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block: document iostat changes for disk busy time accounting 

Since commit 5b18b5a737 ("block: delete part_round_stats and switch to
less precise counting") io_ticks is approximated by adding one at each
start and end of requests if jiffies has changed.

This works perfectly for requests shorter than a jiffy. If requests runs
more than 2 jiffies some I/O time will not be accounted unless there are
other reuqests.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Konstantin Khlebnikov 2019-06-09 14:14:36 +03:00 committed by Jonathan Corbet
parent 3175320232
commit 9d9b889540
1 changed files with 4 additions and 0 deletions
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Documentation/iostats.txt
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@ -97,6 +97,10 @@ Field 9 -- # of I/Os currently in progress
Field 10 -- # of milliseconds spent doing I/Os
This field increases so long as field 9 is nonzero.
Since 5.0 this field counts jiffies when at least one request was
started or completed. If request runs more than 2 jiffies then some
I/O time will not be accounted unless there are other requests.
Field 11 -- weighted # of milliseconds spent doing I/Os
This field is incremented at each I/O start, I/O completion, I/O
merge, or read of these stats by the number of I/Os in progress