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Commit f1afb36a authored by Mike Snitzer's avatar Mike Snitzer
Browse files

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



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: default avatarMike Snitzer <snitzer@redhat.com>
parent b155aa0e

Documentation/device-mapper/cache-policies.txt

+11 −5
Original line number Diff line number Diff line
@@ -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

drivers/md/dm-cache-policy-mq.c

+4 −3
Original line number Diff line number Diff line
@@ -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 @@ static struct dm_cache_policy *mq_create(dm_cblock_t cache_size, 


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,