15.8.3.5 Configuring InnoDB Buffer Pool Flushing

InnoDB performs certain tasks in the background, including flushing of dirty pages (those pages that have been changed but are not yet written to the database files) from the buffer pool.

InnoDB starts flushing buffer pool pages when the percentage of dirty pages in the buffer pool reaches the low water mark setting defined by innodb_max_dirty_pages_pct_lwm. This option is intended to control the ratio of dirty pages in the buffer pool and ideally prevent the percentage of dirty pages from reaching innodb_max_dirty_pages_pct. If the percentage of dirty pages in the buffer pool exceeds innodb_max_dirty_pages_pct, InnoDB begins to aggressively flush buffer pool pages.

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 if 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 workload.

Internal benchmarking has 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 configuration parameter lets you turn off this feature. The default value for innodb_adaptive_flushing is ON, 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 statement, which requires privileges sufficient to set global system variables. See Section 5.1.9.1, “System Variable Privileges”.

For information about fine-tuning InnoDB buffer pool flushing behavior, see Section 15.8.3.6, “Fine-tuning InnoDB Buffer Pool Flushing”.

For more information about InnoDB I/O performance, see Section 8.5.8, “Optimizing InnoDB Disk I/O”.