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5.4.2 The Error Log

This section discusses how to configure the MySQL server for logging of diagnostic messages to the error log. For information about selecting the error message character set and language, see Section 10.6, “Error Message Character Set”, and Section 10.12, “Setting the Error Message Language”.

The error log contains a record of mysqld startup and shutdown times. It also contains diagnostic messages such as errors, warnings, and notes that occur during server startup and shutdown, and while the server is running. For example, if mysqld notices that a table needs to be automatically checked or repaired, it writes a message to the error log.

Depending on error log configuration, error messages may also populate the Performance Schema error_log table, to provide an SQL interface to the log and enable its contents to be queried. See Section 26.12.19.1, “The error_log Table”.

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 5.9, “Debugging MySQL”.

If used to start mysqld, mysqld_safe may write messages to the error log. For example, when mysqld_safe notices abnormal mysqld exits, it restarts mysqld and writes a mysqld restarted message to the error log.

The following sections discuss aspects of configuring error logging.