server 刷新relay_log，正要更新master.info文件的master读位置时，server宕机，master.info未更新，导致replication启动时，导致
时间：2013-04-18 11:32
故障：数据库服务器无故重启，当Slave启动后，报relay_log损坏，SQL线程无法进行。
故障原因：OS的重启，可能会导致relay_log 无法正常同步
此时的解决办法是重新进行change master to ，但master log pos 采用的是Exec_Master_Log_Pos（show slave status\G变量输出)从slave已经执行到的位置开始重新接受数据。
本以为万事ok，但slave上却报出duplicate key的错误，之后可想而知 是设置sql_slave_skip_counter =1 不断跳过。
对duplicate key 原因解释为MYSQL 本身问题
Slave 提交事务和更新复制信息并不是原子性操作：
I/O线程同步：对于中继日志刷新到磁盘，和更新master.info 分别是fsync调用。
发生如下情况可能会造成接受重复事件：
server 刷新relay_log，正要更新master.info文件的master读位置时，server宕机，master.info未更新，导致replication启动时，导致事件重复。
SQL线程同步：SQL线程是先将事务提交到存储引擎，然后再更新relay_log.info文件
事件在数据库上已经应用，，但未更新relay-log.info。恢复时，SQL会重复执行某些事件。
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