MySQL Slave状态参数详解

## MySQL同步机制
MySQL同步功能由3个线程(master上1个，slave上2个)来实现。

执行 `mysql>START SLAVE;` 语句后，slave就创建一个I/O线程。I/O线程连接到master上，并请求master发送二进制日志中的语句。

`show slave status;` 用于提供有关从属服务器线程的关键参数的信息:

```
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 192.168.27.239
                  Master_User: copyuser
                  Master_Port: 53306
                Connect_Retry: 60
              Master_Log_File: log.000005
          Read_Master_Log_Pos: 585437746
               Relay_Log_File: host-192-168-29-228-relay-bin.000013
                Relay_Log_Pos: 585437947
        Relay_Master_Log_File: log.000005
             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: 585437746
              Relay_Log_Space: 585438209
              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: 110
                  Master_UUID: 66eafb32-a34f-11eb-af0a-020017006b3d
             Master_Info_File: /data/mysql/datadir/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:
            Executed_Gtid_Set:
                Auto_Position: 0
         Replicate_Rewrite_DB:
                 Channel_Name:
           Master_TLS_Version:
```

## 配置项详细说明
1.  `Slave_IO_State` : `SHOW PROCESSLIST`输出的State字段的拷贝。`SHOW PROCESSLIST`用于从属I/O线程。如果线程正在试图连接到主服务器，正在等待来自主服务器的时间或正在连接到主服务器等，本语句会通知您

2. `Master_Host` : 被用于连接主服务器的当前地址。

3. `Master_User` : 被用于连接主服务器的当前用户。

4. `Master_Port` : 当前的主服务器接口。

5. `Connect_Retry` :  `--master-connect-retry`选项的当前值

6. `Master_Log_File` : I/O线程当前正在读取的主服务器二进制日志文件的名称。例如：log.000005
         
7. `Read_Master_Log_Pos` : 在当前的主服务器二进制日志中，I/O线程已经读取的位置。例如：585437746
              
8. `Relay_Log_File` : SQL线程当前正在读取和执行的中继日志文件的名称。例如：localhost-relay-bin.000013
                Relay_Log_Pos: 585437947
        Relay_Master_Log_File: log.000005
             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: 585437746
              Relay_Log_Space: 585438209
              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: 110
                  Master_UUID: 66eafb32-a34f-11eb-af0a-020017006b3d
             Master_Info_File: /data/mysql/datadir/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:
            Executed_Gtid_Set:
                Auto_Position: 0
         Replicate_Rewrite_DB:
                 Channel_Name:
           Master_TLS_Version: