MySQL 5.7 Release Notes
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
innodb_read_io_threads and
innodb_write_io_threads
configuration parameters. 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 values
for these parameters in the MySQL option file
(my.cnf or my.ini); you
cannot change values dynamically. The default value for these
parameters is 4 and permissible values range
from 1-64.
The purpose of these configuration options 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
read-ahead requests.
InnoDB tries to balance the load of incoming
requests in such way that most 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. 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
output, you might improve performance by increasing the value of
innodb_read_io_threads.
On Linux systems, InnoDB uses the asynchronous
I/O subsystem by default to perform read-ahead and write requests
for data file pages, which changes the way that
InnoDB background threads service these types
of I/O requests. For more information, see
Section 14.8.7, “Using Asynchronous I/O on Linux”.
For more information about InnoDB I/O
performance, see Section 8.5.8, “Optimizing InnoDB Disk I/O”.