It is a good idea to save the database server's log output
somewhere, rather than just discarding it via /dev/null.
The log output is invaluable when diagnosing
problems.
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 ERROR
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.
Log output tends to be voluminous (especially at higher debug levels) so you won't want to save it indefinitely. You need to rotate the log files so that new log files are started and old ones removed after a reasonable period of time.
If you simply direct the stderr of
postgres 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
PostgreSQL in a development environment,
but few production servers would find this behavior acceptable.
A better approach is to send the server's
stderr 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 logging_collector to
true in postgresql.conf. The control
parameters for this program are described in Section 19.8.1. You can also use this approach
to capture the log data in machine readable CSV
(comma-separated values) format.
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 rotatelogs
tool included in the Apache distribution
can be used with PostgreSQL. One way to
do this is to pipe the server's
stderr output to the desired program.
If you start the server with
pg_ctl, then stderr
is already redirected to stdout, so you just need a
pipe command, for example:
pg_ctl start | rotatelogs /var/log/pgsql_log 86400
You can combine these approaches by setting up logrotate
to collect log files produced by PostgreSQL built-in
logging collector. In this case, the logging collector defines the names and
location of the log files, while logrotate
periodically archives these files. When initiating log rotation,
logrotate must ensure that the application
sends further output to the new file. This is commonly done with a
postrotate script that sends a SIGHUP
signal to the application, which then reopens the log file.
In PostgreSQL, you can run pg_ctl
with the logrotate 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 Section 19.8.1).
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 logrotate is
configured to compress the log file and delete it, the server may lose
the messages logged in this timeframe. To avoid this issue, you can
configure the logging collector to dynamically assign log file names
and use a prerotate script to ignore open log files.
Another production-grade approach to managing log output is to
send it to syslog and let
syslog deal with file rotation. To do this, set the
configuration parameter log_destination to syslog
(to log to syslog only) in
postgresql.conf. Then you can send a SIGHUP
signal to the syslog daemon whenever you want to force it
to start writing a new log file. If you want to automate log
rotation, the logrotate program can be
configured to work with log files from
syslog.
On many systems, however, syslog is not very reliable,
particularly with large log messages; it might truncate or drop messages
just when you need them the most. Also, on Linux,
syslog will flush each message to disk, yielding poor
performance. (You can use a “-” at the start of the file name
in the syslog configuration file to disable syncing.)
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.
pgBadger is an external project that does sophisticated log file analysis. check_postgres provides Nagios alerts when important messages appear in the log files, as well as detection of many other extraordinary conditions.