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 the option is given with no
file_name]file_name value,
mysqld uses the name
host_name.err
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 have a source of
MySQL. You cannot disable writing information
to the Windows Event Log.
If you flush the logs using
FLUSH LOGS or
mysqladmin flush-logs and
mysqld is writing the error log to a file (for
example, if it was started with the
--log-error option), the effect is
version dependent:
As of MySQL 5.5.7, the server closes and reopens the log file. To rename the file, you can do so manually before flushing. Then flushing the logs reopens a new file with the original file name. For example, you can rename the file and create a new one using the following commands:
shell>
mvshell>host_name.errhost_name.err-oldmysqladmin flush-logsshell>mvhost_name.err-oldbackup-directoryOn Windows, use rename rather than mv.
Prior to MySQL 5.5.7, the server renames the current log file with the suffix
-old, then creates a new empty log file. Be aware that a second log-flushing operation thus causes the original error log file to be lost unless you save it under a different name. On Windows, you cannot rename the error log while the server has it open before MySQL 5.5.7. To avoid a restart, flush the logs first to cause the server to rename the original file and create a new one, then save the renamed file. That also works on Unix, or you can use the commands shown earlier.
No error log renaming occurs when the logs are flushed in any case if the server is not writing to a named file.
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, and access-denied errors for new
connection attempts are written. See
Section C.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 or to syslog
mysqld_safe has three error-logging options,
--syslog,
--skip-syslog,
and --log-error. The default
with no logging options or with
--skip-syslog
is to use the default log file. To explicitly specify use of an
error log file, specify
--log-error=
to mysqld_safe, and
mysqld_safe will arrange for
mysqld to write messages to a log file. To use
file_namesyslog instead, specify the
--syslog option.
If you specify --log-error in an
option file in a section that mysqld reads,
mysqld_safe also will find and use the option.
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.