Slave_IO_State: Waiting for master to send event
Last_Error: Relay log read failure: Could not parse relay log event entry. The possible reasons are: the master's binary log is corrupted (you can check this by running 'mysqlbinlog' on the binary log), the slave's relay log is corrupted (you can check this by running 'mysqlbinlog' on the relay log), a network problem, or a bug in the master's or slave's MySQL code. If you want to check the master's binary log or slave's relay log, you will be able to know their names by issuing 'SHOW SLAVE STATUS' on this slave.
是因为从库的 relay log 损坏导致的从库停止执行同步, 后面从 MySQL 的错误日志发现是由几个没经过优化的大查询导致从库内存使用较大，mysqld_safe 被迫重启了。