主库故障，从库切主库的几种情况：

1、完全同步的状态，直接将完全同步的从库切成主库

2、不同步状态，a、服务器宕机，直接将从库切成主库，可能存在数据不一致；b、数据库宕机，把从库没有应用完的主库binlog，传至从库中应用后再切

但过程大概一致，略有不同。以完全同步的情况为例：环境：一主两从，在同一服务器上，多个实例(多实例的安装，有时间补充上)

主库端口：3306

从库端口：3307、3308

1)模拟主库故障(默认端口，并已添加mysqld服务至系统)

service mysqld stop

2)查看从库状态

mysql -S /home/mysql/run/mysql3307.sock -uroot -p123456

show slave status\G

*************************** 1. row ***************************

Slave_IO_State: Reconnecting after a failed master event read

Master_Host: 192.168.7.221

Master_User: slave

Master_Port: 3306

Connect_Retry: 60

Master_Log_File: mysql-bin.000044

Read_Master_Log_Pos: 120

Relay_Log_File: mysqld-relay-bin.000090

Relay_Log_Pos: 283

Relay_Master_Log_File: mysql-bin.000044

Slave_IO_Running: Connecting

Slave_SQL_Running: Yes

Last_IO_Errno: 2003

Last_IO_Error: error reconnecting to master 'slave@192.168.7.221:3306' - retry-time: 60 retries: 1

show processlist;

+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+

| Id | User | Host | db | Command | Time | State | Info |

+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+

| 1 | system user | | NULL | Connect | 262412 | Reconnecting after a failed master event read | NULL |

| 2 | system user | | NULL | Connect | 62439 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL

3)两个从库都处于同步状态，将其中一个从库切为主库，这里将端口3307从库切为主库

stop slave;

4) 修改my.cnf

vi /home/mysql/3307/my.cnf

log-bin=mysql-bin

log-bin-index=binlogs.index

注释掉

relay-log=mysqld-relay-bin

relay-log-index=mysqld-relay-bin.index说明：如果之前开启过binlog，又关闭了，而且日志也清除了，则之后重启报错

mysqld: File './mysql-bin.000004' not found (Errcode: 2 - No such file or directory)

2015-08-13 10:39:09 32790 [ERROR] Failed to open log (file './mysql-bin.000004', errno 2)

2015-08-13 10:39:09 32790 [ERROR] Could not open log file这里删除旧的binlogs.index即可解决：rm -f /home/mysql/3307/binlogs.index

5)重启3307从库

mysqladmin shutdown -S /home/mysql/run/mysql3307.sock -uroot -p123456

mysqld --defaults-file=/home/mysql/3307/my.cnf --user=mysql &6)创建复制用户(最好与原主库的用户密码一致)

grant replication slave on *.* to 'slave'@'192.168.7.221' IDENTIFIED BY 'mysql';7)如果有其他从库，处理一下端口3308从库

stop slave;

change master to master_port= 3307;

start slave;

说明：a、change master中其它信息都没有变动，只改一下端口即可

b、由于3307binlog刚打开，所以从mysql-bin.000001开始记录，此时不用担心，其它从库change master不用添加master_log_file及master_log_pos，其它从库会自动识别，如下状态 8)查看端口3308从库状态

mysql -S /home/mysql/run/mysql3308.sock -uroot -p123456

show slave status\G;

*************************** 1. row ***************************

Slave_IO_State: Waiting for master to send event

Master_Host: 192.168.7.221

Master_User: slave

Master_Port: 3307

Connect_Retry: 60

Master_Log_File: mysql-bin.000001

Read_Master_Log_Pos: 331

Relay_Log_File: mysqld3308-relay-bin.000002

Relay_Log_Pos: 494

Relay_Master_Log_File: mysql-bin.000001

Slave_IO_Running: Yes

Slave_SQL_Running: Yes