The master thread in InnoDB is a thread that performs various tasks in the background. Most of these tasks are I/O related, such as flushing dirty pages from the buffer pool or writing changes from the insert buffer to the appropriate secondary indexes. The master thread attempts to perform these tasks in a way that does not adversely affect the normal working of the server. It tries to estimate the free I/O bandwidth available and tune its activities to take advantage of this free capacity. Historically, InnoDB has used a hard coded value of 100 IOPs (input/output operations per second) as the total I/O capacity of the server.
indicates the overall I/O capacity available to InnoDB. This
parameter should be set to approximately the number of I/O
operations that the system can perform per second. The value
depends on your system configuration. When
innodb_io_capacity is set, the
master threads estimates the I/O bandwidth available for
background tasks based on the set value. Setting the value to
100 reverts to the old behavior.
You can set the value of
innodb_io_capacity to any number
100 or greater. The default value is
reflecting that the performance of typical modern I/O devices is
higher than in the early days of MySQL. Typically, values around
the previous default of 100 are appropriate for consumer-level
storage devices, such as hard drives up to 7200 RPMs. Faster hard
drives, RAID configurations, and SSDs benefit from higher values.
innodb_io_capacity setting is
a total limit for all buffer pool instances. When dirty pages are
limit is divided equally among buffer pool instances. For more
information, see the
You can set the value of this parameter in the MySQL option file
my.ini) or change
it dynamically with the
SET GLOBAL command,
which requires the
In earlier MySQL releases, the
thread also performed any needed
purge operations. Those I/O
operations are now performed by other background threads, whose
number is controlled by the
For more information about InnoDB I/O performance, see Section 8.5.8, “Optimizing InnoDB Disk I/O”.