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When is the error log filename not the right filename

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When evaluating a MySQL system one of the first things to look at is the MySQL error log. This is defined by the log[_-]error variable in the MySQL Configuration file. Generally found like:

grep log.error /etc/my.cnf
log_error=/var/lib/mysql/logs/mysql_error_log
log-error=/var/lib/mysql/logs/mysql_error_log

It is possible to find multiple rows because this could be defined in the [mysqld] and [mysqld_safe] sections. It is also possible it is incorrectly defined twice in any given section.

Immediately I see a problem here, and the following describes why. If you look at this file name, in this case it’s actually found, but the file is empty.

$ ls -l /var/lib/mysql/logs/mysql_error_log
-rw-r----- 1 mysql mysql 0 Feb 19 20:35 /var/lib/mysql/logs/mysql_error_log

An error log should never exist and be empty, because starting the instance producing messages. An error log could be empty because the system does a log rotate (BTW, never rotate the error log, see The correct approach to rolling MySQL logs). However it is empty in this case because MySQL is not writing to the error log filename as defined, because it does not have file extension. NOTE: there is no .log or similar extension. Looking more closely.

$ ls -l /var/lib/mysql/logs/mysql_error_log*
-rw-r----- 1 mysql mysql      0 Feb 19 20:35 /var/lib/mysql/logs/mysql_error_log
-rw-rw---- 1 mysql root  394530 Feb 19 20:35 /var/lib/mysql/logs/mysql_error_log.err

As you can see, MySQL has overwritten your parameter and given the file an extension, as verified by the runtime value.

mysql> show global variables like 'log_error';
+---------------+-----------------------------------------+
| Variable_name | Value                                   |
+---------------+-----------------------------------------+
| log_error     | /var/lib/mysql/logs/mysql_error_log.err |
+---------------+-----------------------------------------+

So, the tip is, always use .log for your MySQL error (and slow query log).

NOTE: Placing the log files in the MySQL datadir (which defaults to /var/lib/mysql) is also a bad idea. A topic for another blog post at a later time.

Monitoring MySQL – The error log

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It is important that you monitor the MySQL error log. There are a few different options available for defining the details of the log. If not specified the default is [datadir]/[hostname].err. This is not an ideal location for 2 reasons.

First, a correctly configured MySQL datadir will have permissions only for the mysql user, and this is generally restrictive access to the user only and no group or world permissions. Other users/groups should have limited access to the mysql error log.

Second, the datadir is for data, not logs, especially logs that can potentially fill file systems. I am referring here to more then just the error log.

I would recommend you create a separate directory for MySQL logs such as the error, slow and general logs. An example I implement for single installation environments using Linux mysql packages is:

mkdir /var/log/mysql
chown mysql:mysql /var/log/mysql
chmod 750 /var/log/mysql

There does not seem to be a consensus over whether to include the hostname or not in the error log filename. My preference is to not include. I would rather the filename to be consistent across multiple servers. The argument is what about when consolidating logs from multiple servers. I discount this because you have to connect to the server to retrieve logs, create a sub directory of that hostname for consolidated logs.

With Linux distributions you may not find log files where you expect. Ubuntu packages for example has the log going to syslog. While the theory is to make system logging and monitoring easier, it makes MySQL specific monitoring more difficult. You also suffer a logrotate problem where you may only have 7 days of log. I prefer to have access to all historical MySQL log information.

The best choice is to define the error log with log-error, in both the [mysqld_safe] and [mysqld] section of your servers my.cnf

[mysqld_safe]
log-error=/var/log/mysql/error.log

[mysqld]
log-error=/var/log/mysql/error.log

In MySQL 5.1 you have the luxury of different output sources, FILE, TABLE or BOTH for the general log and the slow log with –log-output. No option exists for the error log.

Other my.cnf options to be aware of include:

  • log-warnings | skip-log-warnings
  • syslog | skip-syslog

There is generally also lacking in the standard monitoring products/plugins that present MySQL status information. In my monitoring MySQL solutions I provide a line count of the MySQL error log, so that a delta can be easily detected and then reviewed more proactively.

One issue with a recent client is the lack of access to the physical box by different parties and therefore the lack of access to the log. The identification that something needs to be viewed, then the ability to be able to view is an important problem to be solved.

References

Some other references for MySQL error log monitoring.