I/O unit size in InnoDB is only one page, even if doing read ahead. 16KB I/O unit size is too small for sequential reads, and much less efficient than 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.
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.
With this feature InnoDB submits multiple page I/O requests.
This variable shows the number of submitted buffered asynchronous I/O requests.
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