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Documentation / device-mapper / cache-policies.txt




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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 EST.

1	Guidance for writing policies
2	=============================
3	
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.
7	
8	Mappings are loaded into the policy at construction time.
9	
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.
12	
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.
16	
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.
23	
24	
25	Overview of supplied cache replacement policies
26	===============================================
27	
28	multiqueue
29	----------
30	
31	This policy is the default.
32	
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).
36	
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.
41	
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>'
48	
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.
54	
55	The sequential and random thresholds default to 512 and 4 respectively.
56	
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.
66	
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.
75	
76	cleaner
77	-------
78	
79	The cleaner writes back all dirty blocks in a cache to decommission it.
80	
81	Examples
82	========
83	
84	The syntax for a table is:
85		cache <metadata dev> <cache dev> <origin dev> <block size>
86		<#feature_args> [<feature arg>]*
87		<policy> <#policy_args> [<policy arg>]*
88	
89	The syntax to send a message using the dmsetup command is:
90		dmsetup message <mapped device> 0 sequential_threshold 1024
91		dmsetup message <mapped device> 0 random_threshold 8
92	
93	Using dmsetup:
94		dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
95		    /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
96		creates a 128GB large mapped device named 'blah' with the
97		sequential threshold set to 1024 and the random_threshold set to 8.
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