show slave status\G中的Read_Master_Log_Pos和Relay_Log_Pos的(大小)关系

By admin

- 2 minutes read - 247 words

Just to clarify, there are three sets of file/position coordinates in SHOW SLAVE STATUS:

  1. The position, ON THE MASTER, from which the I/O thread is reading: Master_Log_File/Read_Master_Log_Pos. —–相对于主库,从库读取主库的二进制日志的位置,是IO线程

  2. The position, IN THE RELAY LOGS, at which the SQL thread is executing: Relay_Log_File/Relay_Log_Pos —-相对于从库,是从库的sql线程执行到的位置

  3. The position, ON THE MASTER, at which the SQL thread is executing: Relay_Master_Log_File/Exec_Master_Log_Pos —-相对于主库,是从库的sql线程执行到的位置

Numbers 2) and 3) are the same thing, but one is on the slave and the other is on the master.

mysql > show slave status \G

Master_Log_File: mysql-bin-m.000329

Read_Master_Log_Pos: 863952156　—-上面二行代表IO线程,相对于主库的二进制文件

Relay_Log_File: mysql-relay.003990

Relay_Log_Pos: _25077069　—-_上面二行代表了sql线程,相对于从库的中继日志文件

Relay_Master_Log_File: mysql-bin-m.000329

…..

Exec_Master_Log_Pos: 863936961—上面二行代表了sql线程,相对主库

(为了方便演示,我把上面这行提前了.)

Relay_Log_Space: 25092264—当前relay-log文件的大小

Slave_IO_Running: Yes

Slave_SQL_Running: Yes

从上面可以看到,read_master_log_pos 始终会大于exec_master_log_pos的值(也有可能相等):因为一个值是代表io线程,一个值代表sql线程;sql线程肯定在io线程之后.(当然,io线程和sql线程要读写同一个文件,否则比较就失去意义了) .

在binlog中,Xid代表了提交的事务号.现在我们分别去主从库看看,验证一下,在主库的mysql-bin-m.000329文件的863936961处是否与从库的mysql-relay.003990文件的25077069处有相同的sql语句.

先看主库的binlog:

# at 863936961

#100111 20:11:39 server id 115000 end_log_pos 863937234 Query thread_id=515886 exec_time=0 error_code=0

use mall00/*!*/;

UPDATE mall00.t_item_sid88 SET item_end_time = 1263816699, item_is_online = 1, item_status = 1 WHERE iid IN (94322390, 94322428, 94322452, 94322473, 94322506, 94322532, 94322604, 94322641, 94322670, 94322706)/!/;

# at 863937234

#100111 20:11:39 server id 115000 end_log_pos 863937261 Xid = 1225244590

COMMIT/*!*/;

# at 863937261

#100111 20:11:39 server id 115000 end_log_pos 863937457 Query thread_id=515886 exec_time=0error_code=0

SET TIMESTAMP=1263211899/*!*/;

再看从库的relaylog:

# at 25077069

#100111 20:11:39 server id 115000 end_log_pos 863937234 Query thread_id=515886 exec_time=0 error_code=0

use mall00/*!*/;

UPDATE mall00.t_item_sid88 SET item_end_time = 1263816699, item_is_online = 1, item_status = 1 WHERE iid IN (94322390, 94322428, 94322452, 94322473, 94322506, 94322532, 94322604, 94322641, 94322670, 94322706)/*!*/;

# at 25077342

#100111 20:11:39 server id 115000 end_log_pos 863937261 Xid = 1225244590

COMMIT/*!*/;

从上面的日志中,可以看到binlog与realy-log的内容是相同的,除了开头的at位置处的偏移量.因为偏移量始终是相对于文件自身来说,主库上相对于binlog本身,从库上相对于relay-log本身.还可以看到,在每个query语句过后,都有一个Xid 的event,提交该事务,也表明在mysql中是自动提交的,每条语句执行完毕后,系统都自动提交了.那么在基于行的复制中,可能会看到多条binlog 语句才对应一次commit,自然说明这是基于行的复制.

还有一点,就是主库和从库的对应位置的event大小是相同的,例如上面的:

25077342-25077069(从库上event大小)  =  863937234-863936961(主库上event大小)

否则,说明从库的relay-log丢失了,有可能是操作系统缓存丢失,也可能是mysql 异常crash导致relay-log buffer中的数据丢失.那么这时候就需要重设主从同步了.