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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. Log File Maintenance</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="routine-reindex.html" title="25.2. Routine Reindexing" /><link rel="next" href="backup.html" title="Chapter 26. Backup and Restore" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">25.3. Log File Maintenance</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="routine-reindex.html" title="25.2. Routine Reindexing">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="maintenance.html" title="Chapter 25. Routine Database Maintenance Tasks">Up</a></td><th width="60%" align="center">Chapter 25. Routine Database Maintenance Tasks</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="backup.html" title="Chapter 26. Backup and Restore">Next</a></td></tr></table><hr /></div><div class="sect1" id="LOGFILE-MAINTENANCE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">25.3. Log File Maintenance <a href="#LOGFILE-MAINTENANCE" class="id_link">#</a></h2></div></div></div><a id="id-1.6.12.12.2" class="indexterm"></a><p>
It is a good idea to save the database server's log output
somewhere, rather than just discarding it via <code class="filename">/dev/null</code>.
The log output is invaluable when diagnosing
problems.
</p><div class="note"><h3 class="title">Note</h3><p>
The server log can contain sensitive information and needs to be protected,
no matter how or where it is stored, or the destination to which it is routed.
For example, some DDL statements might contain plaintext passwords or other
authentication details. Logged statements at the <code class="literal">ERROR</code>
level might show the SQL source code for applications
and might also contain some parts of data rows. Recording data, events and
related information is the intended function of this facility, so this is
not a leakage or a bug. Please ensure the server logs are visible only to
appropriately authorized people.
</p></div><p>
Log output tends to be voluminous
(especially at higher debug levels) so you won't want to save it
indefinitely. You need to <span class="emphasis"><em>rotate</em></span> the log files so that
new log files are started and old ones removed after a reasonable
period of time.
</p><p>
If you simply direct the <span class="systemitem">stderr</span> of
<code class="command">postgres</code> into a
file, you will have log output, but
the only way to truncate the log file is to stop and restart
the server. This might be acceptable if you are using
<span class="productname">PostgreSQL</span> in a development environment,
but few production servers would find this behavior acceptable.
</p><p>
A better approach is to send the server's
<span class="systemitem">stderr</span> output to some type of log rotation program.
There is a built-in log rotation facility, which you can use by
setting the configuration parameter <code class="varname">logging_collector</code> to
<code class="literal">true</code> in <code class="filename">postgresql.conf</code>. The control
parameters for this program are described in <a class="xref" href="runtime-config-logging.html#RUNTIME-CONFIG-LOGGING-WHERE" title="20.8.1. Where to Log">Section 20.8.1</a>. You can also use this approach
to capture the log data in machine readable <acronym class="acronym">CSV</acronym>
(comma-separated values) format.
</p><p>
Alternatively, you might prefer to use an external log rotation
program if you have one that you are already using with other
server software. For example, the <span class="application">rotatelogs</span>
tool included in the <span class="productname">Apache</span> distribution
can be used with <span class="productname">PostgreSQL</span>. One way to
do this is to pipe the server's
<span class="systemitem">stderr</span> output to the desired program.
If you start the server with
<code class="command">pg_ctl</code>, then <span class="systemitem">stderr</span>
is already redirected to <span class="systemitem">stdout</span>, so you just need a
pipe command, for example:
</p><pre class="programlisting">
pg_ctl start | rotatelogs /var/log/pgsql_log 86400
</pre><p>
</p><p>
You can combine these approaches by setting up <span class="application">logrotate</span>
to collect log files produced by <span class="productname">PostgreSQL</span> built-in
logging collector. In this case, the logging collector defines the names and
location of the log files, while <span class="application">logrotate</span>
periodically archives these files. When initiating log rotation,
<span class="application">logrotate</span> must ensure that the application
sends further output to the new file. This is commonly done with a
<code class="literal">postrotate</code> script that sends a <code class="literal">SIGHUP</code>
signal to the application, which then reopens the log file.
In <span class="productname">PostgreSQL</span>, you can run <code class="command">pg_ctl</code>
with the <code class="literal">logrotate</code> option instead. When the server receives
this command, the server either switches to a new log file or reopens the
existing file, depending on the logging configuration
(see <a class="xref" href="runtime-config-logging.html#RUNTIME-CONFIG-LOGGING-WHERE" title="20.8.1. Where to Log">Section 20.8.1</a>).
</p><div class="note"><h3 class="title">Note</h3><p>
When using static log file names, the server might fail to reopen the log
file if the max open file limit is reached or a file table overflow occurs.
In this case, log messages are sent to the old log file until a
successful log rotation. If <span class="application">logrotate</span> is
configured to compress the log file and delete it, the server may lose
the messages logged in this time frame. To avoid this issue, you can
configure the logging collector to dynamically assign log file names
and use a <code class="literal">prerotate</code> script to ignore open log files.
</p></div><p>
Another production-grade approach to managing log output is to
send it to <span class="application">syslog</span> and let
<span class="application">syslog</span> deal with file rotation. To do this, set the
configuration parameter <code class="varname">log_destination</code> to <code class="literal">syslog</code>
(to log to <span class="application">syslog</span> only) in
<code class="filename">postgresql.conf</code>. Then you can send a <code class="literal">SIGHUP</code>
signal to the <span class="application">syslog</span> daemon whenever you want to force it
to start writing a new log file. If you want to automate log
rotation, the <span class="application">logrotate</span> program can be
configured to work with log files from
<span class="application">syslog</span>.
</p><p>
On many systems, however, <span class="application">syslog</span> is not very reliable,
particularly with large log messages; it might truncate or drop messages
just when you need them the most. Also, on <span class="productname">Linux</span>,
<span class="application">syslog</span> will flush each message to disk, yielding poor
performance. (You can use a <span class="quote">“<span class="quote"><code class="literal">-</code></span>”</span> at the start of the file name
in the <span class="application">syslog</span> configuration file to disable syncing.)
</p><p>
Note that all the solutions described above take care of starting new
log files at configurable intervals, but they do not handle deletion
of old, no-longer-useful log files. You will probably want to set
up a batch job to periodically delete old log files. Another possibility
is to configure the rotation program so that old log files are overwritten
cyclically.
</p><p>
<a class="ulink" href="https://pgbadger.darold.net/" target="_top"><span class="productname">pgBadger</span></a>
is an external project that does sophisticated log file analysis.
<a class="ulink" href="https://bucardo.org/check_postgres/" target="_top"><span class="productname">check_postgres</span></a>
provides Nagios alerts when important messages appear in the log
files, as well as detection of many other extraordinary conditions.
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