Configuring the Rate of InnoDB Buffer Pool Flushing

叁歲伎倆 2022-08-27 13:58 134阅读 0赞

本文转载自：http://dev.mysql.com/doc/refman/5.5/en/innodb-performance-adaptive\_flushing.html

#### Configuring the Rate of `InnoDB` Buffer Pool Flushing ####

InnoDB performs certain tasks in the background, including [ flushing][flushing] of [ dirty pages][dirty pages] (those pages that have been changed but are not yet written to the database files) from the[buffer pool][], a task performed by the [ master thread][master thread]. Currently, InnoDB aggressively flushes buffer pool pages if the percentage of dirty pages in the buffer pool exceeds[`innodb_max_dirty_pages_pct`][innodb_max_dirty_pages_pct].

InnoDB uses an algorithm to estimate the required rate of flushing, based on the speed of redo log generation and the current rate of flushing. The intent is to smooth overall performance by ensuring that buffer flush activity keeps up with the need to keep the buffer pool “clean”. Automatically adjusting the rate of flushing can help to avoid sudden dips in throughput, when excessive buffer pool flushing limits the I/O capacity available for ordinary read and write activity.

InnoDB uses its log files in a circular fashion. Before reusing a portion of a log file, InnoDB flushes to disk all dirty buffer pool pages whose redo entries are contained in that portion of the log file, a process known as a[sharp checkpoint][]. If a workload is write-intensive, it generates a lot of redo information, all written to the log file. If all available space in the log files is used up, a sharp checkpoint occurs, causing a temporary reduction in throughput. This situation can happen even though [ `innodb_max_dirty_pages_pct`][innodb_max_dirty_pages_pct] is not reached.

InnoDB uses a heuristic-based algorithm to avoid such a scenario, by measuring the number of dirty pages in the buffer pool and the rate at which redo is being generated. Based on these numbers, InnoDB decides how many dirty pages to flush from the buffer pool each second. This self-adapting algorithm is able to deal with sudden changes in the workload.

Internal benchmarking has also shown that this algorithm not only maintains throughput over time, but can also improve overall throughput significantly.

Because adaptive flushing can significantly affect the I/O pattern of a workload, the[`innodb_adaptive_flushing`][innodb_adaptive_flushing] configuration parameter lets you turn off this feature. The default value for[`innodb_adaptive_flushing`][innodb_adaptive_flushing] is `TRUE`, enabling the adaptive flushing algorithm. 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` command, which requires the `SUPER` privilege.

For more information about InnoDB I/O performance, see [ Section 8.5.7, “Optimizing `InnoDB` Disk I/O”][Section_8.5.7_ _Optimizing _InnoDB_ Disk I_O].

注：近期参加MySQL运维学习，老师推荐该文章作为学习和技术提高的扩展阅读，先记录到自己的博客中，随后慢慢消化、学习、提高。本文与MySQL数据库 “性能优化”主题有关。

[flushing]: http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_flush
[dirty pages]: http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_dirty_page
[buffer pool]: http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_buffer_pool
[master thread]: http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_master_thread
[innodb_max_dirty_pages_pct]: http://dev.mysql.com/doc/refman/5.5/en/innodb-parameters.html#sysvar_innodb_max_dirty_pages_pct
[sharp checkpoint]: http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_sharp_checkpoint
[innodb_adaptive_flushing]: http://dev.mysql.com/doc/refman/5.5/en/innodb-parameters.html#sysvar_innodb_adaptive_flushing
[Section_8.5.7_ _Optimizing _InnoDB_ Disk I_O]: http://dev.mysql.com/doc/refman/5.5/en/optimizing-innodb-diskio.html