MySQL 9.1 Release Notes
A read-ahead request is
an I/O request to prefetch multiple pages in the
buffer pool
asynchronously, in anticipation of impending need for these
pages. The requests bring in all the pages in one
extent.
InnoDB uses two read-ahead algorithms to
improve I/O performance:
Linear read-ahead is a
technique that predicts what pages might be needed soon based on
pages in the buffer pool being accessed sequentially. You
control when InnoDB performs a read-ahead
operation by adjusting the number of sequential page accesses
required to trigger an asynchronous read request, using the
configuration parameter
innodb_read_ahead_threshold.
Before this parameter was added, InnoDB would
only calculate whether to issue an asynchronous prefetch request
for the entire next extent when it read the last page of the
current extent.
The configuration parameter
innodb_read_ahead_threshold
controls how sensitive InnoDB is in detecting
patterns of sequential page access. If the number of pages read
sequentially from an extent is greater than or equal to
innodb_read_ahead_threshold,
InnoDB initiates an asynchronous read-ahead
operation of the entire following extent.
innodb_read_ahead_threshold can
be set to any value from 0-64. The default value is 56. The
higher the value, the more strict the access pattern check. For
example, if you set the value to 48, InnoDB
triggers a linear read-ahead request only when 48 pages in the
current extent have been accessed sequentially. If the value is
8, InnoDB triggers an asynchronous read-ahead
even if as few as 8 pages in the extent are accessed
sequentially. You can set the value of this parameter in the
MySQL configuration
file, or change it dynamically with the
SET
GLOBAL statement, which requires privileges sufficient
to set global system variables. See
Section 7.1.9.1, “System Variable Privileges”.
Random read-ahead is a
technique that predicts when pages might be needed soon based on
pages already in the buffer pool, regardless of the order in
which those pages were read. If 13 consecutive pages from the
same extent are found in the buffer pool,
InnoDB asynchronously issues a request to
prefetch the remaining pages of the extent. To enable this
feature, set the configuration variable
innodb_random_read_ahead to
ON.
The SHOW ENGINE INNODB STATUS statement
displays statistics to help you evaluate the effectiveness of
the read-ahead algorithm. Statistics include counter information
for the following global status variables:
This information can be useful when fine-tuning the
innodb_random_read_ahead
setting.
For more information about I/O performance, see Section 10.5.8, “Optimizing InnoDB Disk I/O” and Section 10.12.1, “Optimizing Disk I/O”.