MySQL高可用方案MHA在线切换的步骤及原理

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在日常工作中，会碰到如下的场景，如 MySQL 数据库升级，主服务器硬件升级等，这个时候就需要将写操作切换到另外一台服务器上，那么如何进行在线切换呢？同时，要求切换过程短，对业务的影响比较小。

MHA 就提供了这样一种优雅的方式，只会堵塞业务 0.5~2s 的时间，在这段时间内，业务无法读取和写入。

集群信息

角色 IP 地址 ServerID 类型

Master 192.168.244.10 1 写入

Candicate master 192.168.244.20 2 读

Slave 192.168.244.30 3 读

Monitor host 192.168.244.40 监控集群组

MHA 具体的搭建步骤和原理，可参考另外一篇文章：

MySQL 高可用方案 MHA 的部署和原理 http://www.linuxidc.com/Linux/2017-05/144086.htm

在线切换的步骤

1. 关闭 MHA 监控

# masterha_stop –conf=/etc/masterha/app1.cnf

2. 在线切换

# /usr/local/bin/masterha_master_switch –conf=/etc/masterha/app1.cnf –master_state=alive –new_master_host=192.168.244.20 –new_master_port=3306 –orig_master_is_new_slave –running_updates_limit=10000

其中，

–orig_master_is_new_slave 是将原 master 切换为新主的 slave，默认情况下，是不添加的。

–running_updates_limit 默认为 1s，即如果主从延迟时间（Seconds_Behind_Master），或 master show processlist 中 dml 操作大于 1s，则不会执行切换。

在线切换的输出

Tue Apr 11 15:28:32 2017 – [info] MHA::MasterRotate version 0.56.
Tue Apr11 15:28:32 2017 – [info] Starting online master switch..
Tue Apr11 15:28:32 2017 – [info]
Tue Apr11 15:28:32 2017 – [info] * Phase 1: Configuration Check Phase..
Tue Apr11 15:28:32 2017 – [info]
Tue Apr11 15:28:32 2017 – [warning] Global configuration file /etc/masterha_default.cnf not found. Skipping.
Tue Apr11 15:28:32 2017 – [info] Reading application default configuration from /etc/masterha/app1.cnf..
Tue Apr11 15:28:32 2017 – [info] Reading server configuration from /etc/masterha/app1.cnf..
Tue Apr11 15:28:34 2017 – [info] GTID failover mode = 0
Tue Apr11 15:28:34 2017 – [info] Current Alive Master: 192.168.244.10(192.168.244.10:3306)
Tue Apr11 15:28:34 2017 – [info] Alive Slaves:
Tue Apr11 15:28:34 2017 – [info] 192.168.244.20(192.168.244.20:3306) Version=5.6.31–log (oldest major version between slaves) log
-bin:enabledTue Apr 11 15:28:34 2017 – [info] Replicating from 192.168.244.10(192.168.244.10:3306)
Tue Apr11 15:28:34 2017 – [info] Primary candidate for the new Master (candidate_master is set)
Tue Apr11 15:28:34 2017 – [info] 192.168.244.30(192.168.244.30:3306) Version=5.6.31–log (oldest major version between slaves) log
-bin:enabledTue Apr 11 15:28:34 2017 – [info] Replicating from 192.168.244.10(192.168.244.10:3306)

It is better to execute FLUSH NO_WRITE_TO_BINLOG TABLES on the master before switching. Is it ok to execute on192.168.244.10(192.168
.244.10:3306)? (YES/no): yes

Tue Apr 11 15:28:47 2017 – [info] Executing FLUSH NO_WRITE_TO_BINLOG TABLES. This may take long time..
Tue Apr11 15:28:47 2017 – [info] ok.
Tue Apr11 15:28:47 2017 – [info] Checking MHA is not monitoring or doing failover..
Tue Apr11 15:28:47 2017 – [info] Checking replication health on 192.168.244.20..
Tue Apr11 15:28:47 2017 – [info] ok.
Tue Apr11 15:28:47 2017 – [info] Checking replication health on 192.168.244.30..
Tue Apr11 15:28:47 2017 – [info] ok.
Tue Apr11 15:28:47 2017 – [info] 192.168.244.20 can be new master.
Tue Apr11 15:28:47 2017 – [info]
From:
192.168.244.10(192.168.244.10:3306) (current master)
+–192.168.244.20(192.168.244.20:3306)
+–192.168.244.30(192.168.244.30:3306)

To:
192.168.244.20(192.168.244.20:3306) (new master)
+–192.168.244.30(192.168.244.30:3306)
+–192.168.244.10(192.168.244.10:3306)

Starting master switch from192.168.244.10(192.168.244.10:3306) to 192.168.244.20(192.168.244.20:3306)? (yes/NO): yes
Tue Apr11 15:29:00 2017 – [info] Checking whether 192.168.244.20(192.168.244.20:3306) is ok for the new master..
Tue Apr11 15:29:00 2017 – [info] ok.
Tue Apr11 15:29:00 2017 – [info] 192.168.244.10(192.168.244.10:3306): SHOW SLAVE STATUS returned empty result. To check replication
filtering rules, temporarily executing CHANGE MASTER to a dummy host.Tue Apr11 15:29:00 2017 – [info] 192.168.244.10(192.168.244.10:3306): Resetting slave pointing to the dummy host.
Tue Apr11 15:29:00 2017 – [info] ** Phase 1: Configuration Check Phase completed.
Tue Apr11 15:29:00 2017 – [info]
Tue Apr11 15:29:00 2017 – [info] * Phase 2: Rejecting updates Phase..
Tue Apr11 15:29:00 2017 – [info]
Tue Apr11 15:29:00 2017 – [info] Executing master ip online change script to disable write on the current master:
Tue Apr11 15:29:00 2017 – [info] /usr/local/bin/master_ip_online_change –command=stop –orig_master_host=192.168.244.10 —orig_ma
ster_ip=192.168.244.10 –orig_master_port=3306 –orig_master_user=‘monitor‘ –orig_master_password=‘monitor123‘ –new_master_host=192.168.244.20 –new_master_ip=192.168.244.20 –new_master_port=3306 –new_master_user=‘monitor‘ –new_master_password=‘monitor123‘ –orig_master_ssh_user=root –new_master_ssh_user=root –orig_master_is_new_slaveTue Apr 11 15:29:00 2017 476501 Set read_only on the new master.. ok.
Tue Apr11 15:29:00 2017 911951 Set read_only=1 on the orig master.. ok.
Tue Apr11 15:29:00 2017 919517 Killing all application threads..
Tue Apr11 15:29:00 2017 919552 done.
Disabling the VIP an old master:192.168.244.10
SIOCSIFFLAGS: Cannot assign requested address
Tue Apr11 15:29:00 2017 – [info] ok.
Tue Apr11 15:29:00 2017 – [info] Locking all tables on the orig master to reject updates from everybody (including root):
Tue Apr11 15:29:00 2017 – [info] Executing FLUSH TABLES WITH READ LOCK..
Tue Apr11 15:29:00 2017 – [info] ok.
Tue Apr11 15:29:00 2017 – [info] Orig master binlog:pos is mysql-bin.000016:211.
Tue Apr11 15:29:00 2017 – [info] Waiting to execute all relay logs on 192.168.244.20(192.168.244.20:3306)..
Tue Apr11 15:29:01 2017 – [info] master_pos_wait(mysql-bin.000016:211) completed on 192.168.244.20(192.168.244.20:3306). Executed 0
events.Tue Apr11 15:29:01 2017 – [info] done.
Tue Apr11 15:29:01 2017 – [info] Getting new master‘s binlog name and position..
Tue Apr 11 15:29:01 2017 – [info] mysql-bin.000009:211
Tue Apr11 15:29:01 2017 – [info] All other slaves should start replication from here. Statement should be: CHANGE MASTER TO MASTER_
HOST=‘192.168.244.20‘, MASTER_PORT=3306, MASTER_LOG_FILE=‘mysql-bin.000009‘, MASTER_LOG_POS=211, MASTER_USER=‘repl‘, MASTER_PASSWORD=‘xxx‘;Tue Apr 11 15:29:01 2017 – [info] Executing master ip online change script to allow write on the new master:
Tue Apr11 15:29:01 2017 – [info] /usr/local/bin/master_ip_online_change –command=start –orig_master_host=192.168.244.10 —orig_m
aster_ip=192.168.244.10 –orig_master_port=3306 –orig_master_user=‘monitor‘ –orig_master_password=‘monitor123‘ –new_master_host=192.168.244.20 –new_master_ip=192.168.244.20 –new_master_port=3306 –new_master_user=‘monitor‘ –new_master_password=‘monitor123‘ –orig_master_ssh_user=root –new_master_ssh_user=root –orig_master_is_new_slaveTue Apr 11 15:29:01 2017 109040 Set read_only=0 on the new master.
Enabling the VIP192.168.244.188 on the new master: 192.168.244.20
Tue Apr11 15:29:01 2017 – [info] ok.
Tue Apr11 15:29:01 2017 – [info]
Tue Apr11 15:29:01 2017 – [info] * Switching slaves in parallel..
Tue Apr11 15:29:01 2017 – [info]
Tue Apr11 15:29:01 2017 – [info] — Slave switch on host 192.168.244.30(192.168.244.30:3306) started, pid: 17651
Tue Apr11 15:29:01 2017 – [info]
Tue Apr11 15:29:02 2017 – [info] Log messages from 192.168.244.30 …
Tue Apr11 15:29:02 2017 – [info]
Tue Apr11 15:29:01 2017 – [info] Waiting to execute all relay logs on 192.168.244.30(192.168.244.30:3306)..
Tue Apr11 15:29:01 2017 – [info] master_pos_wait(mysql-bin.000016:211) completed on 192.168.244.30(192.168.244.30:3306). Executed 0
events.Tue Apr11 15:29:01 2017 – [info] done.
Tue Apr11 15:29:01 2017 – [info] Resetting slave 192.168.244.30(192.168.244.30:3306) and starting replication from the new master 1
92.168.244.20(192.168.244.20:3306)..Tue Apr 11 15:29:01 2017 – [info] Executed CHANGE MASTER.
Tue Apr11 15:29:01 2017 – [info] Slave started.
Tue Apr11 15:29:02 2017 – [info] End of log messages from 192.168.244.30 …
Tue Apr11 15:29:02 2017 – [info]
Tue Apr11 15:29:02 2017 – [info] — Slave switch on host 192.168.244.30(192.168.244.30:3306) succeeded.
Tue Apr11 15:29:02 2017 – [info] Unlocking all tables on the orig master:
Tue Apr11 15:29:02 2017 – [info] Executing UNLOCK TABLES..
Tue Apr11 15:29:02 2017 – [info] ok.
Tue Apr11 15:29:02 2017 – [info] Starting orig master as a new slave..
Tue Apr11 15:29:02 2017 – [info] Resetting slave 192.168.244.10(192.168.244.10:3306) and starting replication from the new master 1
92.168.244.20(192.168.244.20:3306)..Tue Apr 11 15:29:02 2017 – [info] Executed CHANGE MASTER.
Tue Apr11 15:29:02 2017 – [info] Slave started.
Tue Apr11 15:29:02 2017 – [info] All new slave servers switched successfully.
Tue Apr11 15:29:02 2017 – [info]
Tue Apr11 15:29:02 2017 – [info] * Phase 5: New master cleanup phase..
Tue Apr11 15:29:02 2017 – [info]
Tue Apr11 15:29:02 2017 – [info] 192.168.244.20: Resetting slave info succeeded.
Tue Apr11 15:29:02 2017 – [info] Switching master to 192.168.244.20(192.168.244.20:3306) completed successfully.

MHA 在线切换的原理

1. 检查当前的配置信息及主从服务器的信息

包括读取 MHA 的配置文件 /etc/masterha/app1.cnf 及检查当前 slave 的健康状态

2. 阻止对当前 master 的更新

主要通过如下步骤：

1> 等待 1.5s（$time_until_kill_threads*100ms），等待当前连接断开。

2> 执行 read_only=1，阻止新的 DML 操作

3> 等待 0.5s，等待当前 DML 操作完成。

4> kill 掉所有连接。

5> FLUSH NO_WRITE_TO_BINLOG TABLES

6> FLUSH TABLES WITH READ LOCK

3. 等待新 master 执行完所有的 relay log

Waiting to execute all relay logs on 192.168.244.20(192.168.244.20:3306)..

4. 将新 master 的 read_only 设置为 off，并添加 VIP

5. slave 切换到新 master 上。

1> 等待 slave（192.168.244.30）应用完原主从复制产生的 relay log，然后执行 change master 操作切换到新 master 上。

2> 释放原 master 上加的锁。

3> 因 masterha_master_switch 命令行中带有 –orig_master_is_new_slave 参数，故原 master 也切��为新 master 的从。

6. 清理新 master 的相关信息。

主要是执行了 reset slave all 操作，清除之前的复制信息。

MHA 在线切换需满足的条件

MHA 在执行在线切换之前，会判断当前的主从复制信息，只有满足了以下条件，才能执行切换动作：

1. 所有 SLAVE 的 IO 线程和 SQL 线程都在运行。

2. 所有 slave 的 Seconds_Behind_Master 小于或等于 running_updates_limit 的值，该参数如果没有显示指定的话，则默认为 1s

3. 在 master 上，通过 show processlist 输出，没有一个 DML 操作的时间大于 running_updates_limit 的值。

在线切换时，打开 general log，各个服务器的操作信息

注：在执行 masterha_master_switch 命令时，会有两次确认操作

1. It is better to execute FLUSH NO_WRITE_TO_BINLOG TABLES on the master before switching. Is it ok to execute on 192.168.244.10(192.168
.244.10:3306)? (YES/no):

2. Starting master switch from 192.168.244.10(192.168.244.10:3306) to 192.168.244.20(192.168.244.20:3306)? (yes/NO):

以下输出中间都有两次空白，其中第一次空白之前的输出对应第一次确认之前，第二次之前的输出对应第二次确认之前。

原 master 192.168.244.10

170412 16:52:38 23 Connect monitor@node4 on
23 Query set autocommit=1
23 Query SELECT CONNECTION_ID() AS Value
170412 16:52:39 24 Connect monitor@node4 on
24 Query set autocommit=1
24 Query SELECT CONNECTION_ID() AS Value
24 Query SET wait_timeout=86400
24 Query SELECT @@global.server_id As Value
24 Query SELECT VERSION() AS Value
24 Query SELECT @@global.gtid_mode As Value
24 Query SHOW GLOBAL VARIABLES LIKE ‘log_bin‘
24 Query SHOW MASTER STATUS
24 Query SELECT @@global.datadir AS Value
24 Query SELECT @@global.slave_parallel_workers AS Value
24 Query SHOW SLAVE STATUS
24 Query SELECT @@global.read_only As Value
24 Query SELECT @@global.relay_log_purge As Value

170412 16:54:06 24 Query FLUSH NO_WRITE_TO_BINLOG TABLES
24 Query SELECT GET_LOCK(‘MHA_Master_High_Availability_Monitor‘, ‘0‘) AS Value
24 Query SHOW PROCESSLIST

170412 16:55:51 24 Query SHOW SLAVE STATUS
24 Query CHANGE MASTER TO MASTER_HOST=‘dummy_host‘
170412 16:55:52 24 Query SHOW SLAVE STATUS
24 Query RESET SLAVE /*!50516 ALL*/
24 Query SELECT RELEASE_LOCK(‘MHA_Master_High_Availability_Monitor‘) As Value
24 Quit
25 Connect monitor@node4 on
25 Query set autocommit=1
25 Query SELECT CONNECTION_ID() AS Value
25 Query SET sql_log_bin=0
25 Query SHOW PROCESSLIST
25 Query SELECT @@global.read_only As Value
25 Query SET GLOBAL read_only=1
25 Query SELECT @@global.read_only As Value
25 Query SHOW PROCESSLIST
25 Query SET sql_log_bin=1
25 Quit
26 Connect monitor@node4 on
26 Query set autocommit=1
26 Query SELECT CONNECTION_ID() AS Value
26 Query SET wait_timeout=86400
26 Query FLUSH TABLES WITH READ LOCK
26 Query SHOW MASTER STATUS
170412 16:55:53 26 Query UNLOCK TABLES
26 Query CHANGE MASTER TO MASTER_HOST = ‘192.168.244.20‘ MASTER_USER = ‘repl‘ MASTER_PASSWORD = <secret> MASTE
R_PORT= 3306 MASTER_LOG_FILE = ‘mysql-bin.000010‘ MASTER_LOG_POS = 120 26 Query SET GLOBAL relay_log_purge=0
26 Query START SLAVE
27 Connect Out repl@192.168.244.20:3306
26 Query SHOW SLAVE STATUS
26 Query SELECT RELEASE_LOCK(‘MHA_Master_High_Availability_Failover‘) As Value
26 Quit

新 master 192.168.244.20

170412 16:52:38    23 Connect    monitor@node4 on 
                   23 Query    set autocommit=1
                   23 Query    SELECT CONNECTION_ID() AS Value
170412 16:52:39    24 Connect    monitor@node4 on 
                   24 Query    set autocommit=1
                   24 Query    SELECT CONNECTION_ID() AS Value
                   24 Query    SET wait_timeout=86400
                   24 Query    SELECT @@global.server_id As Value
                   24 Query    SELECT VERSION() AS Value
                   24 Query    SELECT @@global.gtid_mode As Value
                   24 Query    SHOW GLOBAL VARIABLES LIKE 'log_bin'
                   24 Query    SHOW MASTER STATUS
                   24 Query    SELECT @@global.datadir AS Value
                   24 Query    SELECT @@global.slave_parallel_workers AS Value
                   24 Query    SHOW SLAVE STATUS
                   24 Query    SELECT @@global.read_only As Value
                   24 Query    SELECT @@global.relay_log_purge As Value
                   24 Query    SELECT @@global.relay_log_info_repository AS Value
                   24 Query    SELECT @@global.datadir AS Value
                   24 Query    SELECT @@global.relay_log_info_file AS Value
                   24 Query    SHOW SLAVE STATUS
                   24 Query    SELECT Repl_slave_priv AS Value FROM mysql.user WHERE user = 'repl'

           
           
170412 16:54:06    24 Query    SELECT GET_LOCK('MHA_Master_High_Availability_Failover', '0') AS Value
                   24 Query    SHOW SLAVE STATUS
                   24 Query    SHOW SLAVE STATUS

           
170412 16:55:52    24 Query    SHOW PROCESSLIST
                   25 Connect    monitor@node4 on 
                   25 Query    set autocommit=1
                   25 Query    SELECT CONNECTION_ID() AS Value
                   25 Query    SELECT @@global.read_only As Value
                   25 Query    SELECT @@global.read_only As Value
                   25 Quit    
                   24 Query    SHOW SLAVE STATUS
                   24 Query    SELECT MASTER_POS_WAIT('mysql-bin.000017','120',0) AS Result
                   24 Query    STOP SLAVE SQL_THREAD
                   24 Query    SHOW SLAVE STATUS
                   24 Query    SHOW MASTER STATUS
                   26 Connect    monitor@node4 on 
                   26 Query    set autocommit=1
                   26 Query    SELECT CONNECTION_ID() AS Value
                   26 Query    SET sql_log_bin=0
                   26 Query    SELECT @@global.read_only As Value
                   26 Query    SET GLOBAL read_only=0
                   26 Query    SET sql_log_bin=1
                   26 Quit    
                   24 Query    SELECT @@global.read_only As Value
                   27 Connect    repl@node3 on 
                   27 Query    SELECT UNIX_TIMESTAMP()
                   27 Query    SHOW VARIABLES LIKE 'SERVER_ID'
                   27 Query    SET @master_heartbeat_period= 1799999979520
                   27 Query    SET @master_binlog_checksum= @@global.binlog_checksum
                   27 Query    SELECT @master_binlog_checksum
                   27 Query    SELECT @@GLOBAL.GTID_MODE
                   27 Query    SHOW VARIABLES LIKE 'SERVER_UUID'
                   27 Query    SET @slave_uuid= '8a1093c8-1d00-11e7-954f-000c299a5715'
                   27 Binlog Dump    Log: 'mysql-bin.000010'  Pos: 120
170412 16:55:53    28 Connect    repl@node1 on 
                   28 Query    SELECT UNIX_TIMESTAMP()
                   28 Query    SHOW VARIABLES LIKE 'SERVER_ID'
                   28 Query    SET @master_heartbeat_period= 1799999979520
                   28 Query    SET @master_binlog_checksum= @@global.binlog_checksum
                   28 Query    SELECT @master_binlog_checksum
                   28 Query    SELECT @@GLOBAL.GTID_MODE
                   28 Query    SHOW VARIABLES LIKE 'SERVER_UUID'
                   24 Query    STOP SLAVE
                   28 Query    SET @slave_uuid= '2a6365e0-1d05-11e7-956d-000c29c64704'
                   28 Binlog Dump    Log: 'mysql-bin.000010'  Pos: 120
                   24 Query    SHOW SLAVE STATUS
                   24 Query    RESET SLAVE /*!50516 ALL */
                   24 Query    SHOW SLAVE STATUS
                   24 Query    SELECT RELEASE_LOCK('MHA_Master_High_Availability_Failover') As Value
                   24 Quit

slave 192.168.244.30

170412 16:52:37    16 Connect    monitor@node4 on 
                   16 Query    set autocommit=1
                   16 Query    SELECT CONNECTION_ID() AS Value
170412 16:52:38    17 Connect    monitor@node4 on 
                   17 Query    set autocommit=1
                   17 Query    SELECT CONNECTION_ID() AS Value
                   17 Query    SET wait_timeout=86400
                   17 Query    SELECT @@global.server_id As Value
                   17 Query    SELECT VERSION() AS Value
                   17 Query    SELECT @@global.gtid_mode As Value
                   17 Query    SHOW GLOBAL VARIABLES LIKE 'log_bin'
                   17 Query    SHOW MASTER STATUS
                   17 Query    SELECT @@global.datadir AS Value
                   17 Query    SELECT @@global.slave_parallel_workers AS Value
                   17 Query    SHOW SLAVE STATUS
                   17 Query    SELECT @@global.read_only As Value
                   17 Query    SELECT @@global.relay_log_purge As Value
                   17 Query    SELECT @@global.relay_log_info_repository AS Value
                   17 Query    SELECT @@global.datadir AS Value
                   17 Query    SELECT @@global.relay_log_info_file AS Value
                   17 Query    SHOW SLAVE STATUS
                   17 Query    SELECT Repl_slave_priv AS Value FROM mysql.user WHERE user = 'repl'

           
170412 16:54:05    17 Query    SELECT GET_LOCK('MHA_Master_High_Availability_Failover', '0') AS Value
                   17 Query    SHOW SLAVE STATUS
                   17 Query    SHOW SLAVE STATUS
                   

170412 16:55:50    17 Query    SHOW SLAVE STATUS
170412 16:55:51    17 Query    SHOW SLAVE STATUS
                   17 Query    SELECT MASTER_POS_WAIT('mysql-bin.000017','120',0) AS Result
                   17 Query    STOP SLAVE SQL_THREAD
                   17 Query    SHOW SLAVE STATUS
                   17 Query    STOP SLAVE
                   17 Query    STOP SLAVE
                   17 Query    SHOW SLAVE STATUS
                   17 Query    RESET SLAVE
                   17 Query    CHANGE MASTER TO MASTER_HOST = '192.168.244.20' MASTER_USER = 'repl' MASTER_PASSWORD = <secret> MASTE
R_PORT = 3306 MASTER_LOG_FILE = 'mysql-bin.000010' MASTER_LOG_POS = 120           17 Query    SET GLOBAL relay_log_purge=0
                   17 Query    START SLAVE
                   18 Connect Out    repl@192.168.244.20:3306
                   17 Query    SHOW SLAVE STATUS
170412 16:55:52    17 Query    SELECT RELEASE_LOCK('MHA_Master_High_Availability_Failover') As Value
                   17 Quit