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dm cache policy mq: simplify ability to promote sequential IO to the cache

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Before, if the user wanted sequential IO to be promoted to the cache
they'd have to set sequential_threshold to some nebulous large value.

Now, the user may easily disable sequential IO detection (and sequential
IO's implicit bypass of the cache) by setting sequential_threshold to 0.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Mike Snitzer 2014-10-30 10:02:01 -04:00
parent b155aa0e5a
commit f1afb36a61
2 changed files with 15 additions and 8 deletions
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16
Documentation/device-mapper/cache-policies.txt
View File

@ -47,16 +47,22 @@ 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.
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

7
drivers/md/dm-cache-policy-mq.c
View File

@ -865,7 +865,8 @@ static int map(struct mq_policy *mq, dm_oblock_t oblock,
if (e && in_cache(mq, e))
r = cache_entry_found(mq, e, result);
else if (iot_pattern(&mq->tracker) == PATTERN_SEQUENTIAL)
else if (mq->tracker.thresholds[PATTERN_SEQUENTIAL] &&
iot_pattern(&mq->tracker) == PATTERN_SEQUENTIAL)
result->op = POLICY_MISS;
else if (e)
@ -1290,7 +1291,7 @@ bad_pre_cache_init:
static struct dm_cache_policy_type mq_policy_type = {
.name = "mq",
.version = {1, 2, 0},
.version = {1, 3, 0},
.hint_size = 4,
.owner = THIS_MODULE,
.create = mq_create
@ -1298,7 +1299,7 @@ static struct dm_cache_policy_type mq_policy_type = {
static struct dm_cache_policy_type default_policy_type = {
.name = "default",
.version = {1, 2, 0},
.version = {1, 3, 0},
.hint_size = 4,
.owner = THIS_MODULE,
.create = mq_create,