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Documentation / device-mapper / cache-policies.txt

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Based on kernel version 4.2. Page generated on 2015-09-09 12:08 EST.

1	Guidance for writing policies
2	=============================
4	Try to keep transactionality out of it.  The core is careful to
5	avoid asking about anything that is migrating.  This is a pain, but
6	makes it easier to write the policies.
8	Mappings are loaded into the policy at construction time.
10	Every bio that is mapped by the target is referred to the policy.
11	The policy can return a simple HIT or MISS or issue a migration.
13	Currently there's no way for the policy to issue background work,
14	e.g. to start writing back dirty blocks that are going to be evicte
15	soon.
17	Because we map bios, rather than requests it's easy for the policy
18	to get fooled by many small bios.  For this reason the core target
19	issues periodic ticks to the policy.  It's suggested that the policy
20	doesn't update states (eg, hit counts) for a block more than once
21	for each tick.  The core ticks by watching bios complete, and so
22	trying to see when the io scheduler has let the ios run.
25	Overview of supplied cache replacement policies
26	===============================================
28	multiqueue (mq)
29	---------------
31	This policy has been deprecated in favor of the smq policy (see below).
33	The multiqueue policy has three sets of 16 queues: one set for entries
34	waiting for the cache and another two for those in the cache (a set for
35	clean entries and a set for dirty entries).
37	Cache entries in the queues are aged based on logical time. Entry into
38	the cache is based on variable thresholds and queue selection is based
39	on hit count on entry. The policy aims to take different cache miss
40	costs into account and to adjust to varying load patterns automatically.
42	Message and constructor argument pairs are:
43		'sequential_threshold <#nr_sequential_ios>'
44		'random_threshold <#nr_random_ios>'
45		'read_promote_adjustment <value>'
46		'write_promote_adjustment <value>'
47		'discard_promote_adjustment <value>'
49	The sequential threshold indicates the number of contiguous I/Os
50	required before a stream is treated as sequential.  Once a stream is
51	considered sequential it will bypass the cache.  The random threshold
52	is the number of intervening non-contiguous I/Os that must be seen
53	before the stream is treated as random again.
55	The sequential and random thresholds default to 512 and 4 respectively.
57	Large, sequential I/Os are probably better left on the origin device
58	since spindles tend to have good sequential I/O bandwidth.  The
59	io_tracker counts contiguous I/Os to try to spot when the I/O is in one
60	of these sequential modes.  But there are use-cases for wanting to
61	promote sequential blocks to the cache (e.g. fast application startup).
62	If sequential threshold is set to 0 the sequential I/O detection is
63	disabled and sequential I/O will no longer implicitly bypass the cache.
64	Setting the random threshold to 0 does _not_ disable the random I/O
65	stream detection.
67	Internally the mq policy determines a promotion threshold.  If the hit
68	count of a block not in the cache goes above this threshold it gets
69	promoted to the cache.  The read, write and discard promote adjustment
70	tunables allow you to tweak the promotion threshold by adding a small
71	value based on the io type.  They default to 4, 8 and 1 respectively.
72	If you're trying to quickly warm a new cache device you may wish to
73	reduce these to encourage promotion.  Remember to switch them back to
74	their defaults after the cache fills though.
76	Stochastic multiqueue (smq)
77	---------------------------
79	This policy is the default.
81	The stochastic multi-queue (smq) policy addresses some of the problems
82	with the multiqueue (mq) policy.
84	The smq policy (vs mq) offers the promise of less memory utilization,
85	improved performance and increased adaptability in the face of changing
86	workloads.  SMQ also does not have any cumbersome tuning knobs.
88	Users may switch from "mq" to "smq" simply by appropriately reloading a
89	DM table that is using the cache target.  Doing so will cause all of the
90	mq policy's hints to be dropped.  Also, performance of the cache may
91	degrade slightly until smq recalculates the origin device's hotspots
92	that should be cached.
94	Memory usage:
95	The mq policy uses a lot of memory; 88 bytes per cache block on a 64
96	bit machine.
98	SMQ uses 28bit indexes to implement it's data structures rather than
99	pointers.  It avoids storing an explicit hit count for each block.  It
100	has a 'hotspot' queue rather than a pre cache which uses a quarter of
101	the entries (each hotspot block covers a larger area than a single
102	cache block).
104	All these mean smq uses ~25bytes per cache block.  Still a lot of
105	memory, but a substantial improvement nontheless.
107	Level balancing:
108	MQ places entries in different levels of the multiqueue structures
109	based on their hit count (~ln(hit count)).  This means the bottom
110	levels generally have the most entries, and the top ones have very
111	few.  Having unbalanced levels like this reduces the efficacy of the
112	multiqueue.
114	SMQ does not maintain a hit count, instead it swaps hit entries with
115	the least recently used entry from the level above.  The over all
116	ordering being a side effect of this stochastic process.  With this
117	scheme we can decide how many entries occupy each multiqueue level,
118	resulting in better promotion/demotion decisions.
120	Adaptability:
121	The MQ policy maintains a hit count for each cache block.  For a
122	different block to get promoted to the cache it's hit count has to
123	exceed the lowest currently in the cache.  This means it can take a
124	long time for the cache to adapt between varying IO patterns.
125	Periodically degrading the hit counts could help with this, but I
126	haven't found a nice general solution.
128	SMQ doesn't maintain hit counts, so a lot of this problem just goes
129	away.  In addition it tracks performance of the hotspot queue, which
130	is used to decide which blocks to promote.  If the hotspot queue is
131	performing badly then it starts moving entries more quickly between
132	levels.  This lets it adapt to new IO patterns very quickly.
134	Performance:
135	Testing SMQ shows substantially better performance than MQ.
137	cleaner
138	-------
140	The cleaner writes back all dirty blocks in a cache to decommission it.
142	Examples
143	========
145	The syntax for a table is:
146		cache <metadata dev> <cache dev> <origin dev> <block size>
147		<#feature_args> [<feature arg>]*
148		<policy> <#policy_args> [<policy arg>]*
150	The syntax to send a message using the dmsetup command is:
151		dmsetup message <mapped device> 0 sequential_threshold 1024
152		dmsetup message <mapped device> 0 random_threshold 8
154	Using dmsetup:
155		dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
156		    /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
157		creates a 128GB large mapped device named 'blah' with the
158		sequential threshold set to 1024 and the random_threshold set to 8.
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