MySQL Replication之主从切换
在生产环境中,我们的架构很多都是一主多从。比如一个主数据库服务器M,两个从数据库服务器S1,S2同时指向主数据库服务器M。当主服务器M因为意外情况宕机,需要将其中的一个从数据库服务器(假设选择S1)切换成主数据库服务器,同时修改另一个从数据库(S2)的配置,使其指向新的主数据库(S1)。此外还需要通知应用修改主数据库的IP地址,如果可能,将出现故障的主数据库(M)修复或者重置成新的从数据库。通常我们还有其他的方案来实现高可用,比如MHA,MySQL Cluster,MMM,这些将在后续的文章中慢慢道来。现在我们先看简单的一主多从切换的情况。^_^
下面详细介绍切换主从的操作步骤。
1.首先要确保所有的从数据库都已经执行了relay log中的全部更新,在每个从库上,执行stop slave io_thread,停止IO线程,然后检查show processlist的输出,直到看到状态是Slave has read all relay log; waiting for the slave I/O thread to update it,表示更新都执行完毕
S1(从库1操作):
mysql> stop slave io_thread; Query OK, 0 rows affected (0.06 sec) mysql> show processlist\G *************************** 1. row *************************** Id: 3 User: system user Host: db: NULL Command: Connect Time: 2601 State: Slave has read all relay log; waiting for the slave I/O thread to update it Info: NULL *************************** 2. row *************************** Id: 4 User: root Host: localhost db: NULL Command: Query Time: 0 State: NULL Info: show processlist 2 rows in set (0.00 sec) mysql>
S2(从库2操作):
mysql> stop slave io_thread; Query OK, 0 rows affected (0.00 sec) mysql> show processlist\G *************************** 1. row *************************** Id: 4 User: system user Host: db: NULL Command: Connect Time: 2721 State: Slave has read all relay log; waiting for the slave I/O thread to update it Info: NULL *************************** 2. row *************************** Id: 5 User: root Host: localhost db: NULL Command: Query Time: 0 State: NULL Info: show processlist 2 rows in set (0.00 sec) mysql>
2.在从库S1上,执行stop slave停止从服务,然后执行reset master以重置成主数据库,并且进行授权账号,让S2(从库2)有权限进行连接
S1(从库1操作):
mysql> stop slave; Query OK, 0 rows affected (0.01 sec) mysql> reset master; Query OK, 0 rows affected (0.06 sec) mysql> grant replication slave on *.* to 'repl'@'192.168.0.100' identified by '123456'; Query OK, 0 rows affected (0.00 sec) mysql>
3.在S2(从库2)上,执行stop slave停止从服务,然后执行change master to master_host='S1'以重新设置主数据库,然后再执行start slave启动复制:
S2(从库2操作):
mysql> stop slave; Query OK, 0 rows affected (0.01 sec) mysql> change master to master_host='192.168.0.20'; Query OK, 0 rows affected (0.06 sec) mysql> start slave; Query OK, 0 rows affected (0.00 sec) mysql>
4.查看S2(从库2)复制状态是否正常:
mysql> show slave status\G *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 192.168.0.20 Master_User: repl Master_Port: 3306 Connect_Retry: 2 Master_Log_File: mysql-bin.000001 Read_Master_Log_Pos: 261 Relay_Log_File: MySQL-02-relay-bin.000002 Relay_Log_Pos: 407 Relay_Master_Log_File: mysql-bin.000001 Slave_IO_Running: Yes Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: yayun.% Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 261 Relay_Log_Space: 566 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: 0 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 2 1 row in set (0.00 sec) mysql>
查看原来的从库S1,现在的主库的show processlist情况:
mysql> show processlist\G *************************** 1. row *************************** Id: 4 User: root Host: localhost db: NULL Command: Query Time: 0 State: NULL Info: show processlist *************************** 2. row *************************** Id: 7 User: repl Host: 192.168.0.100:60235 db: NULL Command: Binlog Dump Time: 184 State: Master has sent all binlog to slave; waiting for binlog to be updated Info: NULL 2 rows in set (0.00 sec) mysql>
5.通知所有的客户端将应用指向S1(已提升为主库),这样客户端发送的所有的更新变化将记录到S1的二进制日志。
6.删除S1(新的主库)服务器上的master.info和relay-log.info文件,否则下次重启时还会按照从库启动。我们也可以设置该参数:
skip_slave_start
7.最后,如果M服务器修复以后,则可以按照S2的方法配置成S1的从库。
总结:
上面的测试步骤中S1默认都是打开log-bin选项的,这样重置成主数据库后可以将二进制日志记录下来,并传送到其他从库,这是提升为主库必须的。其次,S1没有打开log-slave-updates参数,否则重置成主库以后,可能会将已经执行过的二进制日志重复传送给S2,导致S2同步错误。