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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>25.3. 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 V1.79.1" /><link rel="prev" href="backup-file.html" title="25.2. File System Level Backup" /><link rel="next" href="high-availability.html" title="Chapter 26. High Availability, Load Balancing, and Replication" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">25.3. Continuous Archiving and Point-in-Time Recovery (PITR)</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="backup-file.html" title="25.2. File System Level Backup">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="backup.html" title="Chapter 25. Backup and Restore">Up</a></td><th width="60%" align="center">Chapter 25. Backup and Restore</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="high-availability.html" title="Chapter 26. High Availability, Load Balancing, and Replication">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="CONTINUOUS-ARCHIVING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">25.3. Continuous Archiving and Point-in-Time Recovery (PITR)</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-ARCHIVING-WAL">25.3.1. Setting Up WAL Archiving</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-BASE-BACKUP">25.3.2. Making a Base Backup</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP">25.3.3. Making a Base Backup Using the Low Level API</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-PITR-RECOVERY">25.3.4. Recovering Using a Continuous Archive Backup</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-TIMELINES">25.3.5. Timelines</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#BACKUP-TIPS">25.3.6. Tips and Examples</a></span></dt><dt><span class="sect2"><a href="continuous-archiving.html#CONTINUOUS-ARCHIVING-CAVEATS">25.3.7. Caveats</a></span></dt></dl></div><a id="id-1.6.12.7.2" class="indexterm"></a><a id="id-1.6.12.7.3" class="indexterm"></a><a id="id-1.6.12.7.4" class="indexterm"></a><p>
   At all times, <span class="productname">PostgreSQL</span> maintains a
   <em class="firstterm">write ahead log</em> (WAL) in the <code class="filename">pg_wal/</code>
   subdirectory of the cluster's data directory. The log records
   every change made to the database's data files.  This log exists
   primarily for crash-safety purposes: if the system crashes, the
   database can be restored to consistency by <span class="quote">“<span class="quote">replaying</span>”</span> the
   log entries made since the last checkpoint.  However, the existence
   of the log makes it possible to use a third strategy for backing up
   databases: we can combine a file-system-level backup with backup of
   the WAL files.  If recovery is needed, we restore the file system backup and
   then replay from the backed-up WAL files to bring the system to a
   current state.  This approach is more complex to administer than
   either of the previous approaches, but it has some significant
   benefits:
  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
     We do not need a perfectly consistent file system backup as the starting point.
     Any internal inconsistency in the backup will be corrected by log
     replay (this is not significantly different from what happens during
     crash recovery).  So we do not need a file system snapshot capability,
     just <span class="application">tar</span> or a similar archiving tool.
    </p></li><li class="listitem"><p>
     Since we can combine an indefinitely long sequence of WAL files
     for replay, continuous backup can be achieved simply by continuing to archive
     the WAL files.  This is particularly valuable for large databases, where
     it might not be convenient to take a full backup frequently.
    </p></li><li class="listitem"><p>
     It is not necessary to replay the WAL entries all the
     way to the end.  We could stop the replay at any point and have a
     consistent snapshot of the database as it was at that time.  Thus,
     this technique supports <em class="firstterm">point-in-time recovery</em>: it is
     possible to restore the database to its state at any time since your base
     backup was taken.
    </p></li><li class="listitem"><p>
     If we continuously feed the series of WAL files to another
     machine that has been loaded with the same base backup file, we
     have a <em class="firstterm">warm standby</em> system: at any point we can bring up
     the second machine and it will have a nearly-current copy of the
     database.
    </p></li></ul></div><p>
  </p><div class="note"><h3 class="title">Note</h3><p>
    <span class="application">pg_dump</span> and
    <span class="application">pg_dumpall</span> do not produce file-system-level
    backups and cannot be used as part of a continuous-archiving solution.
    Such dumps are <span class="emphasis"><em>logical</em></span> and do not contain enough
    information to be used by WAL replay.
   </p></div><p>
   As with the plain file-system-backup technique, this method can only
   support restoration of an entire database cluster, not a subset.
   Also, it requires a lot of archival storage: the base backup might be bulky,
   and a busy system will generate many megabytes of WAL traffic that
   have to be archived.  Still, it is the preferred backup technique in
   many situations where high reliability is needed.
  </p><p>
   To recover successfully using continuous archiving (also called
   <span class="quote">“<span class="quote">online backup</span>”</span> by many database vendors), you need a continuous
   sequence of archived WAL files that extends back at least as far as the
   start time of your backup.  So to get started, you should set up and test
   your procedure for archiving WAL files <span class="emphasis"><em>before</em></span> you take your
   first base backup.  Accordingly, we first discuss the mechanics of
   archiving WAL files.
  </p><div class="sect2" id="BACKUP-ARCHIVING-WAL"><div class="titlepage"><div><div><h3 class="title">25.3.1. Setting Up WAL Archiving</h3></div></div></div><p>
    In an abstract sense, a running <span class="productname">PostgreSQL</span> system
    produces an indefinitely long sequence of WAL records.  The system
    physically divides this sequence into WAL <em class="firstterm">segment
    files</em>, which are normally 16MB apiece (although the segment size
    can be altered during <span class="application">initdb</span>).  The segment
    files are given numeric names that reflect their position in the
    abstract WAL sequence.  When not using WAL archiving, the system
    normally creates just a few segment files and then
    <span class="quote">“<span class="quote">recycles</span>”</span> them by renaming no-longer-needed segment files
    to higher segment numbers.  It's assumed that segment files whose
    contents precede the last checkpoint are no longer of
    interest and can be recycled.
   </p><p>
    When archiving WAL data, we need to capture the contents of each segment
    file once it is filled, and save that data somewhere before the segment
    file is recycled for reuse.  Depending on the application and the
    available hardware, there could be many different ways of <span class="quote">“<span class="quote">saving
    the data somewhere</span>”</span>: we could copy the segment files to an NFS-mounted
    directory on another machine, write them onto a tape drive (ensuring that
    you have a way of identifying the original name of each file), or batch
    them together and burn them onto CDs, or something else entirely.  To
    provide the database administrator with flexibility,
    <span class="productname">PostgreSQL</span> tries not to make any assumptions about how
    the archiving will be done.  Instead, <span class="productname">PostgreSQL</span> lets
    the administrator specify a shell command to be executed to copy a
    completed segment file to wherever it needs to go.  The command could be
    as simple as a <code class="literal">cp</code>, or it could invoke a complex shell
    script — it's all up to you.
   </p><p>
    To enable WAL archiving, set the <a class="xref" href="runtime-config-wal.html#GUC-WAL-LEVEL">wal_level</a>
    configuration parameter to <code class="literal">replica</code> or higher,
    <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-MODE">archive_mode</a> to <code class="literal">on</code>,
    and specify the shell command to use in the <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-COMMAND">archive_command</a> configuration parameter.  In practice
    these settings will always be placed in the
    <code class="filename">postgresql.conf</code> file.
    In <code class="varname">archive_command</code>,
    <code class="literal">%p</code> is replaced by the path name of the file to
    archive, while <code class="literal">%f</code> is replaced by only the file name.
    (The path name is relative to the current working directory,
    i.e., the cluster's data directory.)
    Use <code class="literal">%%</code> if you need to embed an actual <code class="literal">%</code>
    character in the command.  The simplest useful command is something
    like:
</p><pre class="programlisting">
archive_command = 'test ! -f /mnt/server/archivedir/%f &amp;&amp; cp %p /mnt/server/archivedir/%f'  # Unix
archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"'  # Windows
</pre><p>
    which will copy archivable WAL segments to the directory
    <code class="filename">/mnt/server/archivedir</code>.  (This is an example, not a
    recommendation, and might not work on all platforms.)  After the
    <code class="literal">%p</code> and <code class="literal">%f</code> parameters have been replaced,
    the actual command executed might look like this:
</p><pre class="programlisting">
test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065
</pre><p>
    A similar command will be generated for each new file to be archived.
   </p><p>
    The archive command will be executed under the ownership of the same
    user that the <span class="productname">PostgreSQL</span> server is running as.  Since
    the series of WAL files being archived contains effectively everything
    in your database, you will want to be sure that the archived data is
    protected from prying eyes; for example, archive into a directory that
    does not have group or world read access.
   </p><p>
    It is important that the archive command return zero exit status if and
    only if it succeeds.  Upon getting a zero result,
    <span class="productname">PostgreSQL</span> will assume that the file has been
    successfully archived, and will remove or recycle it.  However, a nonzero
    status tells <span class="productname">PostgreSQL</span> that the file was not archived;
    it will try again periodically until it succeeds.
   </p><p>
    The archive command should generally be designed to refuse to overwrite
    any pre-existing archive file.  This is an important safety feature to
    preserve the integrity of your archive in case of administrator error
    (such as sending the output of two different servers to the same archive
    directory).
   </p><p>
    It is advisable to test your proposed archive command to ensure that it
    indeed does not overwrite an existing file, <span class="emphasis"><em>and that it returns
    nonzero status in this case</em></span>.
    The example command above for Unix ensures this by including a separate
    <code class="command">test</code> step.  On some Unix platforms, <code class="command">cp</code> has
    switches such as <code class="option">-i</code> that can be used to do the same thing
    less verbosely, but you should not rely on these without verifying that
    the right exit status is returned.  (In particular, GNU <code class="command">cp</code>
    will return status zero when <code class="option">-i</code> is used and the target file
    already exists, which is <span class="emphasis"><em>not</em></span> the desired behavior.)
   </p><p>
    While designing your archiving setup, consider what will happen if
    the archive command fails repeatedly because some aspect requires
    operator intervention or the archive runs out of space. For example, this
    could occur if you write to tape without an autochanger; when the tape
    fills, nothing further can be archived until the tape is swapped.
    You should ensure that any error condition or request to a human operator
    is reported appropriately so that the situation can be
    resolved reasonably quickly. The <code class="filename">pg_wal/</code> directory will
    continue to fill with WAL segment files until the situation is resolved.
    (If the file system containing <code class="filename">pg_wal/</code> fills up,
    <span class="productname">PostgreSQL</span> will do a PANIC shutdown.  No committed
    transactions will be lost, but the database will remain offline until
    you free some space.)
   </p><p>
    The speed of the archiving command is unimportant as long as it can keep up
    with the average rate at which your server generates WAL data.  Normal
    operation continues even if the archiving process falls a little behind.
    If archiving falls significantly behind, this will increase the amount of
    data that would be lost in the event of a disaster. It will also mean that
    the <code class="filename">pg_wal/</code> directory will contain large numbers of
    not-yet-archived segment files, which could eventually exceed available
    disk space. You are advised to monitor the archiving process to ensure that
    it is working as you intend.
   </p><p>
    In writing your archive command, you should assume that the file names to
    be archived can be up to 64 characters long and can contain any
    combination of ASCII letters, digits, and dots.  It is not necessary to
    preserve the original relative path (<code class="literal">%p</code>) but it is necessary to
    preserve the file name (<code class="literal">%f</code>).
   </p><p>
    Note that although WAL archiving will allow you to restore any
    modifications made to the data in your <span class="productname">PostgreSQL</span> database,
    it will not restore changes made to configuration files (that is,
    <code class="filename">postgresql.conf</code>, <code class="filename">pg_hba.conf</code> and
    <code class="filename">pg_ident.conf</code>), since those are edited manually rather
    than through SQL operations.
    You might wish to keep the configuration files in a location that will
    be backed up by your regular file system backup procedures.  See
    <a class="xref" href="runtime-config-file-locations.html" title="19.2. File Locations">Section 19.2</a> for how to relocate the
    configuration files.
   </p><p>
    The archive command is only invoked on completed WAL segments.  Hence,
    if your server generates only little WAL traffic (or has slack periods
    where it does so), there could be a long delay between the completion
    of a transaction and its safe recording in archive storage.  To put
    a limit on how old unarchived data can be, you can set
    <a class="xref" href="runtime-config-wal.html#GUC-ARCHIVE-TIMEOUT">archive_timeout</a> to force the server to switch
    to a new WAL segment file at least that often.  Note that archived
    files that are archived early due to a forced switch are still the same
    length as completely full files.  It is therefore unwise to set a very
    short <code class="varname">archive_timeout</code> — it will bloat your archive
    storage.  <code class="varname">archive_timeout</code> settings of a minute or so are
    usually reasonable.
   </p><p>
    Also, you can force a segment switch manually with
    <code class="function">pg_switch_wal</code> if you want to ensure that a
    just-finished transaction is archived as soon as possible.  Other utility
    functions related to WAL management are listed in <a class="xref" href="functions-admin.html#FUNCTIONS-ADMIN-BACKUP-TABLE" title="Table 9.84. Backup Control Functions">Table 9.84</a>.
   </p><p>
    When <code class="varname">wal_level</code> is <code class="literal">minimal</code> some SQL commands
    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 were
    turned on during execution of one of these statements, WAL would not
    contain enough information for archive recovery.  (Crash recovery is
    unaffected.)  For this reason, <code class="varname">wal_level</code> can only be changed at
    server start.  However, <code class="varname">archive_command</code> can be changed with a
    configuration file reload.  If you wish to temporarily stop archiving,
    one way to do it is to set <code class="varname">archive_command</code> to the empty
    string (<code class="literal">''</code>).
    This will cause WAL files to accumulate in <code class="filename">pg_wal/</code> until a
    working <code class="varname">archive_command</code> is re-established.
   </p></div><div class="sect2" id="BACKUP-BASE-BACKUP"><div class="titlepage"><div><div><h3 class="title">25.3.2. Making a Base Backup</h3></div></div></div><p>
    The easiest way to perform a base backup is to use the
    <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle">pg_basebackup</span></a> tool. It can create
    a base backup either as regular files or as a tar archive. If more
    flexibility than <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle">pg_basebackup</span></a> can provide is
    required, you can also make a base backup using the low level API
    (see <a class="xref" href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP" title="25.3.3. Making a Base Backup Using the Low Level API">Section 25.3.3</a>).
   </p><p>
    It is not necessary to be concerned about the amount of time it takes
    to make a base backup. However, if you normally run the
    server with <code class="varname">full_page_writes</code> disabled, you might notice a drop
    in performance while the backup runs since <code class="varname">full_page_writes</code> is
    effectively forced on during backup mode.
   </p><p>
    To make use of the backup, you will need to keep all the WAL
    segment files generated during and after the file system backup.
    To aid you in doing this, the base backup process
    creates a <em class="firstterm">backup history file</em> that is immediately
    stored into the WAL archive area. This file is named after the first
    WAL segment file that you need for the file system backup.
    For example, if the starting WAL file is
    <code class="literal">0000000100001234000055CD</code> the backup history file will be
    named something like
    <code class="literal">0000000100001234000055CD.007C9330.backup</code>. (The second
    part of the file name stands for an exact position within the WAL
    file, and can ordinarily be ignored.) Once you have safely archived
    the file system backup and the WAL segment files used during the
    backup (as specified in the backup history file), all archived WAL
    segments with names numerically less are no longer needed to recover
    the file system backup and can be deleted. However, you should
    consider keeping several backup sets to be absolutely certain that
    you can recover your data.
   </p><p>
    The backup history file is just a small text file. It contains the
    label string you gave to <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle">pg_basebackup</span></a>, as well as
    the starting and ending times and WAL segments of the backup.
    If you used the label to identify the associated dump file,
    then the archived history file is enough to tell you which dump file to
    restore.
   </p><p>
    Since you have to keep around all the archived WAL files back to your
    last base backup, the interval between base backups should usually be
    chosen based on how much storage you want to expend on archived WAL
    files.  You should also consider how long you are prepared to spend
    recovering, if recovery should be necessary — the system will have to
    replay all those WAL segments, and that could take awhile if it has
    been a long time since the last base backup.
   </p></div><div class="sect2" id="BACKUP-LOWLEVEL-BASE-BACKUP"><div class="titlepage"><div><div><h3 class="title">25.3.3. Making a Base Backup Using the Low Level API</h3></div></div></div><p>
    The procedure for making a base backup using the low level
    APIs contains a few more steps than
    the <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle">pg_basebackup</span></a> method, but is relatively
    simple. It is very important that these steps are executed in
    sequence, and that the success of a step is verified before
    proceeding to the next step.
   </p><p>
    Low level base backups can be made in a non-exclusive or an exclusive
    way. The non-exclusive method is recommended and the exclusive one is
    deprecated and will eventually be removed.
   </p><div class="sect3" id="BACKUP-LOWLEVEL-BASE-BACKUP-NONEXCLUSIVE"><div class="titlepage"><div><div><h4 class="title">25.3.3.1. Making a Non-Exclusive Low-Level Backup</h4></div></div></div><p>
     A non-exclusive low level backup is one that allows other
     concurrent backups to be running (both those started using
     the same backup API and those started using
     <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle">pg_basebackup</span></a>).
    </p><p>
  </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
     Ensure that WAL archiving is enabled and working.
    </p></li><li class="listitem"><p>
     Connect to the server (it does not matter which database) as a user with
     rights to run pg_start_backup (superuser, or a user who has been granted
     EXECUTE on the function) and issue the command:
</p><pre class="programlisting">
SELECT pg_start_backup('label', false, false);
</pre><p>
     where <code class="literal">label</code> is any string you want to use to uniquely
     identify this backup operation. The connection
     calling <code class="function">pg_start_backup</code> must be maintained until the end of
     the backup, or the backup will be automatically aborted.
    </p><p>
     By default, <code class="function">pg_start_backup</code> can take a long time to finish.
     This is because it performs a checkpoint, and the I/O
     required for the checkpoint will be spread out over a significant
     period of time, by default half your inter-checkpoint interval
     (see the configuration parameter
     <a class="xref" href="runtime-config-wal.html#GUC-CHECKPOINT-COMPLETION-TARGET">checkpoint_completion_target</a>).  This is
     usually what you want, because it minimizes the impact on query
     processing.  If you want to start the backup as soon as
     possible, change the second parameter to <code class="literal">true</code>, which will
     issue an immediate checkpoint using as much I/O as available.
    </p><p>
     The third parameter being <code class="literal">false</code> tells
     <code class="function">pg_start_backup</code> to initiate a non-exclusive base backup.
    </p></li><li class="listitem"><p>
     Perform the backup, using any convenient file-system-backup tool
     such as <span class="application">tar</span> or <span class="application">cpio</span> (not
     <span class="application">pg_dump</span> or
     <span class="application">pg_dumpall</span>).  It is neither
     necessary nor desirable to stop normal operation of the database
     while you do this. See
     <a class="xref" href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP-DATA" title="25.3.3.3. Backing Up the Data Directory">Section 25.3.3.3</a> for things to
     consider during this backup.
    </p></li><li class="listitem"><p>
     In the same connection as before, issue the command:
</p><pre class="programlisting">
SELECT * FROM pg_stop_backup(false, true);
</pre><p>
     This terminates backup mode. On a primary, it also performs an automatic
     switch to the next WAL segment.  On a standby, it is not possible to
     automatically switch WAL segments, so you may wish to run
     <code class="function">pg_switch_wal</code> on the primary to perform a manual
     switch. The reason for the switch is to arrange for
     the last WAL segment file written during the backup interval to be
     ready to archive.
    </p><p>
     The <code class="function">pg_stop_backup</code> will return one row with three
     values. The second of these fields should be written to a file named
     <code class="filename">backup_label</code> in the root directory of the backup. The
     third field should be written to a file named
     <code class="filename">tablespace_map</code> unless the field is empty. These files are
     vital to the backup working, and must be written without modification.
    </p></li><li class="listitem"><p>
     Once the WAL segment files active during the backup are archived, you are
     done.  The file identified by <code class="function">pg_stop_backup</code>'s first return
     value is the last segment that is required to form a complete set of
     backup files.  On a primary, if <code class="varname">archive_mode</code> is enabled and the
     <code class="literal">wait_for_archive</code> parameter is <code class="literal">true</code>,
     <code class="function">pg_stop_backup</code> does not return until the last segment has
     been archived.
     On a standby, <code class="varname">archive_mode</code> must be <code class="literal">always</code> in order
     for <code class="function">pg_stop_backup</code> to wait.
     Archiving of these files happens automatically since you have
     already configured <code class="varname">archive_command</code>. In most cases this
     happens quickly, but you are advised to monitor your archive
     system to ensure there are no delays.
     If the archive process has fallen behind
     because of failures of the archive command, it will keep retrying
     until the archive succeeds and the backup is complete.
     If you wish to place a time limit on the execution of
     <code class="function">pg_stop_backup</code>, set an appropriate
     <code class="varname">statement_timeout</code> value, but make note that if
     <code class="function">pg_stop_backup</code> terminates because of this your backup
     may not be valid.
    </p><p>
     If the backup process monitors and ensures that all WAL segment files
     required for the backup are successfully archived then the
     <code class="literal">wait_for_archive</code> parameter (which defaults to true) can be set
     to false to have
     <code class="function">pg_stop_backup</code> return as soon as the stop backup record is
     written to the WAL.  By default, <code class="function">pg_stop_backup</code> will wait
     until all WAL has been archived, which can take some time.  This option
     must be used with caution: if WAL archiving is not monitored correctly
     then the backup might not include all of the WAL files and will
     therefore be incomplete and not able to be restored.
    </p></li></ol></div><p>
    </p></div><div class="sect3" id="BACKUP-LOWLEVEL-BASE-BACKUP-EXCLUSIVE"><div class="titlepage"><div><div><h4 class="title">25.3.3.2. Making an Exclusive Low-Level Backup</h4></div></div></div><div class="note"><h3 class="title">Note</h3><p>
      The exclusive backup method is deprecated and should be avoided.
      Prior to <span class="productname">PostgreSQL</span> 9.6, this was the only
      low-level method available, but it is now recommended that all users
      upgrade their scripts to use non-exclusive backups.
     </p></div><p>
     The process for an exclusive backup is mostly the same as for a
     non-exclusive one, but it differs in a few key steps. This type of
     backup can only be taken on a primary and does not allow concurrent
     backups.  Moreover, because it creates a backup label file, as
     described below, it can block automatic restart of the master server
     after a crash.  On the other hand, the erroneous removal of this
     file from a backup or standby is a common mistake, which can result
     in serious data corruption.  If it is necessary to use this method,
     the following steps may be used.
    </p><p>
  </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
     Ensure that WAL archiving is enabled and working.
    </p></li><li class="listitem"><p>
     Connect to the server (it does not matter which database) as a user with
     rights to run pg_start_backup (superuser, or a user who has been granted
     EXECUTE on the function) and issue the command:
</p><pre class="programlisting">
SELECT pg_start_backup('label');
</pre><p>
     where <code class="literal">label</code> is any string you want to use to uniquely
     identify this backup operation.
     <code class="function">pg_start_backup</code> creates a <em class="firstterm">backup label</em> file,
     called <code class="filename">backup_label</code>, in the cluster directory with
     information about your backup, including the start time and label string.
     The function also creates a <em class="firstterm">tablespace map</em> file,
     called <code class="filename">tablespace_map</code>, in the cluster directory with
     information about tablespace symbolic links in <code class="filename">pg_tblspc/</code> if
     one or more such link is present.  Both files are critical to the
     integrity of the backup, should you need to restore from it.
    </p><p>
     By default, <code class="function">pg_start_backup</code> can take a long time to finish.
     This is because it performs a checkpoint, and the I/O
     required for the checkpoint will be spread out over a significant
     period of time, by default half your inter-checkpoint interval
     (see the configuration parameter
     <a class="xref" href="runtime-config-wal.html#GUC-CHECKPOINT-COMPLETION-TARGET">checkpoint_completion_target</a>).  This is
     usually what you want, because it minimizes the impact on query
     processing.  If you want to start the backup as soon as
     possible, use:
</p><pre class="programlisting">
SELECT pg_start_backup('label', true);
</pre><p>
     This forces the checkpoint to be done as quickly as possible.
    </p></li><li class="listitem"><p>
     Perform the backup, using any convenient file-system-backup tool
     such as <span class="application">tar</span> or <span class="application">cpio</span> (not
     <span class="application">pg_dump</span> or
     <span class="application">pg_dumpall</span>).  It is neither
     necessary nor desirable to stop normal operation of the database
     while you do this. See
     <a class="xref" href="continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP-DATA" title="25.3.3.3. Backing Up the Data Directory">Section 25.3.3.3</a> for things to
     consider during this backup.
    </p><p>
     As noted above, if the server crashes during the backup it may not be
     possible to restart until the <code class="filename">backup_label</code> file has
     been manually deleted from the <code class="envar">PGDATA</code> directory.  Note
     that it is very important to never remove the
     <code class="filename">backup_label</code> file when restoring a backup, because
     this will result in corruption.  Confusion about when it is appropriate
     to remove this file is a common cause of data corruption when using this
     method; be very certain that you remove the file only on an existing
     master and never when building a standby or restoring a backup, even if
     you are building a standby that will subsequently be promoted to a new
     master.
    </p></li><li class="listitem"><p>
     Again connect to the database as a user with rights to run
     pg_stop_backup (superuser, or a user who has been granted EXECUTE on
     the function), and issue the command:
</p><pre class="programlisting">
SELECT pg_stop_backup();
</pre><p>
     This function terminates backup mode and
     performs an automatic switch to the next WAL segment. The reason for the
     switch is to arrange for the last WAL segment written during the backup
     interval to be ready to archive.
    </p></li><li class="listitem"><p>
     Once the WAL segment files active during the backup are archived, you are
     done.  The file identified by <code class="function">pg_stop_backup</code>'s result is
     the last segment that is required to form a complete set of backup files.
     If <code class="varname">archive_mode</code> is enabled,
     <code class="function">pg_stop_backup</code> does not return until the last segment has
     been archived.
     Archiving of these files happens automatically since you have
     already configured <code class="varname">archive_command</code>. In most cases this
     happens quickly, but you are advised to monitor your archive
     system to ensure there are no delays.
     If the archive process has fallen behind
     because of failures of the archive command, it will keep retrying
     until the archive succeeds and the backup is complete.
    </p><p>
     When using exclusive backup mode, it is absolutely imperative to ensure
     that <code class="function">pg_stop_backup</code> completes successfully at the
     end of the backup.  Even if the backup itself fails, for example due to
     lack of disk space, failure to call <code class="function">pg_stop_backup</code>
     will leave the server in backup mode indefinitely, causing future backups
     to fail and increasing the risk of a restart failure during the time that
     <code class="filename">backup_label</code> exists.
    </p></li></ol></div><p>
    </p></div><div class="sect3" id="BACKUP-LOWLEVEL-BASE-BACKUP-DATA"><div class="titlepage"><div><div><h4 class="title">25.3.3.3. Backing Up the Data Directory</h4></div></div></div><p>
    Some file system backup tools emit warnings or errors
    if the files they are trying to copy change while the copy proceeds.
    When taking a base backup of an active database, this situation is normal
    and not an error.  However, you need to ensure that you can distinguish
    complaints of this sort from real errors.  For example, some versions
    of <span class="application">rsync</span> return a separate exit code for
    <span class="quote">“<span class="quote">vanished source files</span>”</span>, and you can write a driver script to
    accept this exit code as a non-error case.  Also, some versions of
    GNU <span class="application">tar</span> return an error code indistinguishable from
    a fatal error if a file was truncated while <span class="application">tar</span> was
    copying it.  Fortunately, GNU <span class="application">tar</span> versions 1.16 and
    later exit with 1 if a file was changed during the backup,
    and 2 for other errors.  With GNU <span class="application">tar</span> version 1.23 and
    later, you can use the warning options <code class="literal">--warning=no-file-changed
    --warning=no-file-removed</code> to hide the related warning messages.
   </p><p>
    Be certain that your backup includes all of the files under
    the database cluster directory (e.g., <code class="filename">/usr/local/pgsql/data</code>).
    If you are using tablespaces that do not reside underneath this directory,
    be careful to include them as well (and be sure that your backup
    archives symbolic links as links, otherwise the restore will corrupt
    your tablespaces).
   </p><p>
    You should, however, omit from the backup the files within the
    cluster's <code class="filename">pg_wal/</code> subdirectory.  This
    slight adjustment is worthwhile because it reduces the risk
    of mistakes when restoring.  This is easy to arrange if
    <code class="filename">pg_wal/</code> is a symbolic link pointing to someplace outside
    the cluster directory, which is a common setup anyway for performance
    reasons.  You might also want to exclude <code class="filename">postmaster.pid</code>
    and <code class="filename">postmaster.opts</code>, which record information
    about the running <span class="application">postmaster</span>, not about the
    <span class="application">postmaster</span> which will eventually use this backup.
    (These files can confuse <span class="application">pg_ctl</span>.)
   </p><p>
    It is often a good idea to also omit from the backup the files
    within the cluster's <code class="filename">pg_replslot/</code> directory, so that
    replication slots that exist on the master do not become part of the
    backup.  Otherwise, the subsequent use of the backup to create a standby
    may result in indefinite retention of WAL files on the standby, and
    possibly bloat on the master if hot standby feedback is enabled, because
    the clients that are using those replication slots will still be connecting
    to and updating the slots on the master, not the standby.  Even if the
    backup is only intended for use in creating a new master, copying the
    replication slots isn't expected to be particularly useful, since the
    contents of those slots will likely be badly out of date by the time
    the new master comes on line.
   </p><p>
    The contents of the directories <code class="filename">pg_dynshmem/</code>,
    <code class="filename">pg_notify/</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> (but not the directories themselves) can be
    omitted from the backup as they will be initialized on postmaster startup.
    If <a class="xref" href="runtime-config-statistics.html#GUC-STATS-TEMP-DIRECTORY">stats_temp_directory</a> is set and is under the data
    directory then the contents of that directory can also be omitted.
   </p><p>
    Any file or directory beginning with <code class="filename">pgsql_tmp</code> can be
    omitted from the backup.  These files are removed on postmaster start and
    the directories will be recreated as needed.
   </p><p>
    <code class="filename">pg_internal.init</code> files can be omitted from the
    backup whenever a file of that name is found.  These files contain
    relation cache data that is always rebuilt when recovering.
   </p><p>
    The backup label
    file includes the label string you gave to <code class="function">pg_start_backup</code>,
    as well as the time at which <code class="function">pg_start_backup</code> was run, and
    the name of the starting WAL file.  In case of confusion it is therefore
    possible to look inside a backup file and determine exactly which
    backup session the dump file came from.  The tablespace map file includes
    the symbolic link names as they exist in the directory
    <code class="filename">pg_tblspc/</code> and the full path of each symbolic link.
    These files are not merely for your information; their presence and
    contents are critical to the proper operation of the system's recovery
    process.
   </p><p>
    It is also possible to make a backup while the server is
    stopped.  In this case, you obviously cannot use
    <code class="function">pg_start_backup</code> or <code class="function">pg_stop_backup</code>, and
    you will therefore be left to your own devices to keep track of which
    backup is which and how far back the associated WAL files go.
    It is generally better to follow the continuous archiving procedure above.
   </p></div></div><div class="sect2" id="BACKUP-PITR-RECOVERY"><div class="titlepage"><div><div><h3 class="title">25.3.4. Recovering Using a Continuous Archive Backup</h3></div></div></div><p>
    Okay, the worst has happened and you need to recover from your backup.
    Here is the procedure:
  </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
     Stop the server, if it's running.
    </p></li><li class="listitem"><p>
     If you have the space to do so,
     copy the whole cluster data directory and any tablespaces to a temporary
     location in case you need them later. Note that this precaution will
     require that you have enough free space on your system to hold two
     copies of your existing database. If you do not have enough space,
     you should at least save the contents of the cluster's <code class="filename">pg_wal</code>
     subdirectory, as it might contain logs which
     were not archived before the system went down.
    </p></li><li class="listitem"><p>
     Remove all existing files and subdirectories under the cluster data
     directory and under the root directories of any tablespaces you are using.
    </p></li><li class="listitem"><p>
     Restore the database files from your file system backup.  Be sure that they
     are restored with the right ownership (the database system user, not
     <code class="literal">root</code>!) and with the right permissions.  If you are using
     tablespaces,
     you should verify that the symbolic links in <code class="filename">pg_tblspc/</code>
     were correctly restored.
    </p></li><li class="listitem"><p>
     Remove any files present in <code class="filename">pg_wal/</code>; these came from the
     file system backup and are therefore probably obsolete rather than current.
     If you didn't archive <code class="filename">pg_wal/</code> at all, then recreate
     it with proper permissions,
     being careful to ensure that you re-establish it as a symbolic link
     if you had it set up that way before.
    </p></li><li class="listitem"><p>
     If you have unarchived WAL segment files that you saved in step 2,
     copy them into <code class="filename">pg_wal/</code>.  (It is best to copy them,
     not move them, so you still have the unmodified files if a
     problem occurs and you have to start over.)
    </p></li><li class="listitem"><p>
     Set recovery configuration settings in
     <code class="filename">postgresql.conf</code> (see <a class="xref" href="runtime-config-wal.html#RUNTIME-CONFIG-WAL-ARCHIVE-RECOVERY" title="19.5.4. Archive Recovery">Section 19.5.4</a>) and create a file
     <code class="filename">recovery.signal</code> in the cluster
     data directory. You might
     also want to temporarily modify <code class="filename">pg_hba.conf</code> to prevent
     ordinary users from connecting until you are sure the recovery was successful.
    </p></li><li class="listitem"><p>
     Start the server.  The server will go into recovery mode and
     proceed to read through the archived WAL files it needs.  Should the
     recovery be terminated because of an external error, the server can
     simply be restarted and it will continue recovery.  Upon completion
     of the recovery process, the server will remove
     <code class="filename">recovery.signal</code> (to prevent
     accidentally re-entering recovery mode later) and then
     commence normal database operations.
    </p></li><li class="listitem"><p>
     Inspect the contents of the database to ensure you have recovered to
     the desired state.  If not, return to step 1.  If all is well,
     allow your users to connect by restoring <code class="filename">pg_hba.conf</code> to normal.
    </p></li></ol></div><p>
   </p><p>
    The key part of all this is to set up a recovery configuration that
    describes how you want to recover and how far the recovery should
    run.  The one thing that you absolutely must specify is the <code class="varname">restore_command</code>,
    which tells <span class="productname">PostgreSQL</span> how to retrieve archived
    WAL file segments.  Like the <code class="varname">archive_command</code>, this is
    a shell command string.  It can contain <code class="literal">%f</code>, which is
    replaced by the name of the desired log file, and <code class="literal">%p</code>,
    which is replaced by the path name to copy the log file to.
    (The path name is relative to the current working directory,
    i.e., the cluster's data directory.)
    Write <code class="literal">%%</code> if you need to embed an actual <code class="literal">%</code>
    character in the command.  The simplest useful command is
    something like:
</p><pre class="programlisting">
restore_command = 'cp /mnt/server/archivedir/%f %p'
</pre><p>
    which will copy previously archived WAL segments from the directory
    <code class="filename">/mnt/server/archivedir</code>.  Of course, you can use something
    much more complicated, perhaps even a shell script that requests the
    operator to mount an appropriate tape.
   </p><p>
    It is important that the command return nonzero exit status on failure.
    The command <span class="emphasis"><em>will</em></span> be called requesting files that are not
    present in the archive; it must return nonzero when so asked.  This is not
    an error condition.  An exception is that if the command was terminated by
    a signal (other than <span class="systemitem">SIGTERM</span>, which is used as
    part of a database server shutdown) or an error by the shell (such as
    command not found), then recovery will abort and the server will not start
    up.
   </p><p>
    Not all of the requested files will be WAL segment
    files; you should also expect requests for files with a suffix of
    <code class="literal">.history</code>. Also be aware that
    the base name of the <code class="literal">%p</code> path will be different from
    <code class="literal">%f</code>; do not expect them to be interchangeable.
   </p><p>
    WAL segments that cannot be found in the archive will be sought in
    <code class="filename">pg_wal/</code>; this allows use of recent un-archived segments.
    However, segments that are available from the archive will be used in
    preference to files in <code class="filename">pg_wal/</code>.
   </p><p>
    Normally, recovery will proceed through all available WAL segments,
    thereby restoring the database to the current point in time (or as
    close as possible given the available WAL segments).  Therefore, a normal
    recovery will end with a <span class="quote">“<span class="quote">file not found</span>”</span> message, the exact text
    of the error message depending upon your choice of
    <code class="varname">restore_command</code>.  You may also see an error message
    at the start of recovery for a file named something like
    <code class="filename">00000001.history</code>.  This is also normal and does not
    indicate a problem in simple recovery situations; see
    <a class="xref" href="continuous-archiving.html#BACKUP-TIMELINES" title="25.3.5. Timelines">Section 25.3.5</a> for discussion.
   </p><p>
    If you want to recover to some previous point in time (say, right before
    the junior DBA dropped your main transaction table), just specify the
    required <a class="link" href="runtime-config-wal.html#RUNTIME-CONFIG-WAL-RECOVERY-TARGET" title="19.5.5. Recovery Target">stopping point</a>.  You can specify
    the stop point, known as the <span class="quote">“<span class="quote">recovery target</span>”</span>, either by
    date/time, named restore point or by completion of a specific transaction
    ID.  As of this writing only the date/time and named restore point options
    are very usable, since there are no tools to help you identify with any
    accuracy which transaction ID to use.
   </p><div class="note"><h3 class="title">Note</h3><p>
      The stop point must be after the ending time of the base backup, i.e.,
      the end time of <code class="function">pg_stop_backup</code>.  You cannot use a base backup
      to recover to a time when that backup was in progress.  (To
      recover to such a time, you must go back to your previous base backup
      and roll forward from there.)
     </p></div><p>
    If recovery finds corrupted WAL data, recovery will
    halt at that point and the server will not start. In such a case the
    recovery process could be re-run from the beginning, specifying a
    <span class="quote">“<span class="quote">recovery target</span>”</span> before the point of corruption so that recovery
    can complete normally.
    If recovery fails for an external reason, such as a system crash or
    if the WAL archive has become inaccessible, then the recovery can simply
    be restarted and it will restart almost from where it failed.
    Recovery restart works much like checkpointing in normal operation:
    the server periodically forces all its state to disk, and then updates
    the <code class="filename">pg_control</code> file to indicate that the already-processed
    WAL data need not be scanned again.
   </p></div><div class="sect2" id="BACKUP-TIMELINES"><div class="titlepage"><div><div><h3 class="title">25.3.5. Timelines</h3></div></div></div><a id="id-1.6.12.7.13.2" class="indexterm"></a><p>
    The ability to restore the database to a previous point in time creates
    some complexities that are akin to science-fiction stories about time
    travel and parallel universes.  For example, in the original history of the database,
    suppose you dropped a critical table at 5:15PM on Tuesday evening, but
    didn't realize your mistake until Wednesday noon.
    Unfazed, you get out your backup, restore to the point-in-time 5:14PM
    Tuesday evening, and are up and running.  In <span class="emphasis"><em>this</em></span> history of
    the database universe, you never dropped the table.  But suppose
    you later realize this wasn't such a great idea, and would like
    to return to sometime Wednesday morning in the original history.
    You won't be able
    to if, while your database was up-and-running, it overwrote some of the
    WAL segment files that led up to the time you now wish you
    could get back to.  Thus, to avoid this, you need to distinguish the series of
    WAL records generated after you've done a point-in-time recovery from
    those that were generated in the original database history.
   </p><p>
    To deal with this problem, <span class="productname">PostgreSQL</span> has a notion
    of <em class="firstterm">timelines</em>.  Whenever an archive recovery completes,
    a new timeline is created to identify the series of WAL records
    generated after that recovery.  The timeline
    ID number is part of WAL segment file names so a new timeline does
    not overwrite the WAL data generated by previous timelines.  It is
    in fact possible to archive many different timelines.  While that might
    seem like a useless feature, it's often a lifesaver.  Consider the
    situation where you aren't quite sure what point-in-time to recover to,
    and so have to do several point-in-time recoveries by trial and error
    until you find the best place to branch off from the old history.  Without
    timelines this process would soon generate an unmanageable mess.  With
    timelines, you can recover to <span class="emphasis"><em>any</em></span> prior state, including
    states in timeline branches that you abandoned earlier.
   </p><p>
    Every time a new timeline is created, <span class="productname">PostgreSQL</span> creates
    a <span class="quote">“<span class="quote">timeline history</span>”</span> file that shows which timeline it branched
    off from and when.  These history files are necessary to allow the system
    to pick the right WAL segment files when recovering from an archive that
    contains multiple timelines.  Therefore, they are archived into the WAL
    archive area just like WAL segment files.  The history files are just
    small text files, so it's cheap and appropriate to keep them around
    indefinitely (unlike the segment files which are large).  You can, if
    you like, add comments to a history file to record your own notes about
    how and why this particular timeline was created.  Such comments will be
    especially valuable when you have a thicket of different timelines as
    a result of experimentation.
   </p><p>
    The default behavior of recovery is to recover along the same timeline
    that was current when the base backup was taken.  If you wish to recover
    into some child timeline (that is, you want to return to some state that
    was itself generated after a recovery attempt), you need to specify the
    target timeline ID in <a class="xref" href="runtime-config-wal.html#GUC-RECOVERY-TARGET-TIMELINE">recovery_target_timeline</a>. You cannot recover into
    timelines that branched off earlier than the base backup.
   </p></div><div class="sect2" id="BACKUP-TIPS"><div class="titlepage"><div><div><h3 class="title">25.3.6. Tips and Examples</h3></div></div></div><p>
    Some tips for configuring continuous archiving are given here.
   </p><div class="sect3" id="BACKUP-STANDALONE"><div class="titlepage"><div><div><h4 class="title">25.3.6.1. Standalone Hot Backups</h4></div></div></div><p>
      It is possible to use <span class="productname">PostgreSQL</span>'s backup facilities to
      produce standalone hot backups. These are backups that cannot be used
      for point-in-time recovery, yet are typically much faster to backup and
      restore than <span class="application">pg_dump</span> dumps.  (They are also much larger
      than <span class="application">pg_dump</span> dumps, so in some cases the speed advantage
      might be negated.)
     </p><p>
      As with base backups, the easiest way to produce a standalone
      hot backup is to use the <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle">pg_basebackup</span></a>
      tool. If you include the <code class="literal">-X</code> parameter when calling
      it, all the write-ahead log required to use the backup will be
      included in the backup automatically, and no special action is
      required to restore the backup.
     </p><p>
      If more flexibility in copying the backup files is needed, a lower
      level process can be used for standalone hot backups as well.
      To prepare for low level standalone hot backups, make sure
      <code class="varname">wal_level</code> is set to
      <code class="literal">replica</code> or higher, <code class="varname">archive_mode</code> to
      <code class="literal">on</code>, and set up an <code class="varname">archive_command</code> that performs
      archiving only when a <span class="emphasis"><em>switch file</em></span> exists.  For example:
</p><pre class="programlisting">
archive_command = 'test ! -f /var/lib/pgsql/backup_in_progress || (test ! -f /var/lib/pgsql/archive/%f &amp;&amp; cp %p /var/lib/pgsql/archive/%f)'
</pre><p>
      This command will perform archiving when
      <code class="filename">/var/lib/pgsql/backup_in_progress</code> exists, and otherwise
      silently return zero exit status (allowing <span class="productname">PostgreSQL</span>
      to recycle the unwanted WAL file).
     </p><p>
      With this preparation, a backup can be taken using a script like the
      following:
</p><pre class="programlisting">
touch /var/lib/pgsql/backup_in_progress
psql -c "select pg_start_backup('hot_backup');"
tar -cf /var/lib/pgsql/backup.tar /var/lib/pgsql/data/
psql -c "select pg_stop_backup();"
rm /var/lib/pgsql/backup_in_progress
tar -rf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/
</pre><p>
      The switch file <code class="filename">/var/lib/pgsql/backup_in_progress</code> is
      created first, enabling archiving of completed WAL files to occur.
      After the backup the switch file is removed. Archived WAL files are
      then added to the backup so that both base backup and all required
      WAL files are part of the same <span class="application">tar</span> file.
      Please remember to add error handling to your backup scripts.
     </p></div><div class="sect3" id="COMPRESSED-ARCHIVE-LOGS"><div class="titlepage"><div><div><h4 class="title">25.3.6.2. Compressed Archive Logs</h4></div></div></div><p>
      If archive storage size is a concern, you can use
      <span class="application">gzip</span> to compress the archive files:
</p><pre class="programlisting">
archive_command = 'gzip &lt; %p &gt; /var/lib/pgsql/archive/%f'
</pre><p>
      You will then need to use <span class="application">gunzip</span> during recovery:
</p><pre class="programlisting">
restore_command = 'gunzip &lt; /mnt/server/archivedir/%f &gt; %p'
</pre><p>
     </p></div><div class="sect3" id="BACKUP-SCRIPTS"><div class="titlepage"><div><div><h4 class="title">25.3.6.3. <code class="varname">archive_command</code> Scripts</h4></div></div></div><p>
      Many people choose to use scripts to define their
      <code class="varname">archive_command</code>, so that their
      <code class="filename">postgresql.conf</code> entry looks very simple:
</p><pre class="programlisting">
archive_command = 'local_backup_script.sh "%p" "%f"'
</pre><p>
      Using a separate script file is advisable any time you want to use
      more than a single command in the archiving process.
      This allows all complexity to be managed within the script, which
      can be written in a popular scripting language such as
      <span class="application">bash</span> or <span class="application">perl</span>.
     </p><p>
      Examples of requirements that might be solved within a script include:
      </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
         Copying data to secure off-site data storage
        </p></li><li class="listitem"><p>
         Batching WAL files so that they are transferred every three hours,
         rather than one at a time
        </p></li><li class="listitem"><p>
         Interfacing with other backup and recovery software
        </p></li><li class="listitem"><p>
         Interfacing with monitoring software to report errors
        </p></li></ul></div><p>
     </p><div class="tip"><h3 class="title">Tip</h3><p>
       When using an <code class="varname">archive_command</code> script, it's desirable
       to enable <a class="xref" href="runtime-config-logging.html#GUC-LOGGING-COLLECTOR">logging_collector</a>.
       Any messages written to <span class="systemitem">stderr</span> from the script will then
       appear in the database server log, allowing complex configurations to
       be diagnosed easily if they fail.
      </p></div></div></div><div class="sect2" id="CONTINUOUS-ARCHIVING-CAVEATS"><div class="titlepage"><div><div><h3 class="title">25.3.7. Caveats</h3></div></div></div><p>
    At this writing, there are several limitations of the continuous archiving
    technique.  These will probably be fixed in future releases:

  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
     If a <a class="xref" href="sql-createdatabase.html" title="CREATE DATABASE"><span class="refentrytitle">CREATE DATABASE</span></a>
     command is executed while a base backup is being taken, and then
     the template database that the <code class="command">CREATE DATABASE</code> copied
     is modified while the base backup is still in progress, it is
     possible that recovery will cause those modifications to be
     propagated into the created database as well.  This is of course
     undesirable.  To avoid this risk, it is best not to modify any
     template databases while taking a base backup.
    </p></li><li class="listitem"><p>
     <a class="xref" href="sql-createtablespace.html" title="CREATE TABLESPACE"><span class="refentrytitle">CREATE TABLESPACE</span></a>
     commands are WAL-logged with the literal absolute path, and will
     therefore be replayed as tablespace creations with the same
     absolute path.  This might be undesirable if the log is being
     replayed on a different machine.  It can be dangerous even if the
     log is being replayed on the same machine, but into a new data
     directory: the replay will still overwrite the contents of the
     original tablespace.  To avoid potential gotchas of this sort,
     the best practice is to take a new base backup after creating or
     dropping tablespaces.
    </p></li></ul></div><p>
   </p><p>
    It should also be noted that the default <acronym class="acronym">WAL</acronym>
    format is fairly bulky since it includes many disk page snapshots.
    These page snapshots are designed to support crash recovery, since
    we might need to fix partially-written disk pages.  Depending on
    your system hardware and software, the risk of partial writes might
    be small enough to ignore, in which case you can significantly
    reduce the total volume of archived logs by turning off page
    snapshots using the <a class="xref" href="runtime-config-wal.html#GUC-FULL-PAGE-WRITES">full_page_writes</a>
    parameter.  (Read the notes and warnings in <a class="xref" href="wal.html" title="Chapter 29. Reliability and the Write-Ahead Log">Chapter 29</a>
    before you do so.)  Turning off page snapshots does not prevent
    use of the logs for PITR operations.  An area for future
    development is to compress archived WAL data by removing
    unnecessary page copies even when <code class="varname">full_page_writes</code> is
    on.  In the meantime, administrators might wish to reduce the number
    of page snapshots included in WAL by increasing the checkpoint
    interval parameters as much as feasible.
   </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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="backup.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="high-availability.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">25.2. File System Level Backup </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 26. High Availability, Load Balancing, and Replication</td></tr></table></div></body></html>