Multiple page asynchronous I/O requests¶
The I/O unit size in InnoDB is only one page, even if the server is doing read ahead.
A 16KB I/O unit size is small for sequential reads and less efficient than a
larger I/O unit size. InnoDB uses Linux asynchronous I/O (
aio) by default.
By submitting multiple consecutive 16KB read requests at once, Linux internally can
merge requests, and reads are more efficient. This feature can submit multiple
page I/O requests and works in the background.
You can manage the feature with the
linear read-ahead technique.
The technique adds pages to the buffer pool based on the buffer pool pages being accessed
sequentially. The configuration parameter,
innodb_read_ahead_threshold controls this process.
On an HDD RAID 1+0 environment, more than 1000MB/s disk reads can be achieved by submitting 64 consecutive pages requests at once, while only 160MB/s disk reads is shown by submitting single page request.
Version Specific Information¶
- Percona Server 5.7.20-18: Feature ported from the Facebook MySQL patch.
Implemented in Percona Server 5.7.20-18.
This variable shows the number of submitted buffered asynchronous I/O requests.