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Diffstat (limited to 'Documentation/device-mapper/cache-policies.txt')
| -rw-r--r-- | Documentation/device-mapper/cache-policies.txt | 24 |
1 files changed, 15 insertions, 9 deletions
diff --git a/Documentation/device-mapper/cache-policies.txt b/Documentation/device-mapper/cache-policies.txt index 66c2774c0c64..0d124a971801 100644 --- a/Documentation/device-mapper/cache-policies.txt +++ b/Documentation/device-mapper/cache-policies.txt @@ -47,20 +47,26 @@ Message and constructor argument pairs are: 'discard_promote_adjustment <value>' The sequential threshold indicates the number of contiguous I/Os -required before a stream is treated as sequential. The random threshold +required before a stream is treated as sequential. Once a stream is +considered sequential it will bypass the cache. The random threshold is the number of intervening non-contiguous I/Os that must be seen before the stream is treated as random again. The sequential and random thresholds default to 512 and 4 respectively. -Large, sequential ios are probably better left on the origin device -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 +Large, sequential I/Os are probably better left on the origin device +since spindles tend to have good sequential I/O bandwidth. The +io_tracker counts contiguous I/Os to try to spot when the I/O is in one +of these sequential modes. But there are use-cases for wanting to +promote sequential blocks to the cache (e.g. fast application startup). +If sequential threshold is set to 0 the sequential I/O detection is +disabled and sequential I/O will no longer implicitly bypass the cache. +Setting the random threshold to 0 does _not_ disable the random I/O +stream detection. + +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 |