问题 现象
发现从库mysql_slave的参数Seconds_Behind_Master越来越大。已排除主从服务器时间不一致;那么主要就判断两点:是io thread慢还是 sql thread慢?先观察show slave status\G 。
判断3个参数(参数后面的值是默认空闲时候的正常值):
Slave_IO_State: Waiting for master to send event
Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
Seconds_Behind_Master: 0
1.sql thread慢的表现:
Seconds_Behind_Master越来越大
Slave_SQL_Running_State: Reading event from the relay log
2.io thread慢的表现:
Seconds_Behind_Master为0
Slave_SQL_Running_State: 显示正常值
Slave_IO_State:显示忙碌状态
本文观察到的值:
Slave_IO_State: Queueing master event to the relay log
Seconds_Behind_Master: 30880
Slave_SQL_Running_State: Reading event from the relay log
上述三个参数,推断是sql thread慢
同步延迟信息详见如下:
mysql> show master status \G
*************************** 1. row ***************************
File: mysql-bin.000057
Position: 214873221
Binlog_Do_DB:
Binlog_Ignore_DB:
Executed_Gtid_Set: 838ad3df-a43d-11e8-8b02-6c92bf7daa8d:1,
efd10b43-a43c-11e8-85d8-6c92bf7d9fd1:1-134202381
1 row in set (0.00 sec)
从库延迟:
mysql> show slave status \G
*************************** 1. row ***************************
Slave_IO_State: Queueing master event to the relay log
Master_Host: 192.168.10.111
Master_User: repl
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.000057
Read_Master_Log_Pos: 214813221
Relay_Log_File: relay-bin.00045
Relay_Log_Pos: 28055325
Relay_Master_Log_File: mysql-bin.000057
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Exec_Master_Log_Pos: 94286319
Relay_Log_Space: 956058939
Seconds_Behind_Master: 30880
Master_Server_Id: 113306
Master_UUID: efd10b43-a43c-11e8-85d8-6c92bf7d9fd1
Master_Info_File: mysql.slave_master_info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Reading event from the relay log
Master_Retry_Count: 86400
Retrieved_Gtid_Set: efd10b43-a43c-11e8-85d8-6c92bf7d9fd1:153917-55474510
Executed_Gtid_Set: 838ad3df-a43d-11e8-8b02-6c92bf7daa8d:1,
efd10b43-a43c-11e8-85d8-6c92bf7d9fd1:1-54003557
Auto_Position: 1
1 row in set (0.00 sec)
注意: Seconds_Behind_Master: 30880
分析步骤:
开始怀疑是参数配置的差异,比对/etc/my.cnf后发现,发现配置并无差异。
从库使用操作系统命令dstat观察,发现繁忙时候,slave的IO写速度上不去
[root@mysql ~]# dstat
You did not select any stats, using -cdngy by default.
----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq| read writ| recv send| in out | int csw
0 0 100 0 0 0| 19k 686k| 0 0 | 0 0 | 231 648
0 0 99 1 0 0| 0 1388k| 798B 396B| 0 0 | 358 1596
0 0 99 1 0 0| 0 1676k| 244B 170B| 0 0 | 397 1949
0 0 99 1 0 0| 0 1796k|1490B 236B| 0 0 | 451 2107
0 0 99 1 0 0| 0 1764k| 244B 170B| 0 0 | 414 2104
0 0 100 0 0 0| 0 1536k|1068B 170B| 0 0 | 390 1649
0 0 99 1 0 0| 0 1516k| 244B 170B| 0 0 | 373 1575
0 0 99 1 0 0| 0 1552k| 34k 500B| 0 0 | 410 1602
0 0 100 0 0 0| 0 1556k| 126B 236B| 0 0 | 363 1527
0 0 99 1 0 0| 0 1384k| 60B 170B| 0 0 | 349 1369
0 0 99 1 0 0| 0 1440k| 13k 434B| 0 0 | 400 1513
0 0 100 0 0 0| 0 1800k| 612B 170B| 0 0 | 418 1905
0 0 99 0 0 0| 0 1484k| 246B 236B| 0 0 | 622 1568
0 0 99 1 0 0| 0 1272k| 60B 170B| 0 0 | 347 1294
0 0 99 1 0 0| 0 1624k| 126B 236B| 0 0 | 400 1815
0 0 99 1 0 0| 0 1580k| 60B 170B| 0 0 | 358 1712
0 0 100 0 0 0| 0 1536k| 22k 302B| 0 0 | 395 1583
0 0 100 0 0 0| 0 1440k| 514B 368B| 0 0 | 383 1765
0 0 99 1 0 0| 0 1596k| 28k 500B| 0 0 | 446 2151
看到从库的写只能达到每秒1.5M左右,IO性能不是很好,也印证了初步的推测。
那么问题来了,
要如何优化IO性能比较差的slave呢?
简单,本文做了如下两个参数的修改:
mysql> set global innodb_flush_log_at_trx_commit=2;
Query OK, 0 rows affected (0.00 sec)
mysql> set global sync_binlog=20 ;
Query OK, 0 rows affected (0.00 sec)
innodb_flush_log_at_trx_commit和sync_binlog这两个参数又是个啥?
innodb_flush_log_at_trx_commit
-
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。
-
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.
-
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"并不是标准意义上的保证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。mysql服务默认是autocommit打开的。
如上sync_binlog,autocommit,innodb_flush_log_at_trx_commit三个参数,详见从库mysql_slave设置如下:
mysql> show variables like '%sync_binlog%';+---------------+-------+| Variable_name | Value |+---------------+-------+| sync_binlog | 0 |+---------------+-------+1 row in set (0.01 sec)
mysql>mysql> show variables like '%innodb_flush_log_at_trx_commit%';+--------------------------------+-------+| Variable_name | Value |+--------------------------------+-------+| innodb_flush_log_at_trx_commit | 1 |+--------------------------------+-------+1 row in set (0.00 sec)
mysql> show variables like '%autocommit%';+---------------+-------+| Variable_name | Value |+---------------+-------+| autocommit | ON |+---------------+-------+1 row in set (0.00 sec)
根据上述分析过程,修改如下值:
mysql> set global sync_binlog=20;
`mysql> set global innodb_flush_log_at_trx_commit=2;`
接下来,我们观察和验证从库同步情况
-
使用dstat命令观察磁盘IO,下图可以看到从库的写有原来的每秒1.5M左右提升为4M以上,
[root@mysql ~]# dstat
You did not select any stats, using -cdngy by default.
----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq| read writ| recv send| in out | int csw
0 0 100 0 0 0| 19k 686k| 0 0 | 0 0 | 231 648
0 0 99 1 0 0| 0 4756k| 882B 462B| 0 0 | 601 6690
0 0 99 1 0 0| 0 4208k| 430B 236B| 0 0 | 642 8771
0 0 99 1 0 0| 0 4452k| 428B 170B| 0 0 | 607 7803
0 0 99 1 0 0| 0 7964k| 26k 500B| 0 0 | 589 6835
0 0 99 1 0 0| 0 3832k|1166B 170B| 0 0 | 368 2467
0 0 99 1 0 0| 0 4584k|2042B 236B| 0 0 | 523 4632
0 0 99 1 0 0| 0 4972k|1296B 170B| 0 0 | 661 9298
0 0 99 1 0 0| 0 4792k|1046B 236B| 0 0 | 606 9634
0 0 99 1 0 0| 0 5244k|3032B 212B| 0 0 | 745 9965
0 0 99 0 0 0| 0 4648k| 796B 170B| 0 0 |1166 19k
0 0 99 1 0 0| 0 4608k| 21k 434B| 0 0 | 827 14k
0 0 99 0 0 0| 0 4552k|1858B 236B| 0 0 | 971 17k
0 0 99 0 0 0| 0 4580k| 980B 170B| 0 0 | 936 17k
0 0 99 1 0 0| 0 8332k| 21k 434B| 0 0 | 940 13k
0 0 99 0 0 0| 0 4136k|1112B 236B| 0 0 |1112 20k
0 0 99 0 0 0| 0 4940k| 796B 170B| 0 0 |1045 19k
0 0 99 0 0 0| 0 4496k| 22k 500B| 0 0 | 951 15k
0 0 99 0 0 0| 0 4652k| 862B 170B| 0 0 |1022 18k
0 0 99 0 0 0| 0 5472k|1546B 236B| 0 0 | 950 15k
0 0 99 0 0 0| 0 2616k| 20k 434B| 0 0 | 901 15k
0 0 99 1 0 0| 0 9564k|2410B 170B| 0 0 | 958 14 -
从库使用 show slave status \G,观察到Seconds_Behind_Master: 21202在持续缩小和主库的差距。Seconds_Behind_Master:由原来的30880降为21202,且在持续减少中。
mysql> show slave status \G
*************************** 1. row ***************************
Slave_IO_State: Queueing master event to the relay log
Master_Host: 192.168.10.111
Master_User: repl
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.000057
Read_Master_Log_Pos: 214833221
Relay_Log_File: relay-bin.00045
Relay_Log_Pos: 28064325
Relay_Master_Log_File: mysql-bin.000057
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Exec_Master_Log_Pos: 544925182
Relay_Log_Space: 973436169
Master_SSL_Allowed: No
Seconds_Behind_Master: 21202
Replicate_Ignore_Server_Ids:
Master_Server_Id: 113306
Master_UUID: efd10b43-a43c-11e8-85d8-6c92bf7d9fd1
Master_Info_File: mysql.slave_master_info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Waiting for dependent transaction to commit
Master_Retry_Count: 86400
Retrieved_Gtid_Set: efd10b43-a43c-11e8-85d8-6c92bf7d9fd1:153917-55504200
Executed_Gtid_Set: 838ad3df-a43d-11e8-8b02-6c92bf7daa8d:1,
efd10b43-a43c-11e8-85d8-6c92bf7d9fd1:1-54771730
Auto_Position: 1
1 row in set (4.98 sec)
至此,Seconds_Behind_Master越来越打的问题得到解决。