MySQL错误日志是记录MySQL 运行过程中较为严重的警告和错误信息，以及MySQL每次启动和关闭的详细信息。错误日志的命名通常为hostname.err。其中，hostname表示服务器主机名。
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”.
查看错误日志的位置
错误日志默认存放位置为数据目录下，你也可以用下面命令查看。例如下面是我测试服务的情况。
mysql> show variables like '%log_error%';
+---------------+------------------------------------------------------+
| Variable_name | Value |
+---------------+------------------------------------------------------+
| log_error | /home/WDPM/MysqlData/mysql/DB-Server.localdomain.err |
+---------------+------------------------------------------------------+
1 row in set (0.00 sec)
mysql>
修改错误日志的位置
错误日志所记录的信息是可以通过log-error和log-warnings来定义的，其中log_error是定义是否启用错误日志的功能和错误日志的存储位置，log-warnings是定义是否将警告信息也定义至错误日志中。可以在启动MySQL时，指定log_error的值。如下所示
[root@DB-Server ~]# /etc/init.d/mysql start --log_error=/tmp/DB-Server.localdomain.err
Starting MySQL..........[ OK ]
[root@DB-Server ~]# mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 5.6.20-enterprise-commercial-advanced-log MySQL Enterprise Server - Advanced Edition (Commercial)
Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show variables like '%log_error%';
+---------------+--------------------------------+
| Variable_name | Value |
+---------------+--------------------------------+
| log_error | /tmp/DB-Server.localdomain.err |
+---------------+--------------------------------+
1 row in set (0.00 sec)
如果关闭MySQL，然后重启MySQL，你会发现log_error的值又变为了/home/WDPM/MysqlData/mysql/DB-Server.localdomain.err 。也就是说这样设置只是对本次有效，如果需要永久修改，那么就必须在my.cnf中指定log_error参数的值。
错误日志可以任意命名吗？ 答案是可以，例如我在/etc/my.cnf配置文件中，添加了参数log_error=/u02/mysql/mysql.err，重新启动MySQL，如下所示
如果配置文件中设置了参数log_error，启动MySQL时也设置参数log_error，那么那个优先级高呢？我测试验证了一下，my.cnf的参数优先级别要高于启动是的参数log_error
/etc/init.d/mysql start log_error=/tmp/DB-Server.localdomain.err
错误日志归档备份
错误日志如果不清理或删除，那么它会一直增长。在MySQL 5.5.7之前，可以通过mysqladmin –uroot –p flush-logs命令删除错误日志。MySQL 5.5.7以及之后，只能通过下面方式来归档、备份错误日志.
shell> mv host_name.err host_name.err-old
shell> mysqladmin -u root -p flush-logs
shell> mv host_name.err-old backup-directory
