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><A
NAME="LOGFILE-MAINTENANCE"
>23.3. Log File Maintenance</A
></H1
><P
>   It is a good idea to save the database server's log output
   somewhere, rather than just discarding it via <TT
CLASS="FILENAME"
>/dev/null</TT
>.
   The log output is invaluable when diagnosing
   problems.  However, the 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"
><I
CLASS="EMPHASIS"
>rotate</I
></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
   <TT
CLASS="COMMAND"
>postgres</TT
> 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 <TT
CLASS="VARNAME"
>logging_collector</TT
> to
   <TT
CLASS="LITERAL"
>true</TT
> in <TT
CLASS="FILENAME"
>postgresql.conf</TT
>.  The control
   parameters for this program are described in <A
HREF="runtime-config-logging.html#RUNTIME-CONFIG-LOGGING-WHERE"
>Section 18.8.1</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
>.  To do this,
   just pipe the server's
   <SPAN
CLASS="SYSTEMITEM"
>stderr</SPAN
> output to the desired program.
   If you start the server with
   <TT
CLASS="COMMAND"
>pg_ctl</TT
>, 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
>   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 <TT
CLASS="VARNAME"
>log_destination</TT
> to <TT
CLASS="LITERAL"
>syslog</TT
>
   (to log to <SPAN
CLASS="APPLICATION"
>syslog</SPAN
> only) in
   <TT
CLASS="FILENAME"
>postgresql.conf</TT
>. Then you can send a <TT
CLASS="LITERAL"
>SIGHUP</TT
>
   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"
>"<TT
CLASS="LITERAL"
>-</TT
>"</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
HREF="http://pgfouine.projects.postgresql.org/"
TARGET="_top"
><SPAN
CLASS="PRODUCTNAME"
>pgFouine</SPAN
></A
>
   is an external project that does sophisticated log file analysis.
   <A
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.
  </P
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