前言:大家平时在检查主从复制的时候基本上都会使用 show slave status来看主从复制的情况，那么你真的读懂每个指标项了吗？

一、status指标解读

root@localhost [(none)]>show slave status\G
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    7
Current database: *** NONE ***

*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 192.168.50.3
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000001
          Read_Master_Log_Pos: 728
               Relay_Log_File: relay-bin.000002
                Relay_Log_Pos: 941
        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: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 728
              Relay_Log_Space: 1142
              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: 1003306
                  Master_UUID: dff47046-3ad5-11e8-86fe-080027de0e0e
             Master_Info_File: /storage/mysql/data/master.info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: dff47046-3ad5-11e8-86fe-080027de0e0e:1-3
            Executed_Gtid_Set: dff47046-3ad5-11e8-86fe-080027de0e0e:1-3
                Auto_Position: 1
         Replicate_Rewrite_DB: 
                 Channel_Name: 
           Master_TLS_Version: 
1 row in set (0.00 sec)

下面我们逐个来解读一下：

二、日常运维注意点

• 在GTID环境下，主从同步延迟观察就非常简单了，我们只需要观察Retrieved_Gtid_Set表示的就是io thread读取到的位置，Executed_Gtid_Set表示的就是sql thread执行到的位置
• 在非GTID环境下，主从复制观察相对来说稍微麻烦一些，(Master_Log_file, Read_Master_Log_Pos): 表示io thread读取到的master binlog位置；
  Relay_Master_Log_File, Exec_Master_Log_Pos): 表示sql thread已经执行到的位置；
  (Relay_Log_File, Relay_Log_Pos): 表示sql thread执行到的relay log位置
• Seconds_Behind_Master表示slave延迟master的时间数，并不是很准确，计算公式是:io_thread.timestamp-sql_thread.timestamp