MySQL Slave库恢复实录
状况描述： 今天登录一个MySQL数据库slave节点主机发现/var/lib/mysql下存放大量的mysql-relay-bin文件，最早的文件创建日期甚至是2018年，我记得在slave库同步完master的日志操作记录后，会删除这些文件（默认设置不会删除，我记错了），于是便查看了slave库的状态，发现如下报错：
mysql> show slave status\G;
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: *.*.*.*
Master_User: dbsync
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.000095
Read_Master_Log_Pos: 869242147
Relay_Log_File: mysqld-relay-bin.000146
Relay_Log_Pos: 871280529
Relay_Master_Log_File: mysql-bin.000075
Slave_IO_Running: Yes
Slave_SQL_Running: No
Replicate_Do_DB: cdb,cdb_admin
Replicate_Ignore_DB: mysql
Replicate_Do_Table:
Replicate_Ignore_Table:
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 1594
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.
Skip_Counter: 0
Exec_Master_Log_Pos: 871280384
Relay_Log_Space: 19994786573
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed: No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: NULL
Master_SSL_Verify_Server_Cert: No
Last_IO_Errno: 0
Last_IO_Error:
Last_SQL_Errno: 1594
Last_SQL_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.
1 row in set (0.00 sec)
ERROR:
No query specified原因： 我在master节点上删除了名称为mysql-bin.00007格式的文件，其中包括mysql-bin.000075，因此，slave库找不到该文件，无法同步。
解决办法：
重做slave库。由于数据差异太大，而且我觉得不光一张表出现了数据不一样的问题，所以干净点，把从库重做。
1）比对master、slave节点库配置信息，保证一致。（我不知道为什么设置了双主模式，实际上我只有一个实例跑在master节点上啊？）
2）在master、slave节点上查看流量情况（show processlist），保证要重做的slave库上没有业务的流量接入。
3）停止master节点上slave进程。（这个停了以后，我就没开过，不知道有没有问题，待观察）
4）记录master节点上库的日志记录位置，之后备份数据库：
mysql> show master status;
+------------------+-----------+-------------------------------+------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+------------------+-----------+-------------------------------+------------------+
| mysql-bin.000095 | 871760173 | cdb,cdb_admin | mysql |
+------------------+-----------+-------------------------------+------------------+
1 row in set (0.01 sec)
mysqldump -u root -p --databases cdb,cdb_admin > bak.master.sql5）保险起见，备份slave节点库：
mysqldump -u root -p --databases cdb,cdb_admin > bak.slave.sql
6）重做开始：把master库备份文件复制到slave节点上，导入该备份文件
mysql -u root -p < bak.master.sql
7）在slave节点上，重新指定读master日志的位置：
slave stop;
CHANGE MASTER TO MASTER_LOG_FILE='mysql-bin.000095',MASTER_LOG_POS=871760173; //POS为刚才记录的master节点日志记录位置
slave start;8）slave节点上 show slave status；此时Slave_IO_Running，Slave_SQL_Running均运行起来了，刷新slave status，Read_Master_Log_Pos数值也开始增加，重新开始同步了。
总结： 清理文件时，要注意mysql-bin文件在master、slave节点日志读取和写的位置啊！，删之前一定要确认日志位置在master和slave断已被读过，不要乱删，否则搞得slave库无法同步了，就算在slave节点上强行指定master日志读取位置或者跳过该错误，也不排除slave库上数据丢失的可能。