5.2.2. The Error Log

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[=file_name] option. If the option is given with no file_name value, mysqld uses the name host_name.err by default. The server creates the file in the data directory unless an absolute path name is given to specify a different directory.

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> mv host_name.err host_name.err-old
    shell> mysqladmin flush-logs
    shell> mv host_name.err-old backup-directory

    On 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=file_name to mysqld_safe, and mysqld_safe will arrange for mysqld to write messages to a log file. To use syslog 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.

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