Rechercher dans le manuel MySQL
15.8.3.4 Configuring InnoDB Buffer Pool Prefetching (Read-Ahead)
A read-ahead request is
an I/O request to prefetch multiple pages in the
buffer pool
asynchronously, in anticipation that these pages will be needed
soon. 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 5.1.9.1, “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 command
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 8.5.8, “Optimizing InnoDB Disk I/O” and Section 8.12.1, “Optimizing Disk I/O”.
Document created the 26/06/2006, last modified the 26/10/2018
Source of the printed document:https://www.gaudry.be/en/mysql-rf-innodb-performance-read-ahead.html
The infobrol is a personal site whose content is my sole responsibility. The text is available under CreativeCommons license (BY-NC-SA). More info on the terms of use and the author.
References
These references and links indicate documents consulted during the writing of this page, or which may provide additional information, but the authors of these sources can not be held responsible for the content of this page.
The author This site is solely responsible for the way in which the various concepts, and the freedoms that are taken with the reference works, are presented here. Remember that you must cross multiple source information to reduce the risk of errors.
