ERROR 1872 (HY000): Slave failed to initialize relay log info structure from the repository

线上一个mysql5.6主从复制被操作系统重启弄崩溃了。重新搭建主从复制时，

start slave io_thead;

start slave sql_thread;

之后，show slave status\G 报错：

ERROR 1872 (HY000): Slave failed to initialize relay log info structure from the repository

原因是，从库已经存在之前的relay log

解决方法：

stop slave;

reset slave;

set global gitd_purged='xxxxxx:xxxx';

start slave;

搞定。