17.8.7 Configuring InnoDB I/O Capacity

The InnoDB master thread and other threads perform various tasks in the background, most of which are I/O related, such as flushing dirty pages from the buffer pool and writing changes from the change buffer to the appropriate secondary indexes. InnoDB attempts to perform these tasks in a way that does not adversely affect the normal working of the server. It tries to estimate the available I/O bandwidth and tune its activities to take advantage of available capacity.

The innodb_io_capacity variable defines the overall I/O capacity available to InnoDB. It should be set to approximately the number of I/O operations that the system can perform per second (IOPS). When innodb_io_capacity is set, InnoDB estimates the I/O bandwidth available for background tasks based on the set value.

You can set innodb_io_capacity to a value of 100 or greater. The default value is 200. Typically, values around 100 are appropriate for consumer-level storage devices, such as hard drives up to 7200 RPMs. Faster hard drives, RAID configurations, and solid state drives (SSDs) benefit from higher values.

Ideally, keep the setting as low as practical, but not so low that background activities fall behind. If the value is too high, data is removed from the buffer pool and change buffer too quickly for caching to provide a significant benefit. For busy systems capable of higher I/O rates, you can set a higher value to help the server handle the background maintenance work associated with a high rate of row changes. Generally, you can increase the value as a function of the number of drives used for InnoDB I/O. For example, you can increase the value on systems that use multiple disks or SSDs.

The default setting of 200 is generally sufficient for a lower-end SSD. For a higher-end, bus-attached SSD, consider a higher setting such as 1000, for example. For systems with individual 5400 RPM or 7200 RPM drives, you might lower the value to 100, which represents an estimated proportion of the I/O operations per second (IOPS) available to older-generation disk drives that can perform about 100 IOPS.

Although you can specify a high value such as a million, in practice such large values have little benefit. Generally, a value higher than 20000 is not recommended unless you are certain that lower values are insufficient for your workload.

Consider write workload when tuning innodb_io_capacity. Systems with large write workloads are likely to benefit from a higher setting. A lower setting may be sufficient for systems with a small write workload.

The innodb_io_capacity setting is not a per buffer pool instance setting. Available I/O capacity is distributed equally among buffer pool instances for flushing activities.

You can set the innodb_io_capacity value in the MySQL option file (my.cnf or my.ini) or modify it at runtime using a SET GLOBAL statement, which requires privileges sufficient to set global system variables. See Section 7.1.9.1, “System Variable Privileges”.

Ignoring I/O Capacity at Checkpoints

The innodb_flush_sync variable, which is enabled by default, causes the innodb_io_capacity setting to be ignored during bursts of I/O activity that occur at checkpoints. To adhere to the I/O rate defined by the innodb_io_capacity and innodb_io_capacity_max settings, disable innodb_flush_sync.

You can set the innodb_flush_sync value in the MySQL option file (my.cnf or my.ini) or modify it at runtime using a SET GLOBAL statement, which requires privileges sufficient to set global system variables. See Section 7.1.9.1, “System Variable Privileges”.

Configuring an I/O Capacity Maximum

If flushing activity falls behind, InnoDB can flush more aggressively, at a higher rate of I/O operations per second (IOPS) than defined by the innodb_io_capacity variable. The innodb_io_capacity_max variable defines a maximum number of IOPS performed by InnoDB background tasks in such situations.

If you specify an innodb_io_capacity setting at startup but do not specify a value for innodb_io_capacity_max, innodb_io_capacity_max defaults to twice the value of innodb_io_capacity or 2000, whichever value is greater.

When configuring innodb_io_capacity_max, twice the innodb_io_capacity is often a good starting point. The default value of 2000 is intended for workloads that use an SSD or more than one regular disk drive. A setting of 2000 is likely too high for workloads that do not use SSDs or multiple disk drives, and could allow too much flushing. For a single regular disk drive, a setting between 200 and 400 is recommended. For a high-end, bus-attached SSD, consider a higher setting such as 2500. As with the innodb_io_capacity setting, keep the setting as low as practical, but not so low that InnoDB cannot sufficiently extend rate of IOPS beyond the innodb_io_capacity setting.

Consider write workload when tuning innodb_io_capacity_max. Systems with large write workloads may benefit from a higher setting. A lower setting may be sufficient for systems with a small write workload.

innodb_io_capacity_max cannot be set to a value lower than the innodb_io_capacity value.

Setting innodb_io_capacity_max to DEFAULT using a SET statement (SET GLOBAL innodb_io_capacity_max=DEFAULT) sets innodb_io_capacity_max to the maximum value.

The innodb_io_capacity_max limit applies to all buffer pool instances. It is not a per buffer pool instance setting.