https://book.2cto.com/201402/40300.html
在MySQL5.1.X版本中，innodb_file_io_threads参数默认是4，该参数在Linux系统上是不可更改的，但Windows系统上可以调整。这个参数的作用是：InnoDB使用后台线程处理数据页上读写I/O（输入输出）请求的数量。
在MySQL5.5.X版本中，或者说是在InnoDB Plugin1.0.4以后，就用两个新的参数，即innodb_read_io_threads和innodb_write_io_threads，取代了innodb_file_io_threads如此调整后，在Linux平台上就可以根据CPU核数来更改相应的参数值了，默认是4。
假如CPU是2颗8核的，那么可以设置：
innodb_read_io_threads = 8
innodb_write_io_threads = 8
如果数据库的读操作比写操作多，那么可以设置：
innodb_read_io_threads = 10
innodb_write_io_threads = 6
也就是说，你可以根据情况加以设置。
注意
这两个参数不支持动态改变，需要把该参数加入my.cnf里，修改完后重启MySQL服务，允许值的范围是1～64。
调整完成后，你可以用命令“show engine innodb status\G;”来查看调整结果，如下所示：
show engine innodb status
--------
FILE I/O
--------
I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
I/O thread 1 state: waiting for completed aio requests (log thread)
I/O thread 2 state: waiting for completed aio requests (read thread)
I/O thread 3 state: waiting for completed aio requests (read thread)
I/O thread 4 state: waiting for completed aio requests (read thread)
I/O thread 5 state: waiting for completed aio requests (read thread)
I/O thread 6 state: waiting for completed aio requests (write thread)
I/O thread 7 state: waiting for completed aio requests (write thread)
I/O thread 8 state: waiting for completed aio requests (write thread)
I/O thread 9 state: waiting for completed aio requests (write thread)
关于innodb_read_io_threads和innodb_write_io_threads参数的含义，请参见MySQL5.5 手册：