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15.8.7 Configuring the InnoDB Master Thread I/O Rate
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.
The parameter innodb_io_capacity
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 200
,
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.
The innodb_io_capacity
setting is
a total limit for all buffer pool instances. When dirty pages are
flushed, the innodb_io_capacity
limit is divided equally among buffer pool instances. For more
information, see the
innodb_io_capacity
system
variable description.
You can set the value of this parameter in the MySQL option file
(my.cnf
or my.ini
) or change
it dynamically with the
SET GLOBAL
statement, which requires privileges sufficient to set global
system variables. See
Section 5.1.9.1, “System Variable Privileges”.
The innodb_flush_sync
configuration option causes the
innodb_io_capacity
setting to be
ignored during bursts of I/O activity that occur at checkpoints.
innodb_flush_sync
is enabled by
default.
In earlier MySQL releases, the InnoDB
master
thread also performed any needed
purge operations. Those I/O
operations are now performed by other background threads, whose
number is controlled by the
innodb_purge_threads
configuration option.
For more information about InnoDB I/O performance, see Section 8.5.8, “Optimizing InnoDB Disk I/O”.
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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