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5.4.2 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 exits abnormally. The trace can be used to determine where mysqld exited. See Section 24.5, “Debugging and Porting MySQL”.

If mysqld_safe is used to start mysqld and mysqld exits abnormally, mysqld_safe notices this, restarts mysqld, and writes a mysqld restarted message to the error log.

In the following discussion, console means stderr, the standard error output; this is your terminal or console window unless the standard error output has been redirected. (For example, if invoked with the --syslog option, mysqld_safe arranges for the server's stderr to be sent to syslog, as described later.)

On Windows, the --log-error, --pid-file, and --console options affect error logging:

  • If no log file name is specified, the default log file is host_name.err in the data directory, unless the --pid-file option is specified. In that case, the default name is the PID file base name with a suffix of .err in the data directory.

  • Without --log-error, mysqld writes error messages to the default log file.

  • With --log-error[=file_name], mysqld writes error messages to an error log file. mysqld writes to the named file if present, creating it in the data directory unless an absolute path name is given to specify a different directory. If no file is named, mysqld writes to the default log file.

  • With --console, mysqld writes error messages to the console, unless --log-error is also given. If both options are present, --console is ignored and has no effect. Their order does not matter: --log-error takes precedence and error messages go to a log file. (Before MySQL 5.5.3, if both options are given, the last one takes precedence.)

In addition, on Windows, the server writes events and error messages to the Windows Event Log within the Application log. Entries marked as Warning and Note are written to the Event Log, but not informational messages such as information statements from individual storage engines. These log entries have a source of MySQL. You cannot disable writing information to the Windows Event Log.

On Unix and Unix-like systems, mysqld writes error log messages as follows:

  • Without --log-error, mysqld writes error messages to the console.

  • With --log-error[=file_name], mysqld writes error messages to an error log file. The server uses the named file if present, creating it in the data directory unless an absolute path name is given to specify a different directory. If no file is named, the default name is host_name.err in the data directory.


    It is common for Yum or APT package installations to configure the error log location to be under /var/log with an entry like log-error=/var/log/mysqld.log in a server configuration file; removing the file name from the entry reverts the error log file to its default setting, which is host_name.err in the data directory.

At runtime, log_error system variable indicates the error log file name if error output is written to a file.

If you specify --log-error in an option file in a [mysqld], [server], or [mysqld_safe] section, mysqld_safe will find and use the option.

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, 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.

If the server is not writing to a named file, no error log renaming occurs when the logs are flushed.

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.

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 B.5.2.11, “Communication Errors and Aborted Connections”.

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