一个有趣的现象----innodb_io_capacity_MySQL

之前公司客户有出现过一种情况，是使用sysbench 进行压力测试，在测试的过程中发生一个现象，如下所示

下面是客户那里的输出结果：

[1310s] threads: 600, tps: 2176.70, reads: 1087.10, writes: 1089.60, response time: 1076.07ms (95%), errors: 0.00, reconnects:  0.00
[1320s] threads: 600, tps: 2292.10, reads: 1144.30, writes: 1147.80, response time: 805.14ms (95%), errors: 0.00, reconnects:  0.00
[1330s] threads: 600, tps: 2205.90, reads: 1103.30, writes: 1102.60, response time: 969.33ms (95%), errors: 0.00, reconnects:  0.00
[1340s] threads: 600, tps: 2038.20, reads: 1015.80, writes: 1022.40, response time: 920.41ms (95%), errors: 0.00, reconnects:  0.00
[1350s] threads: 600, tps: 2002.90, reads: 998.90, writes: 1004.00, response time: 1096.88ms (95%), errors: 0.00, reconnects:  0.00
[1360s] threads: 600, tps: 2002.90, reads: 1000.10, writes: 1002.80, response time: 1108.77ms (95%), errors: 0.00, reconnects:  0.00
[1370s] threads: 600, tps: 2114.90, reads: 1057.60, writes: 1057.30, response time: 930.94ms (95%), errors: 0.00, reconnects:  0.00
[1380s] threads: 600, tps: 2073.30, reads: 1033.90, writes: 1039.40, response time: 967.59ms (95%), errors: 0.00, reconnects:  0.00
[1390s] threads: 600, tps: 2314.09, reads: 1153.99, writes: 1160.09, response time: 1016.58ms (95%), errors: 0.00, reconnects:  0.00
[1400s] threads: 600, tps: 1850.91, reads: 924.21, writes: 926.71, response time: 1543.45ms (95%), errors: 0.00, reconnects:  0.00
[1410s] threads: 600, tps: 2493.41, reads: 1243.81, writes: 1249.91, response time: 1124.14ms (95%), errors: 0.00, reconnects:  0.00
[1420s] threads: 600, tps: 1628.29, reads: 815.40, writes: 812.60, response time: 1302.12ms (95%), errors: 0.00, reconnects:  0.00
[1430s] threads: 600, tps: 1737.90, reads: 865.30, writes: 872.60, response time: 1128.86ms (95%), errors: 0.00, reconnects:  0.00
[1440s] threads: 600, tps: 1576.90, reads: 787.60, writes: 789.30, response time: 1375.44ms (95%), errors: 0.00, reconnects:  0.00
[1450s] threads: 600, tps: 1773.60, reads: 884.00, writes: 889.60, response time: 1374.20ms (95%), errors: 0.00, reconnects:  0.00
[1460s] threads: 600, tps: 1845.71, reads: 922.71, writes: 923.01, response time: 1252.42ms (95%), errors: 0.00, reconnects:  0.00
[1470s] threads: 600, tps: 2229.28, reads: 1111.89, writes: 1117.39, response time: 1001.47ms (95%), errors: 0.00, reconnects:  0.00
[1480s] threads: 600, tps: 2510.32, reads: 1254.31, writes: 1256.71, response time: 918.75ms (95%), errors: 0.00, reconnects:  0.00
[1490s] threads: 600, tps: 1908.09, reads: 951.79, writes: 955.59, response time: 1148.29ms (95%), errors: 0.00, reconnects:  0.00
[1500s] threads: 600, tps: 2327.93, reads: 1161.71, writes: 1166.41, response time: 1395.34ms (95%), errors: 0.00, reconnects:  0.00
[1510s] threads: 600, tps: 2329.08, reads: 1162.89, writes: 1165.99, response time: 988.08ms (95%), errors: 0.00, reconnects:  0.00
[1520s] threads: 600, tps: 2036.43, reads: 1017.81, writes: 1018.61, response time: 938.21ms (95%), errors: 0.00, reconnects:  0.00
[1530s] threads: 600, tps: 787.59, reads: 393.19, writes: 394.39, response time: 1060.72ms (95%), errors: 0.00, reconnects:  0.00
[1540s] threads: 600, tps: 0.00, reads: 0.00, writes: 0.00, response time: 0.00ms (95%), errors: 0.00, reconnects:  0.00
[2120s] threads: 600, tps: 0.00, reads: 0.00, writes: 0.00, response time: 0.00ms (95%), errors: 0.00, reconnects:  0.00
[2130s] threads: 600, tps: 0.00, reads: 0.00, writes: 0.00, response time: 0.00ms (95%), errors: 0.00, reconnects:  0.00
[2140s] threads: 600, tps: 219.00, reads: 108.30, writes: 110.70, response time: 615414.74ms (95%), errors: 0.00, reconnects:  0.00
[2150s] threads: 600, tps: 2046.80, reads: 1023.90, writes: 1022.90, response time: 1158.65ms (95%), errors: 0.00, reconnects:  0.00
[2160s] threads: 600, tps: 2560.12, reads: 1275.81, writes: 1284.31, response time: 854.55ms (95%), errors: 0.00, reconnects:  0.00
[2170s] threads: 600, tps: 3093.08, reads: 1542.49, writes: 1550.59, response time: 783.97ms (95%), errors: 0.00, reconnects:  0.00
[2180s] threads: 600, tps: 3234.00, reads: 1616.00, writes: 1618.00, response time: 698.42ms (95%), errors: 0.00, reconnects:  0.00
[2190s] threads: 600, tps: 3709.84, reads: 1851.62, writes: 1858.62, response time: 772.09ms (95%), errors: 0.00, reconnects:  0.00
[2200s] threads: 600, tps: 3492.39, reads: 1741.19, writes: 1750.79, response time: 762.67ms (95%), errors: 0.00, reconnects:  0.00
[2210s] threads: 600, tps: 3282.96, reads: 1639.88, writes: 1643.08, response time: 889.00ms (95%), errors: 0.00, reconnects:  0.00
[2220s] threads: 600, tps: 3922.43, reads: 1958.12, writes: 1964.32, response time: 690.32ms (95%), errors: 0.00, reconnects:  0.00
[2230s] threads: 600, tps: 3949.69, reads: 1972.60, writes: 1977.10, response time: 836.58ms (95%), errors: 0.00, reconnects:  0.00
[2240s] threads: 600, tps: 4091.38, reads: 2042.09, writes: 2049.29, response time: 617.39ms (95%), errors: 0.00, reconnects:  0.00

在中途会有一阵TPS为零，为什么会出现上述的情况呢，是因为脏页过多，MySQL 必须先将脏页刷到磁盘才能继续工作．

要想了解脏页与redo log 之间的关系，请看　http://blog.csdn.net/yaoqinglin/article/details/46646267

当脏页刷新的速度不及事务提交的速度，导致脏页过多时，就会触发MySQL 的保护机制，停止写入的操作，只刷盘，直到MySQL认为OK了才好．

配置文件如下

innodb_log_file_size = 1000M 
innodb_log_files_in_group = 4
innodb_max_dirty_pages_pct = 75 
innodb_io_capacity  = 200

问题的原因找到了，那怎么解决这个问题呢？

个人觉得应该：最为重要的是 减小 innodb_io_capacity

原理分析：

首先看下图

当Log Pad 占了redo log 的75%以上，MySQL会异步的将Log pad所表示的脏页刷到磁盘中，但是此时MySQL不会停止事务的提交以及写入redo log.

当Log Pad 占了redo log 的90%时，MySQL会停止全部的写入操作，将Log Pad 刷新到磁盘．

造成这种情况的原因呢，自然是刷新的速度比不上事务的提交的速度．但是我们在发生问题之间的监控表示，磁盘的I/O并没有被大量的使用，那么MySQL为甚么不

在发生问题之前使用磁盘I/O开始刷，以减轻发生问题时的压力．

原因是MySQL有一种自适应的刷盘方式，控制整个刷新进程．innodb_adaptive_flushing，innodb_io_capacity, innodb_max_dirty_pages_pct, redo log 大小来判断什么时候

开始刷新脏页．怎么判断呢，大致上MySQL 会根据innodb_io_capacity来判断更新的速度能不能在可控的范围内．如果innodb_io_capacity设置过大，则会造成MySQL高估了

磁盘的能力，导致脏页堆积，就会出现本文所说的问题．如果设置过低，则会出现MySQL低估了磁盘的能力，使得数据库能够单位时间内提交的事务数(tps)降低．

我们的服务器的磁盘是7200rpm，属于比较低级的磁盘，根据MySQL 官方的建议，应该将innodb_io_capacity降低到100.

[root@t1 bin]# ./sg_vpd /dev/sda --page=0xb1
Block device characteristics VPD page (SBC):
  <strong>Nominal rotation rate: 7200 rpm</strong>
  Product type: Not specified
  WABEREQ=0
  WACEREQ=0
  Nominal form factor: 3.5 inch
  HAW_ZBC=0
  FUAB=0
  VBULS=0

官方的建议：

For systems with individual 5400 RPM or 7200 RPM drives, you might lower the value to the former　default of 100.

修改之后重新测试，发现不会出现问题．解决的很漂亮有木有．