错误日志文件对MySQL的启动、运行、关闭过程进行了记录。MySQL DBA在遇到问题时应该首先查看该文件以便定位问题。该文件不仅记录了所有的错误信息，也记录一些警告信息或正确的信息。用户可以通过命令SHOW VARIABLES LIKE 'log_error'来定位该文件，如：
mysql> SHOW VARIABLES LIKE 'log_error'\G;
*************************** 1. row ***************************
Variable_name: log_error
Value: /mysql_data_2/stargazer.log
1 row in set (0.00 sec)
mysql> system hostname
stargazer
可以看到错误文件的路径和文件名，在默认情况下错误文件的文件名为服务器的主机名。如上面看到的，该主机名为stargazer，所以错误文件名为startgazer.err。当出现MySQL数据库不能正常启动时，第一个必须查找的文件应该就是错误日志文件，该文件记录了错误信息，能很好地指导用户发现问题。当数据库不能重启时，通过查错误日志文件可以得到如下内容：
[root@nineyou0-43 data]# tail -n 50 nineyou0-43.err
090924 11:31:18 mysqld started
090924 11:31:18 InnoDB: Started; log sequence number 8 2801063331
090924 11:31:19 [ERROR] Fatal error: Can't open and lock privilege tables: Table 'mysql.host' doesn't exist
090924 11:31:19 mysqld ended
这里，错误日志文件提示了找不到权限库mysql，所以启动失败。有时用户可以直接在错误日志文件中得到优化的帮助，因为有些警告（warning）很好地说明了问题所在。而这时可以不需要通过查看数据库状态来得知，例如，下面的错误文件中的信息可能告诉用户需要增大InnoDB存储引擎的redo log：
090924 11:39:44 InnoDB: ERROR: the age of the last checkpoint is 9433712,
InnoDB: which exceeds the log group capacity 9433498.
InnoDB: If you are using big BLOB or TEXT rows, you must set the
InnoDB: combined size of log files at least 10 times bigger than the
InnoDB: largest such row.
090924 11:40:00 InnoDB: ERROR: the age of the last checkpoint is 9433823,
InnoDB: which exceeds the log group capacity 9433498.
InnoDB: If you are using big BLOB or TEXT rows, you must set the
InnoDB: combined size of log files at least 10 times bigger than the
InnoDB: largest such row.
090924 11:40:16 InnoDB: ERROR: the age of the last checkpoint is 9433645,
InnoDB: which exceeds the log group capacity 9433498.
InnoDB: If you are using big BLOB or TEXT rows, you must set the
InnoDB: combined size of log files at least 10 times bigger than the
InnoDB: largest such row.
