转 ： 请问mysql如何确定一个库是主库还是从库。

select user,host from mysql.user;
SELECT Repl_slave_priv,Repl_client_priv,super_priv,host FROM mysql.USER WHERE USER='db';

##如果是从库，该 输出 会有 记录： 

mysql> show slave statusG;
*************************** 1. row ***************************
Slave_IO_State:
Master_Host: 10.200.212.66
Master_User: repl
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.000004
Read_Master_Log_Pos: 150641419
Relay_Log_File: mysql-relay-bin.000009
Relay_Log_Pos: 4
Relay_Master_Log_File: mysql-bin.000004
Slave_IO_Running: No
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: 150641419
Relay_Log_Space: 240
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@10.200.212.66:3306' - retry-time: 60 retries: 86400
Last_SQL_Errno: 0
Last_SQL_Error:
Replicate_Ignore_Server_Ids:
Master_Server_Id: 0
Master_UUID: 2e3c5fc9-8b01-11e8-a7f2-005056a6ac0b
Master_Info_File: /db/mysql/data/mydata/master.info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
Master_Retry_Count: 86400
Master_Bind:
Last_IO_Error_Timestamp: 191016 18:07:25
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set:
Executed_Gtid_Set: 2e3c5fc9-8b01-11e8-a7f2-005056a6ac0b:1-163031,
3d838987-8b2e-11e8-a918-005056a671cc:1-48721749
Auto_Position: 1
1 row in set (0.00 sec)

ERROR:
No query specified

###如果是主库，这条命令会有输出。并且read_only 状态是off.

mysql> SHOW MASTER STATUS ;
+------------------+-----------+--------------+------------------+------------------------------------------------------------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+------------------+-----------+--------------+------------------+------------------------------------------------------------------------------------------------+
| mysql-bin.000259 | 416932786 | | | 2e3c5fc9-8b01-11e8-a7f2-005056a6ac0b:1-163031,
3d838987-8b2e-11e8-a918-005056a671cc:1-48730726 |
+------------------+-----------+--------------+------------------+------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show variables like 'read_only';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| read_only | OFF |
+---------------+-------+
1 row in set (0.00 sec)

mysql> show variables like 'read_only';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| read_only | OFF |
+---------------+-------+
1 row in set (0.00 sec)

###

极少情况下，