在MySQL的master-slave或dual master的架构中，要重新开始复制可以使用change master to命令，而参数的选择可以来自简单的show slave status命令，举例说明如下：

mysql> show slave status\G

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

Slave_IO_State: Connecting to master

Master_Host: 192.168.2.216

Master_User: repl

Master_Port: 3306

Connect_Retry: 60

Master_Log_File: bin.000001

Read_Master_Log_Pos: 2218353

Relay_Log_File: relaylog.000003

Relay_Log_Pos: 4

Relay_Master_Log_File: bin.000001

Slave_IO_Running: Connecting

Slave_SQL_Running: Yes

Replicate_Do_DB:

Replicate_Ignore_DB: mysql,performance_schema,information_schema

Replicate_Do_Table:

Replicate_Ignore_Table:

Replicate_Wild_Do_Table:

Replicate_Wild_Ignore_Table:

Last_Errno: 0

Last_Error:

Skip_Counter: 0

Exec_Master_Log_Pos: 2218353

Relay_Log_Space: 107

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: 2003

Last_IO_Error: error connecting to master ‘repl@192.168.2.216:3306′ – retry-time: 60  retries: 86400

Last_SQL_Errno: 0

Last_SQL_Error:

Replicate_Ignore_Server_Ids:

Master_Server_Id: 0

1 row in set (0.00 sec)mysql> show slave status\G

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

Slave_IO_State: Connecting to master

Master_Host: 192.168.2.216

Master_User: repl

Master_Port: 3306

Connect_Retry: 60

Master_Log_File: bin.000001

Read_Master_Log_Pos: 2218353

Relay_Log_File: relaylog.000003

Relay_Log_Pos: 4

Relay_Master_Log_File: bin.000001

Slave_IO_Running: Connecting

Slave_SQL_Running: Yes

Replicate_Do_DB:

Replicate_Ignore_DB: mysql,performance_schema,information_schema

Replicate_Do_Table:

Replicate_Ignore_Table:

Replicate_Wild_Do_Table:

Replicate_Wild_Ignore_Table:

Last_Errno: 0

Last_Error:

Skip_Counter: 0

Exec_Master_Log_Pos: 2218353

Relay_Log_Space: 107

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: 2003

Last_IO_Error: error connecting to master ‘repl@192.168.2.216:3306′ – retry-time: 60  retries: 86400

Last_SQL_Errno: 0

Last_SQL_Error:

Replicate_Ignore_Server_Ids:

Master_Server_Id: 0

1 row in set (0.00 sec)

我们看到这个库无法和Master数据库连接了，这时可以先停掉复制：mysql> stop slave;

Query OK, 0 rows affected (0.00 sec)

我们都知道目前MySQL的版本slave端是通过2个线程实现复制的，一个是I/O Thread负责连接到master接收binlog信息，在从服务器端这部分信息叫做relay log。另一个SQL Thread负责读取relay log并执行其中的event(在statement format的时候其实就是SQL语句)。这从以上输出的信息也能看出来，IO线程在连接但是连不上，而SQL线程正在执行：Slave_IO_Running: Connecting

Slave_SQL_Running: Yes

接下来我们再用change master to命令修改连接参数和复制起始点，其中重要的MASTER_LOG_FILE和MASTER_LOG_POS参数，这两个参数对应的是IO线程生成relay log的信息，因此和Relay_Master_Log_File和Exec_Master_Log_Pos是对应的，具体命令如下：mysql> CHANGE MASTER TO

-> MASTER_HOST=’192.168.0.35′,

-> MASTER_PORT = 3306,

-> MASTER_USER=’banping_repl’,

-> MASTER_PASSWORD=’slavepassword’,

-> MASTER_LOG_FILE=’bin.000001′,

-> MASTER_LOG_POS=2218353;

Query OK, 0 rows affected (0.00 sec)

然后再开始复制：mysql> start slave;

Query OK, 0 rows affected (0.00 sec)

再查看slave的状态：mysql> show slave status\G

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

Slave_IO_State: Waiting for master to send event

Master_Host: 192.168.0.35

Master_User: repl

Master_Port: 3306

Connect_Retry: 60

Master_Log_File: bin.000002

Read_Master_Log_Pos: 1819899

Relay_Log_File: relaylog.000003

Relay_Log_Pos: 76625

Relay_Master_Log_File: bin.000002

Slave_IO_Running: Yes

Slave_SQL_Running: Yes

Replicate_Do_DB:

Replicate_Ignore_DB: mysql,performance_schema,information_schema

Replicate_Do_Table:

Replicate_Ignore_Table:

Replicate_Wild_Do_Table:

Replicate_Wild_Ignore_Table:

Last_Errno: 0

Last_Error:

Skip_Counter: 0

Exec_Master_Log_Pos: 76485

Relay_Log_Space: 1820328

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: 50236

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: 1

1 row in set (0.00 sec)

可见已经恢复了复制。完整的命令如下：

CHANGE MASTER TO option [, option] ...

option:

MASTER_HOST = 'host_name'

| MASTER_USER = 'user_name'

| MASTER_PASSWORD = 'password'

| MASTER_PORT = port_num

| MASTER_CONNECT_RETRY = interval

| MASTER_HEARTBEAT_PERIOD = interval

| MASTER_LOG_FILE = 'master_log_name'

| MASTER_LOG_POS = master_log_pos

| RELAY_LOG_FILE = 'relay_log_name'

| RELAY_LOG_POS = relay_log_pos

| MASTER_SSL = {0|1}

| MASTER_SSL_CA = 'ca_file_name'

| MASTER_SSL_CAPATH = 'ca_directory_name'

| MASTER_SSL_CERT = 'cert_file_name'

| MASTER_SSL_KEY = 'key_file_name'

| MASTER_SSL_CIPHER = 'cipher_list'

| MASTER_SSL_VERIFY_SERVER_CERT = {0|1}

| IGNORE_SERVER_IDS = (server_id_list)