MySQL主从延迟高，Reading event from the relay log

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今天收到邮件报警，mysql主从延迟>1h，导致新的数据无法进行同步。
mysql工作目录下积累了很多的mysqld-relay文件

Seconds_Behind_Master: 越来越大。
Slave_SQL_Running_State: 一直处于Reading event from the relay log 状态

mysql> show  slave status\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 10.52.35.185
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.011374
          Read_Master_Log_Pos: 88856621
               Relay_Log_File: mysqld-relay-bin.034139
                Relay_Log_Pos: 7119391
        Relay_Master_Log_File: mysql-bin.011371
             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: performance_schema.*,test.%,information_schema.%
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 7119228
              Relay_Log_Space: 2116679529
              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: 5430
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: 185
                  Master_UUID: 870ab79e-2a14-11ea-ba96-00505697e312
             Master_Info_File: /data/mysql/db/master.info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Reading event from the relay log
           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
1 row in set (0.00 sec)

如果innodb_flush_log_at_trx_commit设置为0，log buffer将每秒一次地写入log file中，并且log file的flush(刷到磁盘)操作同时进行.该模式下，在事务提交的时候，不会主动触发写入磁盘的操作。

如果innodb_flush_log_at_trx_commit设置为1，每次事务提交时MySQL都会把log buffer的数据写入log file，并且flush(刷到磁盘)中去.

如果innodb_flush_log_at_trx_commit设置为2，每次事务提交时MySQL都会把log buffer的数据写入log file.但是flush(刷到磁盘)操作并不会同时进行。该模式下,MySQL会每秒执行一次 flush(刷到磁盘)操作。

注意：

由于进程调度策略问题,这个“每秒执行一次 flush(刷到磁盘)操作”并不是保证100%的“每秒”。

sync_binlog

sync_binlog 的默认值是0，像操作系统刷其他文件的机制一样，MySQL不会同步到磁盘中去而是依赖操作系统来刷新binary log。

当sync_binlog =N (N>0) ，MySQL 在每写 N次 二进制日志binary log时，会使用fdatasync()函数将它的写二进制日志binary log同步到磁盘中去。

注意:

如果启用了autocommit，那么每一个语句statement就会有一次写操作；否则每个事务对应一个写操作。

而且mysql服务默认是autocommit打开的

mysql> set global sync_binlog=20;
Query OK, 0 rows affected (0.00 sec)

mysql> set global innodb_flush_log_at_trx_commit=2;
Query OK, 0 rows affected (0.00 sec)

修改配置文件my.cnf 重启生效。
sync_binlog=20
innodb_flush_log_at_trx_commit=2

mysql> show  slave status\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 10.52.35.185
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.011387
          Read_Master_Log_Pos: 52242714
               Relay_Log_File: mysqld-relay-bin.034269
                Relay_Log_Pos: 52242877
        Relay_Master_Log_File: mysql-bin.011387
             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: performance_schema.*,test.%,information_schema.%
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 52242714
              Relay_Log_Space: 52243098
              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: 185
                  Master_UUID: 870ab79e-2a14-11ea-ba96-00505697e312
             Master_Info_File: /data/mysql/db/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: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: 
            Executed_Gtid_Set: 
                Auto_Position: 0
1 row in set (0.00 sec)

↑ 可以看到Seconds_Behind_Master 已经0延迟了

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