InnoDB uses background threads to service various types of I/O requests. Starting from InnoDB storage engine 1.0.4, the number of background threads tasked with servicing read and write I/O on data pages is configurable. In previous versions of InnoDB, there was only one thread each for read and write on non-Windows platforms. On Windows, the number of background threads was controlled by innodb_file_io_threads. The configuration parameter innodb_file_io_threads has been removed in InnoDB storage engine 1.0.4. If you try to set a value for this parameter, a warning is written to the log file and the value is ignored.

In place of innodb_file_io_threads, two new configuration parameters are introduced in the InnoDB storage engine 1.0.4, which are effective on all supported platforms. The two parameters innodb_read_io_threads and innodb_write_io_threads signify the number of background threads used for read and write requests respectively. You can set the value of these parameters in the MySQL option file (my.cnf or my.ini). These parameters cannot be changed dynamically. The default value for these parameters is 4 and the permissible values range from 1-64.

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 times innodb_read_io_threads pending read requests in SHOW ENGINE INNODB STATUS, then you may gain by increasing the value of innodb_read_io_threads.