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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>26.3. Continuous Archiving and Point-in-Time Recovery (PITR)</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-file.html" title="26.2. File System Level Backup" /><link rel="next" href="high-availability.html" title="Chapter 27. High Availability, Load Balancing, and Replication" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">26.3. Continuous Archiving and Point-in-Time Recovery (PITR)</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="backup-file.html" title="26.2. File System Level Backup">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="high-availability.html" title="Chapter 27. High Availability, Load Balancing, and Replication">Next</a></td></tr></table><hr /></div><div class="sect1" id="CONTINUOUS-ARCHIVING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">26.3. Continuous Archiving and Point-in-Time Recovery (PITR) <a href="#CONTINUOUS-ARCHIVING" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-ARCHIVING-WAL">26.3.1. Setting Up WAL Archiving</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-BASE-BACKUP">26.3.2. Making a Base Backup</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP">26.3.3. Making a Base Backup Using the Low Level API</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-PITR-RECOVERY">26.3.4. Recovering Using a Continuous Archive Backup</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-TIMELINES">26.3.5. Timelines</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-TIPS">26.3.6. Tips and Examples</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#CONTINUOUS-ARCHIVING-CAVEATS">26.3.7. Caveats</a></span></dt></dl></div><a id="id-1.6.13.7.2" class="indexterm"></a><a id="id-1.6.13.7.3" class="indexterm"></a><a id="id-1.6.13.7.4" class="indexterm"></a><p>3   At all times, <span class="productname">PostgreSQL</span> maintains a4   <em class="firstterm">write ahead log</em> (WAL) in the <code class="filename">pg_wal/</code>5   subdirectory of the cluster's data directory. The log records6   every change made to the database's data files.  This log exists7   primarily for crash-safety purposes: if the system crashes, the8   database can be restored to consistency by <span class="quote">“<span class="quote">replaying</span>”</span> the9   log entries made since the last checkpoint.  However, the existence10   of the log makes it possible to use a third strategy for backing up11   databases: we can combine a file-system-level backup with backup of12   the WAL files.  If recovery is needed, we restore the file system backup and13   then replay from the backed-up WAL files to bring the system to a14   current state.  This approach is more complex to administer than15   either of the previous approaches, but it has some significant16   benefits:17  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>18     We do not need a perfectly consistent file system backup as the starting point.19     Any internal inconsistency in the backup will be corrected by log20     replay (this is not significantly different from what happens during21     crash recovery).  So we do not need a file system snapshot capability,22     just <span class="application">tar</span> or a similar archiving tool.23    </p></li><li class="listitem"><p>24     Since we can combine an indefinitely long sequence of WAL files25     for replay, continuous backup can be achieved simply by continuing to archive26     the WAL files.  This is particularly valuable for large databases, where27     it might not be convenient to take a full backup frequently.28    </p></li><li class="listitem"><p>29     It is not necessary to replay the WAL entries all the30     way to the end.  We could stop the replay at any point and have a31     consistent snapshot of the database as it was at that time.  Thus,32     this technique supports <em class="firstterm">point-in-time recovery</em>: it is33     possible to restore the database to its state at any time since your base34     backup was taken.35    </p></li><li class="listitem"><p>36     If we continuously feed the series of WAL files to another37     machine that has been loaded with the same base backup file, we38     have a <em class="firstterm">warm standby</em> system: at any point we can bring up39     the second machine and it will have a nearly-current copy of the40     database.41    </p></li></ul></div><p>42  </p><div class="note"><h3 class="title">Note</h3><p>43    <span class="application">pg_dump</span> and44    <span class="application">pg_dumpall</span> do not produce file-system-level45    backups and cannot be used as part of a continuous-archiving solution.46    Such dumps are <span class="emphasis"><em>logical</em></span> and do not contain enough47    information to be used by WAL replay.48   </p></div><p>49   As with the plain file-system-backup technique, this method can only50   support restoration of an entire database cluster, not a subset.51   Also, it requires a lot of archival storage: the base backup might be bulky,52   and a busy system will generate many megabytes of WAL traffic that53   have to be archived.  Still, it is the preferred backup technique in54   many situations where high reliability is needed.55  </p><p>56   To recover successfully using continuous archiving (also called57   <span class="quote">“<span class="quote">online backup</span>”</span> by many database vendors), you need a continuous58   sequence of archived WAL files that extends back at least as far as the59   start time of your backup.  So to get started, you should set up and test60   your procedure for archiving WAL files <span class="emphasis"><em>before</em></span> you take your61   first base backup.  Accordingly, we first discuss the mechanics of62   archiving WAL files.63  </p><div class="sect2" id="BACKUP-ARCHIVING-WAL"><div class="titlepage"><div><div><h3 class="title">26.3.1. Setting Up WAL Archiving <a href="#BACKUP-ARCHIVING-WAL" class="id_link">#</a></h3></div></div></div><p>64    In an abstract sense, a running <span class="productname">PostgreSQL</span> system65    produces an indefinitely long sequence of WAL records.  The system66    physically divides this sequence into WAL <em class="firstterm">segment67    files</em>, which are normally 16MB apiece (although the segment size68    can be altered during <span class="application">initdb</span>).  The segment69    files are given numeric names that reflect their position in the70    abstract WAL sequence.  When not using WAL archiving, the system71    normally creates just a few segment files and then72    <span class="quote">“<span class="quote">recycles</span>”</span> them by renaming no-longer-needed segment files73    to higher segment numbers.  It's assumed that segment files whose74    contents precede the last checkpoint are no longer of75    interest and can be recycled.76   </p><p>77    When archiving WAL data, we need to capture the contents of each segment78    file once it is filled, and save that data somewhere before the segment79    file is recycled for reuse.  Depending on the application and the80    available hardware, there could be many different ways of <span class="quote">“<span class="quote">saving81    the data somewhere</span>”</span>: we could copy the segment files to an NFS-mounted82    directory on another machine, write them onto a tape drive (ensuring that83    you have a way of identifying the original name of each file), or batch84    them together and burn them onto CDs, or something else entirely.  To85    provide the database administrator with flexibility,86    <span class="productname">PostgreSQL</span> tries not to make any assumptions about how87    the archiving will be done.  Instead, <span class="productname">PostgreSQL</span> lets88    the administrator specify a shell command or an archive library to be executed to copy a89    completed segment file to wherever it needs to go.  This could be as simple90    as a shell command that uses <code class="literal">cp</code>, or it could invoke a91    complex C function — it's all up to you.92   </p><p>93    To enable WAL archiving, set the <a class="xref" href="runtime-config-wal.html#GUC-WAL-LEVEL">wal_level</a>94    configuration parameter to <code class="literal">replica</code> or higher,95    <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-MODE">archive_mode</a> to <code class="literal">on</code>,96    specify the shell command to use in the <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-COMMAND">archive_command</a> configuration parameter97    or specify the library to use in the <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-LIBRARY">archive_library</a> configuration parameter.  In practice98    these settings will always be placed in the99    <code class="filename">postgresql.conf</code> file.100   </p><p>101    In <code class="varname">archive_command</code>,102    <code class="literal">%p</code> is replaced by the path name of the file to103    archive, while <code class="literal">%f</code> is replaced by only the file name.104    (The path name is relative to the current working directory,105    i.e., the cluster's data directory.)106    Use <code class="literal">%%</code> if you need to embed an actual <code class="literal">%</code>107    character in the command.  The simplest useful command is something108    like:109</p><pre class="programlisting">110archive_command = 'test ! -f /mnt/server/archivedir/%f &amp;&amp; cp %p /mnt/server/archivedir/%f'  # Unix111archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"'  # Windows112</pre><p>113    which will copy archivable WAL segments to the directory114    <code class="filename">/mnt/server/archivedir</code>.  (This is an example, not a115    recommendation, and might not work on all platforms.)  After the116    <code class="literal">%p</code> and <code class="literal">%f</code> parameters have been replaced,117    the actual command executed might look like this:118</p><pre class="programlisting">119test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065120</pre><p>121    A similar command will be generated for each new file to be archived.122   </p><p>123    The archive command will be executed under the ownership of the same124    user that the <span class="productname">PostgreSQL</span> server is running as.  Since125    the series of WAL files being archived contains effectively everything126    in your database, you will want to be sure that the archived data is127    protected from prying eyes; for example, archive into a directory that128    does not have group or world read access.129   </p><p>130    It is important that the archive command return zero exit status if and131    only if it succeeds.  Upon getting a zero result,132    <span class="productname">PostgreSQL</span> will assume that the file has been133    successfully archived, and will remove or recycle it.  However, a nonzero134    status tells <span class="productname">PostgreSQL</span> that the file was not archived;135    it will try again periodically until it succeeds.136   </p><p>137    Another way to archive is to use a custom archive module as the138    <code class="varname">archive_library</code>.  Since such modules are written in139    <code class="literal">C</code>, creating your own may require considerably more effort140    than writing a shell command.  However, archive modules can be more141    performant than archiving via shell, and they will have access to many142    useful server resources.  For more information about archive modules, see143    <a class="xref" href="archive-modules.html" title="Chapter 51. Archive Modules">Chapter 51</a>.144   </p><p>145    When the archive command is terminated by a signal (other than146    <span class="systemitem">SIGTERM</span> that is used as part of a server147    shutdown) or an error by the shell with an exit status greater than148    125 (such as command not found), or if the archive function emits an149    <code class="literal">ERROR</code> or <code class="literal">FATAL</code>, the archiver process150    aborts and gets restarted by the postmaster. In such cases, the failure is151    not reported in <a class="xref" href="monitoring-stats.html#PG-STAT-ARCHIVER-VIEW" title="Table 28.22. pg_stat_archiver View">pg_stat_archiver</a>.152   </p><p>153    Archive commands and libraries should generally be designed to refuse to overwrite154    any pre-existing archive file.  This is an important safety feature to155    preserve the integrity of your archive in case of administrator error156    (such as sending the output of two different servers to the same archive157    directory).  It is advisable to test your proposed archive library to ensure158    that it does not overwrite an existing file.159   </p><p>160    In rare cases, <span class="productname">PostgreSQL</span> may attempt to161    re-archive a WAL file that was previously archived.  For example, if the162    system crashes before the server makes a durable record of archival163    success, the server will attempt to archive the file again after164    restarting (provided archiving is still enabled).  When an archive command or library165    encounters a pre-existing file, it should return a zero status or <code class="literal">true</code>, respectively,166    if the WAL file has identical contents to the pre-existing archive and the167    pre-existing archive is fully persisted to storage.  If a pre-existing168    file contains different contents than the WAL file being archived, the169    archive command or library <span class="emphasis"><em>must</em></span> return a nonzero status or170    <code class="literal">false</code>, respectively.171   </p><p>172    The example command above for Unix avoids overwriting a pre-existing archive173    by including a separate174    <code class="command">test</code> step.  On some Unix platforms, <code class="command">cp</code> has175    switches such as <code class="option">-i</code> that can be used to do the same thing176    less verbosely, but you should not rely on these without verifying that177    the right exit status is returned.  (In particular, GNU <code class="command">cp</code>178    will return status zero when <code class="option">-i</code> is used and the target file179    already exists, which is <span class="emphasis"><em>not</em></span> the desired behavior.)180   </p><p>181    While designing your archiving setup, consider what will happen if182    the archive command or library fails repeatedly because some aspect requires183    operator intervention or the archive runs out of space. For example, this184    could occur if you write to tape without an autochanger; when the tape185    fills, nothing further can be archived until the tape is swapped.186    You should ensure that any error condition or request to a human operator187    is reported appropriately so that the situation can be188    resolved reasonably quickly. The <code class="filename">pg_wal/</code> directory will189    continue to fill with WAL segment files until the situation is resolved.190    (If the file system containing <code class="filename">pg_wal/</code> fills up,191    <span class="productname">PostgreSQL</span> will do a PANIC shutdown.  No committed192    transactions will be lost, but the database will remain offline until193    you free some space.)194   </p><p>195    The speed of the archive command or library is unimportant as long as it can keep up196    with the average rate at which your server generates WAL data.  Normal197    operation continues even if the archiving process falls a little behind.198    If archiving falls significantly behind, this will increase the amount of199    data that would be lost in the event of a disaster. It will also mean that200    the <code class="filename">pg_wal/</code> directory will contain large numbers of201    not-yet-archived segment files, which could eventually exceed available202    disk space. You are advised to monitor the archiving process to ensure that203    it is working as you intend.204   </p><p>205    In writing your archive command or library, you should assume that the file names to206    be archived can be up to 64 characters long and can contain any207    combination of ASCII letters, digits, and dots.  It is not necessary to208    preserve the original relative path (<code class="literal">%p</code>) but it is necessary to209    preserve the file name (<code class="literal">%f</code>).210   </p><p>211    Note that although WAL archiving will allow you to restore any212    modifications made to the data in your <span class="productname">PostgreSQL</span> database,213    it will not restore changes made to configuration files (that is,214    <code class="filename">postgresql.conf</code>, <code class="filename">pg_hba.conf</code> and215    <code class="filename">pg_ident.conf</code>), since those are edited manually rather216    than through SQL operations.217    You might wish to keep the configuration files in a location that will218    be backed up by your regular file system backup procedures.  See219    <a class="xref" href="runtime-config-file-locations.html" title="20.2. File Locations">Section 20.2</a> for how to relocate the220    configuration files.221   </p><p>222    The archive command or function is only invoked on completed WAL segments.  Hence,223    if your server generates only little WAL traffic (or has slack periods224    where it does so), there could be a long delay between the completion225    of a transaction and its safe recording in archive storage.  To put226    a limit on how old unarchived data can be, you can set227    <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-TIMEOUT">archive_timeout</a> to force the server to switch228    to a new WAL segment file at least that often.  Note that archived229    files that are archived early due to a forced switch are still the same230    length as completely full files.  It is therefore unwise to set a very231    short <code class="varname">archive_timeout</code> — it will bloat your archive232    storage.  <code class="varname">archive_timeout</code> settings of a minute or so are233    usually reasonable.234   </p><p>235    Also, you can force a segment switch manually with236    <code class="function">pg_switch_wal</code> if you want to ensure that a237    just-finished transaction is archived as soon as possible.  Other utility238    functions related to WAL management are listed in <a class="xref" href="functions-admin.html#FUNCTIONS-ADMIN-BACKUP-TABLE" title="Table 9.91. Backup Control Functions">Table 9.91</a>.239   </p><p>240    When <code class="varname">wal_level</code> is <code class="literal">minimal</code> some SQL commands241    are optimized to avoid WAL logging, as described in <a class="xref" href="populate.html#POPULATE-PITR" title="14.4.7. Disable WAL Archival and Streaming Replication">Section 14.4.7</a>.  If archiving or streaming replication were242    turned on during execution of one of these statements, WAL would not243    contain enough information for archive recovery.  (Crash recovery is244    unaffected.)  For this reason, <code class="varname">wal_level</code> can only be changed at245    server start.  However, <code class="varname">archive_command</code> and <code class="varname">archive_library</code> can be changed with a246    configuration file reload.  If you are archiving via shell and wish to247    temporarily stop archiving,248    one way to do it is to set <code class="varname">archive_command</code> to the empty249    string (<code class="literal">''</code>).250    This will cause WAL files to accumulate in <code class="filename">pg_wal/</code> until a251    working <code class="varname">archive_command</code> is re-established.252   </p></div><div class="sect2" id="BACKUP-BASE-BACKUP"><div class="titlepage"><div><div><h3 class="title">26.3.2. Making a Base Backup <a href="#BACKUP-BASE-BACKUP" class="id_link">#</a></h3></div></div></div><p>253    The easiest way to perform a base backup is to use the254    <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a> tool. It can create255    a base backup either as regular files or as a tar archive. If more256    flexibility than <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a> can provide is257    required, you can also make a base backup using the low level API258    (see <a class="xref" href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP" title="26.3.3. Making a Base Backup Using the Low Level API">Section 26.3.3</a>).259   </p><p>260    It is not necessary to be concerned about the amount of time it takes261    to make a base backup. However, if you normally run the262    server with <code class="varname">full_page_writes</code> disabled, you might notice a drop263    in performance while the backup runs since <code class="varname">full_page_writes</code> is264    effectively forced on during backup mode.265   </p><p>266    To make use of the backup, you will need to keep all the WAL267    segment files generated during and after the file system backup.268    To aid you in doing this, the base backup process269    creates a <em class="firstterm">backup history file</em> that is immediately270    stored into the WAL archive area. This file is named after the first271    WAL segment file that you need for the file system backup.272    For example, if the starting WAL file is273    <code class="literal">0000000100001234000055CD</code> the backup history file will be274    named something like275    <code class="literal">0000000100001234000055CD.007C9330.backup</code>. (The second276    part of the file name stands for an exact position within the WAL277    file, and can ordinarily be ignored.) Once you have safely archived278    the file system backup and the WAL segment files used during the279    backup (as specified in the backup history file), all archived WAL280    segments with names numerically less are no longer needed to recover281    the file system backup and can be deleted. However, you should282    consider keeping several backup sets to be absolutely certain that283    you can recover your data.284   </p><p>285    The backup history file is just a small text file. It contains the286    label string you gave to <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a>, as well as287    the starting and ending times and WAL segments of the backup.288    If you used the label to identify the associated dump file,289    then the archived history file is enough to tell you which dump file to290    restore.291   </p><p>292    Since you have to keep around all the archived WAL files back to your293    last base backup, the interval between base backups should usually be294    chosen based on how much storage you want to expend on archived WAL295    files.  You should also consider how long you are prepared to spend296    recovering, if recovery should be necessary — the system will have to297    replay all those WAL segments, and that could take awhile if it has298    been a long time since the last base backup.299   </p></div><div class="sect2" id="BACKUP-LOWLEVEL-BASE-BACKUP"><div class="titlepage"><div><div><h3 class="title">26.3.3. Making a Base Backup Using the Low Level API <a href="#BACKUP-LOWLEVEL-BASE-BACKUP" class="id_link">#</a></h3></div></div></div><p>300    The procedure for making a base backup using the low level301    APIs contains a few more steps than302    the <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a> method, but is relatively303    simple. It is very important that these steps are executed in304    sequence, and that the success of a step is verified before305    proceeding to the next step.306   </p><p>307     Multiple backups are able to be run concurrently (both those308     started using this backup API and those started using309     <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a>).310    </p><p>311  </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>312     Ensure that WAL archiving is enabled and working.313    </p></li><li class="listitem"><p>314     Connect to the server (it does not matter which database) as a user with315     rights to run <code class="function">pg_backup_start</code> (superuser,316     or a user who has been granted <code class="literal">EXECUTE</code> on the317     function) and issue the command:318</p><pre class="programlisting">319SELECT pg_backup_start(label =&gt; 'label', fast =&gt; false);320</pre><p>321     where <code class="literal">label</code> is any string you want to use to uniquely322     identify this backup operation. The connection323     calling <code class="function">pg_backup_start</code> must be maintained until the end of324     the backup, or the backup will be automatically aborted.325    </p><p>326     Online backups are always started at the beginning of a checkpoint.327     By default, <code class="function">pg_backup_start</code> will wait for the next328     regularly scheduled checkpoint to complete, which may take a long time (see the329     configuration parameters <a class="xref" href="runtime-config-wal.html#GUC-CHECKPOINT-TIMEOUT">checkpoint_timeout</a> and330     <a class="xref" href="runtime-config-wal.html#GUC-CHECKPOINT-COMPLETION-TARGET">checkpoint_completion_target</a>).  This is331     usually preferable as it minimizes the impact on the running system.  If you332     want to start the backup as soon as possible, pass <code class="literal">true</code> as333     the second parameter to <code class="function">pg_backup_start</code> and it will334     request an immediate checkpoint, which will finish as fast as possible using335     as much I/O as possible.336    </p></li><li class="listitem"><p>337     Perform the backup, using any convenient file-system-backup tool338     such as <span class="application">tar</span> or <span class="application">cpio</span> (not339     <span class="application">pg_dump</span> or340     <span class="application">pg_dumpall</span>).  It is neither341     necessary nor desirable to stop normal operation of the database342     while you do this. See343     <a class="xref" href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP-DATA" title="26.3.3.1. Backing Up the Data Directory">Section 26.3.3.1</a> for things to344     consider during this backup.345    </p></li><li class="listitem"><p>346     In the same connection as before, issue the command:347</p><pre class="programlisting">348SELECT * FROM pg_backup_stop(wait_for_archive =&gt; true);349</pre><p>350     This terminates backup mode. On a primary, it also performs an automatic351     switch to the next WAL segment.  On a standby, it is not possible to352     automatically switch WAL segments, so you may wish to run353     <code class="function">pg_switch_wal</code> on the primary to perform a manual354     switch. The reason for the switch is to arrange for355     the last WAL segment file written during the backup interval to be356     ready to archive.357    </p><p>358     <code class="function">pg_backup_stop</code> will return one row with three359     values. The second of these fields should be written to a file named360     <code class="filename">backup_label</code> in the root directory of the backup. The361     third field should be written to a file named362     <code class="filename">tablespace_map</code> unless the field is empty. These files are363     vital to the backup working and must be written byte for byte without364     modification, which may require opening the file in binary mode.365    </p></li><li class="listitem"><p>366     Once the WAL segment files active during the backup are archived, you are367     done.  The file identified by <code class="function">pg_backup_stop</code>'s first return368     value is the last segment that is required to form a complete set of369     backup files.  On a primary, if <code class="varname">archive_mode</code> is enabled and the370     <code class="literal">wait_for_archive</code> parameter is <code class="literal">true</code>,371     <code class="function">pg_backup_stop</code> does not return until the last segment has372     been archived.373     On a standby, <code class="varname">archive_mode</code> must be <code class="literal">always</code> in order374     for <code class="function">pg_backup_stop</code> to wait.375     Archiving of these files happens automatically since you have376     already configured <code class="varname">archive_command</code> or <code class="varname">archive_library</code>.377     In most cases this happens quickly, but you are advised to monitor your378     archive system to ensure there are no delays.379     If the archive process has fallen behind because of failures of the380     archive command or library, it will keep retrying381     until the archive succeeds and the backup is complete.382     If you wish to place a time limit on the execution of383     <code class="function">pg_backup_stop</code>, set an appropriate384     <code class="varname">statement_timeout</code> value, but make note that if385     <code class="function">pg_backup_stop</code> terminates because of this your backup386     may not be valid.387    </p><p>388     If the backup process monitors and ensures that all WAL segment files389     required for the backup are successfully archived then the390     <code class="literal">wait_for_archive</code> parameter (which defaults to true) can be set391     to false to have392     <code class="function">pg_backup_stop</code> return as soon as the stop backup record is393     written to the WAL.  By default, <code class="function">pg_backup_stop</code> will wait394     until all WAL has been archived, which can take some time.  This option395     must be used with caution: if WAL archiving is not monitored correctly396     then the backup might not include all of the WAL files and will397     therefore be incomplete and not able to be restored.398    </p></li></ol></div><p>399    </p><div class="sect3" id="BACKUP-LOWLEVEL-BASE-BACKUP-DATA"><div class="titlepage"><div><div><h4 class="title">26.3.3.1. Backing Up the Data Directory <a href="#BACKUP-LOWLEVEL-BASE-BACKUP-DATA" class="id_link">#</a></h4></div></div></div><p>400    Some file system backup tools emit warnings or errors401    if the files they are trying to copy change while the copy proceeds.402    When taking a base backup of an active database, this situation is normal403    and not an error.  However, you need to ensure that you can distinguish404    complaints of this sort from real errors.  For example, some versions405    of <span class="application">rsync</span> return a separate exit code for406    <span class="quote">“<span class="quote">vanished source files</span>”</span>, and you can write a driver script to407    accept this exit code as a non-error case.  Also, some versions of408    GNU <span class="application">tar</span> return an error code indistinguishable from409    a fatal error if a file was truncated while <span class="application">tar</span> was410    copying it.  Fortunately, GNU <span class="application">tar</span> versions 1.16 and411    later exit with 1 if a file was changed during the backup,412    and 2 for other errors.  With GNU <span class="application">tar</span> version 1.23 and413    later, you can use the warning options <code class="literal">--warning=no-file-changed414    --warning=no-file-removed</code> to hide the related warning messages.415   </p><p>416    Be certain that your backup includes all of the files under417    the database cluster directory (e.g., <code class="filename">/usr/local/pgsql/data</code>).418    If you are using tablespaces that do not reside underneath this directory,419    be careful to include them as well (and be sure that your backup420    archives symbolic links as links, otherwise the restore will corrupt421    your tablespaces).422   </p><p>423    You should, however, omit from the backup the files within the424    cluster's <code class="filename">pg_wal/</code> subdirectory.  This425    slight adjustment is worthwhile because it reduces the risk426    of mistakes when restoring.  This is easy to arrange if427    <code class="filename">pg_wal/</code> is a symbolic link pointing to someplace outside428    the cluster directory, which is a common setup anyway for performance429    reasons.  You might also want to exclude <code class="filename">postmaster.pid</code>430    and <code class="filename">postmaster.opts</code>, which record information431    about the running <span class="application">postmaster</span>, not about the432    <span class="application">postmaster</span> which will eventually use this backup.433    (These files can confuse <span class="application">pg_ctl</span>.)434   </p><p>435    It is often a good idea to also omit from the backup the files436    within the cluster's <code class="filename">pg_replslot/</code> directory, so that437    replication slots that exist on the primary do not become part of the438    backup.  Otherwise, the subsequent use of the backup to create a standby439    may result in indefinite retention of WAL files on the standby, and440    possibly bloat on the primary if hot standby feedback is enabled, because441    the clients that are using those replication slots will still be connecting442    to and updating the slots on the primary, not the standby.  Even if the443    backup is only intended for use in creating a new primary, copying the444    replication slots isn't expected to be particularly useful, since the445    contents of those slots will likely be badly out of date by the time446    the new primary comes on line.447   </p><p>448    The contents of the directories <code class="filename">pg_dynshmem/</code>,449    <code class="filename">pg_notify/</code>, <code class="filename">pg_serial/</code>,450    <code class="filename">pg_snapshots/</code>, <code class="filename">pg_stat_tmp/</code>,451    and <code class="filename">pg_subtrans/</code> (but not the directories themselves) can be452    omitted from the backup as they will be initialized on postmaster startup.453   </p><p>454    Any file or directory beginning with <code class="filename">pgsql_tmp</code> can be455    omitted from the backup.  These files are removed on postmaster start and456    the directories will be recreated as needed.457   </p><p>458    <code class="filename">pg_internal.init</code> files can be omitted from the459    backup whenever a file of that name is found.  These files contain460    relation cache data that is always rebuilt when recovering.461   </p><p>462    The backup label463    file includes the label string you gave to <code class="function">pg_backup_start</code>,464    as well as the time at which <code class="function">pg_backup_start</code> was run, and465    the name of the starting WAL file.  In case of confusion it is therefore466    possible to look inside a backup file and determine exactly which467    backup session the dump file came from.  The tablespace map file includes468    the symbolic link names as they exist in the directory469    <code class="filename">pg_tblspc/</code> and the full path of each symbolic link.470    These files are not merely for your information; their presence and471    contents are critical to the proper operation of the system's recovery472    process.473   </p><p>474    It is also possible to make a backup while the server is475    stopped.  In this case, you obviously cannot use476    <code class="function">pg_backup_start</code> or <code class="function">pg_backup_stop</code>, and477    you will therefore be left to your own devices to keep track of which478    backup is which and how far back the associated WAL files go.479    It is generally better to follow the continuous archiving procedure above.480   </p></div></div><div class="sect2" id="BACKUP-PITR-RECOVERY"><div class="titlepage"><div><div><h3 class="title">26.3.4. Recovering Using a Continuous Archive Backup <a href="#BACKUP-PITR-RECOVERY" class="id_link">#</a></h3></div></div></div><p>481    Okay, the worst has happened and you need to recover from your backup.482    Here is the procedure:483  </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>484     Stop the server, if it's running.485    </p></li><li class="listitem"><p>486     If you have the space to do so,487     copy the whole cluster data directory and any tablespaces to a temporary488     location in case you need them later. Note that this precaution will489     require that you have enough free space on your system to hold two490     copies of your existing database. If you do not have enough space,491     you should at least save the contents of the cluster's <code class="filename">pg_wal</code>492     subdirectory, as it might contain WAL files which493     were not archived before the system went down.494    </p></li><li class="listitem"><p>495     Remove all existing files and subdirectories under the cluster data496     directory and under the root directories of any tablespaces you are using.497    </p></li><li class="listitem"><p>498     Restore the database files from your file system backup.  Be sure that they499     are restored with the right ownership (the database system user, not500     <code class="literal">root</code>!) and with the right permissions.  If you are using501     tablespaces,502     you should verify that the symbolic links in <code class="filename">pg_tblspc/</code>503     were correctly restored.504    </p></li><li class="listitem"><p>505     Remove any files present in <code class="filename">pg_wal/</code>; these came from the506     file system backup and are therefore probably obsolete rather than current.507     If you didn't archive <code class="filename">pg_wal/</code> at all, then recreate508     it with proper permissions,509     being careful to ensure that you re-establish it as a symbolic link510     if you had it set up that way before.511    </p></li><li class="listitem"><p>512     If you have unarchived WAL segment files that you saved in step 2,513     copy them into <code class="filename">pg_wal/</code>.  (It is best to copy them,514     not move them, so you still have the unmodified files if a515     problem occurs and you have to start over.)516    </p></li><li class="listitem"><p>517     Set recovery configuration settings in518     <code class="filename">postgresql.conf</code> (see <a class="xref" href="runtime-config-wal.html#RUNTIME-CONFIG-WAL-ARCHIVE-RECOVERY" title="20.5.5. Archive Recovery">Section 20.5.5</a>) and create a file519     <code class="filename">recovery.signal</code> in the cluster520     data directory. You might521     also want to temporarily modify <code class="filename">pg_hba.conf</code> to prevent522     ordinary users from connecting until you are sure the recovery was successful.523    </p></li><li class="listitem"><p>524     Start the server.  The server will go into recovery mode and525     proceed to read through the archived WAL files it needs.  Should the526     recovery be terminated because of an external error, the server can527     simply be restarted and it will continue recovery.  Upon completion528     of the recovery process, the server will remove529     <code class="filename">recovery.signal</code> (to prevent530     accidentally re-entering recovery mode later) and then531     commence normal database operations.532    </p></li><li class="listitem"><p>533     Inspect the contents of the database to ensure you have recovered to534     the desired state.  If not, return to step 1.  If all is well,535     allow your users to connect by restoring <code class="filename">pg_hba.conf</code> to normal.536    </p></li></ol></div><p>537   </p><p>538    The key part of all this is to set up a recovery configuration that539    describes how you want to recover and how far the recovery should540    run.  The one thing that you absolutely must specify is the <code class="varname">restore_command</code>,541    which tells <span class="productname">PostgreSQL</span> how to retrieve archived542    WAL file segments.  Like the <code class="varname">archive_command</code>, this is543    a shell command string.  It can contain <code class="literal">%f</code>, which is544    replaced by the name of the desired WAL file, and <code class="literal">%p</code>,545    which is replaced by the path name to copy the WAL file to.546    (The path name is relative to the current working directory,547    i.e., the cluster's data directory.)548    Write <code class="literal">%%</code> if you need to embed an actual <code class="literal">%</code>549    character in the command.  The simplest useful command is550    something like:551</p><pre class="programlisting">552restore_command = 'cp /mnt/server/archivedir/%f %p'553</pre><p>554    which will copy previously archived WAL segments from the directory555    <code class="filename">/mnt/server/archivedir</code>.  Of course, you can use something556    much more complicated, perhaps even a shell script that requests the557    operator to mount an appropriate tape.558   </p><p>559    It is important that the command return nonzero exit status on failure.560    The command <span class="emphasis"><em>will</em></span> be called requesting files that are not561    present in the archive; it must return nonzero when so asked.  This is not562    an error condition.  An exception is that if the command was terminated by563    a signal (other than <span class="systemitem">SIGTERM</span>, which is used as564    part of a database server shutdown) or an error by the shell (such as565    command not found), then recovery will abort and the server will not start566    up.567   </p><p>568    Not all of the requested files will be WAL segment569    files; you should also expect requests for files with a suffix of570    <code class="literal">.history</code>. Also be aware that571    the base name of the <code class="literal">%p</code> path will be different from572    <code class="literal">%f</code>; do not expect them to be interchangeable.573   </p><p>574    WAL segments that cannot be found in the archive will be sought in575    <code class="filename">pg_wal/</code>; this allows use of recent un-archived segments.576    However, segments that are available from the archive will be used in577    preference to files in <code class="filename">pg_wal/</code>.578   </p><p>579    Normally, recovery will proceed through all available WAL segments,580    thereby restoring the database to the current point in time (or as581    close as possible given the available WAL segments).  Therefore, a normal582    recovery will end with a <span class="quote">“<span class="quote">file not found</span>”</span> message, the exact text583    of the error message depending upon your choice of584    <code class="varname">restore_command</code>.  You may also see an error message585    at the start of recovery for a file named something like586    <code class="filename">00000001.history</code>.  This is also normal and does not587    indicate a problem in simple recovery situations; see588    <a class="xref" href="continuous-archiving.html#BACKUP-TIMELINES" title="26.3.5. Timelines">Section 26.3.5</a> for discussion.589   </p><p>590    If you want to recover to some previous point in time (say, right before591    the junior DBA dropped your main transaction table), just specify the592    required <a class="link" href="runtime-config-wal.html#RUNTIME-CONFIG-WAL-RECOVERY-TARGET" title="20.5.6. Recovery Target">stopping point</a>.  You can specify593    the stop point, known as the <span class="quote">“<span class="quote">recovery target</span>”</span>, either by594    date/time, named restore point or by completion of a specific transaction595    ID.  As of this writing only the date/time and named restore point options596    are very usable, since there are no tools to help you identify with any597    accuracy which transaction ID to use.598   </p><div class="note"><h3 class="title">Note</h3><p>599      The stop point must be after the ending time of the base backup, i.e.,600      the end time of <code class="function">pg_backup_stop</code>.  You cannot use a base backup601      to recover to a time when that backup was in progress.  (To602      recover to such a time, you must go back to your previous base backup603      and roll forward from there.)604     </p></div><p>605    If recovery finds corrupted WAL data, recovery will606    halt at that point and the server will not start. In such a case the607    recovery process could be re-run from the beginning, specifying a608    <span class="quote">“<span class="quote">recovery target</span>”</span> before the point of corruption so that recovery609    can complete normally.610    If recovery fails for an external reason, such as a system crash or611    if the WAL archive has become inaccessible, then the recovery can simply612    be restarted and it will restart almost from where it failed.613    Recovery restart works much like checkpointing in normal operation:614    the server periodically forces all its state to disk, and then updates615    the <code class="filename">pg_control</code> file to indicate that the already-processed616    WAL data need not be scanned again.617   </p></div><div class="sect2" id="BACKUP-TIMELINES"><div class="titlepage"><div><div><h3 class="title">26.3.5. Timelines <a href="#BACKUP-TIMELINES" class="id_link">#</a></h3></div></div></div><a id="id-1.6.13.7.13.2" class="indexterm"></a><p>618    The ability to restore the database to a previous point in time creates619    some complexities that are akin to science-fiction stories about time620    travel and parallel universes.  For example, in the original history of the database,621    suppose you dropped a critical table at 5:15PM on Tuesday evening, but622    didn't realize your mistake until Wednesday noon.623    Unfazed, you get out your backup, restore to the point-in-time 5:14PM624    Tuesday evening, and are up and running.  In <span class="emphasis"><em>this</em></span> history of625    the database universe, you never dropped the table.  But suppose626    you later realize this wasn't such a great idea, and would like627    to return to sometime Wednesday morning in the original history.628    You won't be able629    to if, while your database was up-and-running, it overwrote some of the630    WAL segment files that led up to the time you now wish you631    could get back to.  Thus, to avoid this, you need to distinguish the series of632    WAL records generated after you've done a point-in-time recovery from633    those that were generated in the original database history.634   </p><p>635    To deal with this problem, <span class="productname">PostgreSQL</span> has a notion636    of <em class="firstterm">timelines</em>.  Whenever an archive recovery completes,637    a new timeline is created to identify the series of WAL records638    generated after that recovery.  The timeline639    ID number is part of WAL segment file names so a new timeline does640    not overwrite the WAL data generated by previous timelines.641    For example, in the WAL file name642    <code class="filename">0000000100001234000055CD</code>, the leading643    <code class="literal">00000001</code> is the timeline ID in hexadecimal.  (Note that644    in other contexts, such as server log messages, timeline IDs are645    usually printed in decimal.)646   </p><p>647    It is648    in fact possible to archive many different timelines.  While that might649    seem like a useless feature, it's often a lifesaver.  Consider the650    situation where you aren't quite sure what point-in-time to recover to,651    and so have to do several point-in-time recoveries by trial and error652    until you find the best place to branch off from the old history.  Without653    timelines this process would soon generate an unmanageable mess.  With654    timelines, you can recover to <span class="emphasis"><em>any</em></span> prior state, including655    states in timeline branches that you abandoned earlier.656   </p><p>657    Every time a new timeline is created, <span class="productname">PostgreSQL</span> creates658    a <span class="quote">“<span class="quote">timeline history</span>”</span> file that shows which timeline it branched659    off from and when.  These history files are necessary to allow the system660    to pick the right WAL segment files when recovering from an archive that661    contains multiple timelines.  Therefore, they are archived into the WAL662    archive area just like WAL segment files.  The history files are just663    small text files, so it's cheap and appropriate to keep them around664    indefinitely (unlike the segment files which are large).  You can, if665    you like, add comments to a history file to record your own notes about666    how and why this particular timeline was created.  Such comments will be667    especially valuable when you have a thicket of different timelines as668    a result of experimentation.669   </p><p>670    The default behavior of recovery is to recover to the latest timeline found671    in the archive. If you wish to recover to the timeline that was current672    when the base backup was taken or into a specific child timeline (that673    is, you want to return to some state that was itself generated after a674    recovery attempt), you need to specify <code class="literal">current</code> or the675    target timeline ID in <a class="xref" href="runtime-config-wal.html#GUC-RECOVERY-TARGET-TIMELINE">recovery_target_timeline</a>. You676    cannot recover into timelines that branched off earlier than the base backup.677   </p></div><div class="sect2" id="BACKUP-TIPS"><div class="titlepage"><div><div><h3 class="title">26.3.6. Tips and Examples <a href="#BACKUP-TIPS" class="id_link">#</a></h3></div></div></div><p>678    Some tips for configuring continuous archiving are given here.679   </p><div class="sect3" id="BACKUP-STANDALONE"><div class="titlepage"><div><div><h4 class="title">26.3.6.1. Standalone Hot Backups <a href="#BACKUP-STANDALONE" class="id_link">#</a></h4></div></div></div><p>680      It is possible to use <span class="productname">PostgreSQL</span>'s backup facilities to681      produce standalone hot backups. These are backups that cannot be used682      for point-in-time recovery, yet are typically much faster to backup and683      restore than <span class="application">pg_dump</span> dumps.  (They are also much larger684      than <span class="application">pg_dump</span> dumps, so in some cases the speed advantage685      might be negated.)686     </p><p>687      As with base backups, the easiest way to produce a standalone688      hot backup is to use the <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a>689      tool. If you include the <code class="literal">-X</code> parameter when calling690      it, all the write-ahead log required to use the backup will be691      included in the backup automatically, and no special action is692      required to restore the backup.693     </p></div><div class="sect3" id="COMPRESSED-ARCHIVE-LOGS"><div class="titlepage"><div><div><h4 class="title">26.3.6.2. Compressed Archive Logs <a href="#COMPRESSED-ARCHIVE-LOGS" class="id_link">#</a></h4></div></div></div><p>694      If archive storage size is a concern, you can use695      <span class="application">gzip</span> to compress the archive files:696</p><pre class="programlisting">697archive_command = 'gzip &lt; %p &gt; /mnt/server/archivedir/%f.gz'698</pre><p>699      You will then need to use <span class="application">gunzip</span> during recovery:700</p><pre class="programlisting">701restore_command = 'gunzip &lt; /mnt/server/archivedir/%f.gz &gt; %p'702</pre><p>703     </p></div><div class="sect3" id="BACKUP-SCRIPTS"><div class="titlepage"><div><div><h4 class="title">26.3.6.3. <code class="varname">archive_command</code> Scripts <a href="#BACKUP-SCRIPTS" class="id_link">#</a></h4></div></div></div><p>704      Many people choose to use scripts to define their705      <code class="varname">archive_command</code>, so that their706      <code class="filename">postgresql.conf</code> entry looks very simple:707</p><pre class="programlisting">708archive_command = 'local_backup_script.sh "%p" "%f"'709</pre><p>710      Using a separate script file is advisable any time you want to use711      more than a single command in the archiving process.712      This allows all complexity to be managed within the script, which713      can be written in a popular scripting language such as714      <span class="application">bash</span> or <span class="application">perl</span>.715     </p><p>716      Examples of requirements that might be solved within a script include:717      </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>718         Copying data to secure off-site data storage719        </p></li><li class="listitem"><p>720         Batching WAL files so that they are transferred every three hours,721         rather than one at a time722        </p></li><li class="listitem"><p>723         Interfacing with other backup and recovery software724        </p></li><li class="listitem"><p>725         Interfacing with monitoring software to report errors726        </p></li></ul></div><p>727     </p><div class="tip"><h3 class="title">Tip</h3><p>728       When using an <code class="varname">archive_command</code> script, it's desirable729       to enable <a class="xref" href="runtime-config-logging.html#GUC-LOGGING-COLLECTOR">logging_collector</a>.730       Any messages written to <span class="systemitem">stderr</span> from the script will then731       appear in the database server log, allowing complex configurations to732       be diagnosed easily if they fail.733      </p></div></div></div><div class="sect2" id="CONTINUOUS-ARCHIVING-CAVEATS"><div class="titlepage"><div><div><h3 class="title">26.3.7. Caveats <a href="#CONTINUOUS-ARCHIVING-CAVEATS" class="id_link">#</a></h3></div></div></div><p>734    At this writing, there are several limitations of the continuous archiving735    technique.  These will probably be fixed in future releases:736 737  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>738     If a <a class="link" href="sql-createdatabase.html" title="CREATE DATABASE"><code class="command">CREATE DATABASE</code></a>739     command is executed while a base backup is being taken, and then740     the template database that the <code class="command">CREATE DATABASE</code> copied741     is modified while the base backup is still in progress, it is742     possible that recovery will cause those modifications to be743     propagated into the created database as well.  This is of course744     undesirable.  To avoid this risk, it is best not to modify any745     template databases while taking a base backup.746    </p></li><li class="listitem"><p>747     <a class="link" href="sql-createtablespace.html" title="CREATE TABLESPACE"><code class="command">CREATE TABLESPACE</code></a>748     commands are WAL-logged with the literal absolute path, and will749     therefore be replayed as tablespace creations with the same750     absolute path.  This might be undesirable if the WAL is being751     replayed on a different machine.  It can be dangerous even if the752     WAL is being replayed on the same machine, but into a new data753     directory: the replay will still overwrite the contents of the754     original tablespace.  To avoid potential gotchas of this sort,755     the best practice is to take a new base backup after creating or756     dropping tablespaces.757    </p></li></ul></div><p>758   </p><p>759    It should also be noted that the default <acronym class="acronym">WAL</acronym>760    format is fairly bulky since it includes many disk page snapshots.761    These page snapshots are designed to support crash recovery, since762    we might need to fix partially-written disk pages.  Depending on763    your system hardware and software, the risk of partial writes might764    be small enough to ignore, in which case you can significantly765    reduce the total volume of archived WAL files by turning off page766    snapshots using the <a class="xref" href="runtime-config-wal.html#GUC-FULL-PAGE-WRITES">full_page_writes</a>767    parameter.  (Read the notes and warnings in <a class="xref" href="wal.html" title="Chapter 30. Reliability and the Write-Ahead Log">Chapter 30</a>768    before you do so.)  Turning off page snapshots does not prevent769    use of the WAL for PITR operations.  An area for future770    development is to compress archived WAL data by removing771    unnecessary page copies even when <code class="varname">full_page_writes</code> is772    on.  In the meantime, administrators might wish to reduce the number773    of page snapshots included in WAL by increasing the checkpoint774    interval parameters as much as feasible.775   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="backup-file.html" title="26.2. File System Level Backup">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="high-availability.html" title="Chapter 27. High Availability, Load Balancing, and Replication">Next</a></td></tr><tr><td width="40%" align="left" valign="top">26.2. File System Level Backup </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"> Chapter 27. High Availability, Load Balancing, and Replication</td></tr></table></div></body></html>
codekingpro/portable-devtools · Team Ai