InnoDB uses background threads to service various types of I/O requests. You can configure the number of background threads that service read and write I/O on data pages, using the configuration parameters innodb_read_io_threads and innodb_write_io_threads. These parameters signify the number of background threads used for read and write requests respectively. They are effective on all supported platforms. You can set the value of these parameters in the MySQL option file (my.cnf or my.ini); you cannot change them dynamically. The default value for these parameters is 4 and the permissible values range from 1-64.

These parameters replace innodb_file_io_threads from earlier versions of MySQL. If you try to set a value for this obsolete parameter, a warning is written to the log file and the value is ignored. This parameter only applied to Windows platforms. (On non-Windows platforms, there was only one thread each for read and write.)

The purpose of this change is to make InnoDB more scalable on high end systems. Each background thread can handle up to 256 pending I/O requests. A major source of background I/O is the read-ahead requests. InnoDB tries to balance the load of incoming requests in such way that most of the background threads share work equally. InnoDB also attempts to allocate read requests from the same extent to the same thread to increase the chances of coalescing the requests together. If you have a high end I/O subsystem and you see more than 64 × innodb_read_io_threads pending read requests in SHOW ENGINE INNODB STATUS, you might gain by increasing the value of innodb_read_io_threads.

For more information about InnoDB I/O performance, see Section 8.5.7, “Optimizing InnoDB Disk I/O”.