Multiple page asynchronous I/O requests¶
The I/O unit size in InnoDB is only one page, even if the server 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 can be done more efficiently. 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 a 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.
- 5.6.38-83.0 – Implemented
This variable shows the total number of submitted buffered asynchronous I/O requests. The variable is updated after the submission request to the kernel and is a counter, which always increases.
The following is an example of a variable call:
mysql> SHOW GLOBAL STATUS like "innodb_buffered_aio_submitted"; innodb_buffered_aio_submitted 12439