The error log contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.
On some operating systems, the error log contains a stack trace if mysqld dies. The trace can be used to determine where mysqld died. See MySQL Internals: Porting.
You can specify where mysqld writes the error
log with the
--log-error[=
option. If no file_name]file_name value is given,
mysqld uses the name
host_name.errFLUSH
LOGS, the error log is renamed with the suffix
-old and mysqld creates a
new empty log file. (No renaming occurs if the
--log-error option was not given to
mysqld.)
If you do not specify --log-error,
or (on Windows) if you use the
--console option, errors are
written to stderr, the standard error output.
Usually this is your terminal.
On Windows, error output is always written to the
.err file if
--console is not given.
In addition, on Windows, events and error messages are written to
the Windows Event Log within the Application log. Entries marked
as Warning and Note are
written to the Event Log, but informational messages (such as
information statements from individual storage engines) are not
copied to the Event Log. The log entries will have a source of
MySQL. You cannot disable writing information
to the Windows Event Log.
The --log-warnings option or
log_warnings system variable can
be used to control warning logging to the error log. The default
value is enabled (1). Warning logging can be disabled using a
value of 0. If the value is greater than 1, aborted connections
are written to the error log. See
Section B.5.2.11, “Communication Errors and Aborted Connections”.
If you use mysqld_safe to start
mysqld, mysqld_safe arranges
for mysqld to write error messages to a log
file. If you specify a file name via
--log-error to
mysqld_safe or mysqld, that
file name is used. Otherwise, mysqld_safe uses
the default error log file.
If mysqld_safe is used to start
mysqld and mysqld dies
unexpectedly, mysqld_safe notices that it needs
to restart mysqld and writes a
restarted mysqld message to the error log.
User Comments
I did some testing with MySQL 4.0.21 this morning. Here's a typical snippet from my "hostname.err" file. To generate this, I did a "NET START MySQL", connected with one session and ran a 2000-entry query, and then did a "NET STOP MySQL" while the query was still returning data.
MySql: ready for connections.
Version: '4.0.21-nt-log' socket: '' port: 3306 Source distribution
040923 10:00:00 MySql: Normal shutdown
040923 10:00:01 MySql: Forcing close of thread 1 user: 'root'
040923 10:00:01 InnoDB: Starting shutdown...
040923 10:00:03 InnoDB: Shutdown completed
040923 10:00:03 MySql: Shutdown Complete
The Windows Application Event Log recorded 3 messages at the same time. All of the messages corresponded with the entries prefixed with the "MySQL:" entries in the hostname.err file. (OK)
However, all 3 messages were logged as ERRORS; this designation is misleading. If anything, the "Normal Shutdown" and "Shutdown Complete" messages should have been logged as INFORMATION and the "Forcing close of thread..." message should have been logged as a WARNING.
Also, it is important to note that the MySQL service startup was NOT LOGGED in the Event Log.
Long story short, if you are a Windows user, it is probably still best (as of 4.0.21) to stick with your existing "parse-the-.err" script rather than rely on the Windows Event Log if you're interested in MySQL service starts, stops and abnormal events.
It seems that if you are using mysqld_safe (whether you specify --log-error= or not), FLUSH LOGS will not rename and recreate the error log, because mysqld_safe does not pass the --log-error= arg to mysqld; rather, it runs mysqld with stderr redirected to the error log file.
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