场景：两台数据库运行一段时间后发现集群挂了，一台服务正常，一台不正常。
日志如下：
[ERROR] InnoDB: Attempted to open a previously opened tablespace. Previous tablespace mysql/innodb_table_stats uses space ID: 1 at filepath: ./mysql/innodb_table_stats.ibd. Cannot open tablespace testdb/dr_actions which uses space ID: 1 at filepath: ./testdb/dr_actions.ibd 2013-11-18 21:25:25 a171e1a8 InnoDB: Operating system error number 2 in a file operation. InnoDB: The error means the system cannot find the path specified. InnoDB: If you are installing InnoDB, remember that you must create InnoDB: directories yourself, InnoDB does not create them. InnoDB: Error: could not open single-table tablespace file ./testdb/dr_actions.ibd InnoDB: We do not continue the crash recovery, because the table may become InnoDB: corrupt if we cannot apply the log records in the InnoDB log to it. InnoDB: To fix the problem and start mysqld: InnoDB: 1) If there is a permission problem in the file and mysqld cannot InnoDB: open the file, you should modify the permissions. InnoDB: 2) If the table is not needed, or you can restore it from a backup, InnoDB: then you can remove the .ibd file, and InnoDB will do a normal InnoDB: crash recovery and ignore that table. InnoDB: 3) If the file system or the disk is broken, and you cannot remove InnoDB: the .ibd file, you can set innodb_force_recovery > 0 in my.cnf InnoDB: and force InnoDB to continue crash recovery here.
解决方法：
编辑/etc/my.cnf，添加
[mysqld] innodb_force_recovery=1
重启数据库服务
启动成功后，在去掉该条记录，再次重启数据库服务，即可。
可参看：http://www.softaculous.com/board/index.php?tid=4700&title=MySQL_not_starting
那是第一种情况，还有一种情况是两端的数据不一致，导致无法启动成功。