dm cache: make the 'mq' policy an alias for 'smq'

multiqueue (mq)

---------------

This policy has been deprecated in favor of the smq policy (see below).

This policy is now an alias for smq (see below).

The multiqueue policy has three sets of 16 queues: one set for entries

waiting for the cache and another two for those in the cache (a set for

clean entries and a set for dirty entries).

The following tunables are accepted, but have no effect:

Cache entries in the queues are aged based on logical time. Entry into

the cache is based on variable thresholds and queue selection is based

on hit count on entry. The policy aims to take different cache miss

costs into account and to adjust to varying load patterns automatically.

Message and constructor argument pairs are:

	'sequential_threshold <#nr_sequential_ios>'

	'random_threshold <#nr_random_ios>'

	'read_promote_adjustment <value>'

	'write_promote_adjustment <value>'

	'discard_promote_adjustment <value>'

The sequential threshold indicates the number of contiguous I/Os

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 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

reduce these to encourage promotion.  Remember to switch them back to

their defaults after the cache fills though.

Stochastic multiqueue (smq)

---------------------------

         algorithms used to select which blocks are promoted, demoted,

         cleaned etc.  It supports writeback and writethrough modes.

config DM_CACHE_MQ

       tristate "MQ Cache Policy (EXPERIMENTAL)"

       depends on DM_CACHE

       default y

       ---help---

         A cache policy that uses a multiqueue ordered by recent hit

         count to select which blocks should be promoted and demoted.

         This is meant to be a general purpose policy.  It prioritises

         reads over writes.

config DM_CACHE_SMQ

       tristate "Stochastic MQ Cache Policy (EXPERIMENTAL)"

       depends on DM_CACHE

		+= dm-log-userspace-base.o dm-log-userspace-transfer.o

dm-thin-pool-y	+= dm-thin.o dm-thin-metadata.o

dm-cache-y	+= dm-cache-target.o dm-cache-metadata.o dm-cache-policy.o

dm-cache-mq-y   += dm-cache-policy-mq.o

dm-cache-smq-y   += dm-cache-policy-smq.o

dm-cache-cleaner-y += dm-cache-policy-cleaner.o

dm-era-y	+= dm-era-target.o

obj-$(CONFIG_DM_THIN_PROVISIONING)	+= dm-thin-pool.o

obj-$(CONFIG_DM_VERITY)		+= dm-verity.o

obj-$(CONFIG_DM_CACHE)		+= dm-cache.o

obj-$(CONFIG_DM_CACHE_MQ)	+= dm-cache-mq.o

obj-$(CONFIG_DM_CACHE_SMQ)	+= dm-cache-smq.o

obj-$(CONFIG_DM_CACHE_CLEANER)	+= dm-cache-cleaner.o

obj-$(CONFIG_DM_ERA)		+= dm-era.o