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dm cache policy mq: tweak algorithm that decides when to promote a block

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Rather than maintaining a separate promote_threshold variable that we
periodically update we now use the hit count of the oldest clean
block.  Also add a fudge factor to discourage demoting dirty blocks.

With some tests this has a sizeable difference, because the old code
was too eager to demote blocks.  For example, device-mapper-test-suite's
git_extract_cache_quick test goes from taking 190 seconds, to 142
(linear on spindle takes 250).

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Joe Thornber
2014-10-22 14:30:58 +01:00
committed by Mike Snitzer
parent 41abc4e1af
commit b155aa0e5a
2 changed files with 54 additions and 29 deletions
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6
Documentation/device-mapper/cache-policies.txt
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@@ -58,9 +58,9 @@ since spindles tend to have good bandwidth. The io_tracker counts
contiguous I/Os to try to spot when the io is in one of these sequential
modes.
Internally the mq policy maintains a promotion threshold variable. If
the hit count of a block not in the cache goes above this threshold it
gets promoted to the cache. The read, write and discard promote adjustment
Internally the mq policy determines a promotion threshold. If the hit
count of a block not in the cache goes above this threshold it gets
promoted to the cache. The read, write and discard promote adjustment
tunables allow you to tweak the promotion threshold by adding a small
value based on the io type. They default to 4, 8 and 1 respectively.
If you're trying to quickly warm a new cache device you may wish to