14.8.3.5 Configuring Buffer Pool Flushing

InnoDB performs certain tasks in the background, including flushing of dirty pages from the buffer pool. Dirty pages are those that have been modified but are not yet written to the data files on disk.

In MySQL 5.7, buffer pool flushing is performed by page cleaner threads. The number of page cleaner threads is controlled by the innodb_page_cleaners variable, which has a default value of 4. However, if the number of page cleaner threads exceeds the number of buffer pool instances, innodb_page_cleaners is automatically set to the same value as innodb_buffer_pool_instances.

Buffer pool flushing is initiated when the percentage of dirty pages reaches the low water mark value defined by the innodb_max_dirty_pages_pct_lwm variable. The default low water mark is 0, which disables this early flushing behaviour.

The purpose of the innodb_max_dirty_pages_pct_lwm threshold is to control the percentage dirty pages in the buffer pool and to prevent the amount of dirty pages from reaching the threshold defined by the innodb_max_dirty_pages_pct variable, which has a default value of 75. InnoDB aggressively flushes buffer pool pages if the percentage of dirty pages in the buffer pool reaches the innodb_max_dirty_pages_pct threshold.

When configuring innodb_max_dirty_pages_pct_lwm, the value should always be lower than the innodb_max_dirty_pages_pct value.

Additional variables permit fine-tuning of buffer pool flushing behavior:

The innodb_flush_neighbors variable defines whether flushing a page from the buffer pool also flushes other dirty pages in the same extent.
- A setting of 0 disables innodb_flush_neighbors. Dirty pages in the same extent are not flushed.
- The default setting of 1 flushes contiguous dirty pages in the same extent.
- A setting of 2 flushes dirty pages in the same extent.
When table data is stored on a traditional HDD storage device, flushing neighbor pages in one operation reduces I/O overhead (primarily for disk seek operations) compared to flushing individual pages at different times. For table data stored on SSD, seek time is not a significant factor and you can disable this setting to spread out write operations.
The innodb_lru_scan_depth variable specifies, per buffer pool instance, how far down the buffer pool LRU list the page cleaner thread scans looking for dirty pages to flush. This is a background operation performed by a page cleaner thread once per second.
A setting smaller than the default is generally suitable for most workloads. A value that is significantly higher than necessary may impact performance. Only consider increasing the value if you have spare I/O capacity under a typical workload. Conversely, if a write-intensive workload saturates your I/O capacity, decrease the value, especially in the case of a large buffer pool.
When tuning innodb_lru_scan_depth, start with a low value and configure the setting upward with the goal of rarely seeing zero free pages. Also, consider adjusting innodb_lru_scan_depth when changing the number of buffer pool instances, since innodb_lru_scan_depth * innodb_buffer_pool_instances defines the amount of work performed by the page cleaner thread each second.

The innodb_flush_neighbors and innodb_lru_scan_depth variables are primarily intended for write-intensive workloads. With heavy DML activity, flushing can fall behind if it is not aggressive enough, or disk writes can saturate I/O capacity if flushing is too aggressive. The ideal settings depend on your workload, data access patterns, and storage configuration (for example, whether data is stored on HDD or SSD devices).

Adaptive Flushing

InnoDB uses an adaptive flushing algorithm to dynamically adjust the 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 flushing activity keeps pace with the current workload. Automatically adjusting the flushing rate helps avoid sudden dips in throughput that can occur when bursts of I/O activity due to buffer pool flushing affects the I/O capacity available for ordinary read and write activity.

Sharp checkpoints, which are typically associated with write-intensive workloads that generate a lot of redo entries, can cause a sudden change in throughput, for example. A sharp checkpoint occurs when InnoDB wants to reuse a portion of a log file. Before doing so, all dirty pages with redo entries in that portion of the log file must be flushed. If log files become full, a sharp checkpoint occurs, causing a temporary reduction in throughput. This scenario can occur even if innodb_max_dirty_pages_pct threshold is not reached.

The adaptive flushing algorithm helps avoid such scenarios by tracking the number of dirty pages in the buffer pool and the rate at which redo log records are being generated. Based on this information, it decides how many dirty pages to flush from the buffer pool each second, which permits it to manage sudden changes in workload.

The innodb_adaptive_flushing_lwm variable defines a low water mark for redo log capacity. When that threshold is crossed, adaptive flushing is enabled, even if the innodb_adaptive_flushing variable is disabled.

Internal benchmarking has shown that the algorithm not only maintains throughput over time, but can also improve overall throughput significantly. However, adaptive flushing can affect the I/O pattern of a workload significantly and may not be appropriate in all cases. It gives the most benefit when the redo log is in danger of filling up. If adaptive flushing is not appropriate to the characteristics of your workload, you can disable it. Adaptive flushing controlled by the innodb_adaptive_flushing variable, which is enabled by default.

innodb_flushing_avg_loops defines the number of iterations that InnoDB keeps the previously calculated snapshot of the flushing state, controlling how quickly adaptive flushing responds to foreground workload changes. A high innodb_flushing_avg_loops value means that InnoDB keeps the previously calculated snapshot longer, so adaptive flushing responds more slowly. When setting a high value it is important to ensure that redo log utilization does not reach 75% (the hardcoded limit at which asynchronous flushing starts), and that the innodb_max_dirty_pages_pct threshold keeps the number of dirty pages to a level that is appropriate for the workload.

Systems with consistent workloads, a large log file size (innodb_log_file_size), and small spikes that do not reach 75% log space utilization should use a high innodb_flushing_avg_loops value to keep flushing as smooth as possible. For systems with extreme load spikes or log files that do not provide a lot of space, a smaller value allows flushing to closely track workload changes, and helps to avoid reaching 75% log space utilization.

Be aware that if flushing falls behind, the rate of buffer pool flushing can exceed the I/O capacity available to InnoDB, as defined by innodb_io_capacity setting. The innodb_io_capacity_max value defines an upper limit on I/O capacity in such situations, so that a spike in I/O activity does not consume the entire I/O capacity of the server.

The innodb_io_capacity setting is applicable to all buffer pool instances. When dirty pages are flushed, I/O capacity is divided equally among buffer pool instances.