When is the error log filename not the right filename

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.

Tagged with: Databases MySQL

Related Posts

Databases MySQL

The correct approach to rolling MySQL logs

I say correct because there are several incorrect approaches to managing MySQL logs. In MySQL you have two important log files, the MySQL error log (configured with –log-error ) and the MySQL slow query log (configured with –log-slow-queries or –slow-query-log and –slow-query-log-file which is available from 5.

Read more
Databases MySQL Open Source Uncategorized

Monitoring MySQL – The error log

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].

Read more
Why Being Proactive Is Always a Winning Approach

Why Being Proactive Is Always a Winning Approach

Many companies manage production infrastructure using a reactive model rather than a proactive one. Organizations typically react to warnings and alerts, then implement corrective actions in response. While some companies have well-designed architectural patterns—such as feature flags and rate limiting—that can quickly mitigate the impact of issues, these are merely temporary solutions, not resolutions.

Read more