not usable without + the commit log files, + <code class="filename">pg_xact/*</code>, which contain the commit status of + all transactions. A table file is only usable with this + information. Of course it is also impossible to restore only a + table and the associated <code class="filename">pg_xact</code> data + because that would render all other tables in the database + cluster useless. So file system backups only work for complete + backup and restoration of an entire database cluster. + </p></li></ol></div><p> + </p><p> + An alternative file-system backup approach is to make a + <span class="quote">“<span class="quote">consistent snapshot</span>”</span> of the data directory, if the + file system supports that functionality (and you are willing to + trust that it is implemented correctly). The typical procedure is + to make a <span class="quote">“<span class="quote">frozen snapshot</span>”</span> of the volume containing the + database, then copy the whole data directory (not just parts, see + above) from the snapshot to a backup device, then release the frozen + snapshot. This will work even while the database server is running. + However, a backup created in this way saves + the database files in a state as if the database server was not + properly shut down; therefore, when you start the database server + on the backed-up data, it will think the previous server instance + crashed and will replay the WAL log. This is not a problem; just + be aware of it (and be sure to include the WAL files in your backup). + You can perform a <code class="command">CHECKPOINT</code> before taking the + snapshot to reduce recovery time. + </p><p> + If your database is spread across multiple file systems, there might not + be any way to obtain exactly-simultaneous frozen snapshots of all + the volumes. For example, if your data files and WAL log are on different + disks, or if tablespaces are on different file systems, it might + not be possible to use snapshot backup because the snapshots + <span class="emphasis"><em>must</em></span> be simultaneous. + Read your file system documentation very carefully before trusting + the consistent-snapshot technique in such situations. + </p><p> + If simultaneous snapshots are not possible, one option is to shut down + the database server long enough to establish all the frozen snapshots. + 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 file + system changes during the backup. This requires enabling continuous + archiving just during the backup process; restore is done using + 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>). + </p><p> + Another option is to use <span class="application">rsync</span> to perform a file + system backup. This is done by first running <span class="application">rsync</span> + while the database server is running, then shutting down the database + server long enough to do an <code class="command">rsync --checksum</code>. + (<code class="option">--checksum</code> is necessary because <code class="command">rsync</code> only + has file modification-time granularity of one second.) The + second <span class="application">rsync</span> will be quicker than the first, + because it has relatively little data to transfer, and the end result + will be consistent because the server was down. This method + allows a file system backup to be performed with minimal downtime. + </p><p> + Note that a file system backup will typically be larger + than an SQL dump. (<span class="application">pg_dump</span> does not need to dump + the contents of indexes for example, just the commands to recreate + them.) However, taking a file system backup might be faster. + </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 15.5 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>
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