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Documentation / device-mapper / cache.txt

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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.

1	Introduction
2	============
4	dm-cache is a device mapper target written by Joe Thornber, Heinz
5	Mauelshagen, and Mike Snitzer.
7	It aims to improve performance of a block device (eg, a spindle) by
8	dynamically migrating some of its data to a faster, smaller device
9	(eg, an SSD).
11	This device-mapper solution allows us to insert this caching at
12	different levels of the dm stack, for instance above the data device for
13	a thin-provisioning pool.  Caching solutions that are integrated more
14	closely with the virtual memory system should give better performance.
16	The target reuses the metadata library used in the thin-provisioning
17	library.
19	The decision as to what data to migrate and when is left to a plug-in
20	policy module.  Several of these have been written as we experiment,
21	and we hope other people will contribute others for specific io
22	scenarios (eg. a vm image server).
24	Glossary
25	========
27	  Migration -  Movement of the primary copy of a logical block from one
28		       device to the other.
29	  Promotion -  Migration from slow device to fast device.
30	  Demotion  -  Migration from fast device to slow device.
32	The origin device always contains a copy of the logical block, which
33	may be out of date or kept in sync with the copy on the cache device
34	(depending on policy).
36	Design
37	======
39	Sub-devices
40	-----------
42	The target is constructed by passing three devices to it (along with
43	other parameters detailed later):
45	1. An origin device - the big, slow one.
47	2. A cache device - the small, fast one.
49	3. A small metadata device - records which blocks are in the cache,
50	   which are dirty, and extra hints for use by the policy object.
51	   This information could be put on the cache device, but having it
52	   separate allows the volume manager to configure it differently,
53	   e.g. as a mirror for extra robustness.  This metadata device may only
54	   be used by a single cache device.
56	Fixed block size
57	----------------
59	The origin is divided up into blocks of a fixed size.  This block size
60	is configurable when you first create the cache.  Typically we've been
61	using block sizes of 256KB - 1024KB.  The block size must be between 64
62	(32KB) and 2097152 (1GB) and a multiple of 64 (32KB).
64	Having a fixed block size simplifies the target a lot.  But it is
65	something of a compromise.  For instance, a small part of a block may be
66	getting hit a lot, yet the whole block will be promoted to the cache.
67	So large block sizes are bad because they waste cache space.  And small
68	block sizes are bad because they increase the amount of metadata (both
69	in core and on disk).
71	Cache operating modes
72	---------------------
74	The cache has three operating modes: writeback, writethrough and
75	passthrough.
77	If writeback, the default, is selected then a write to a block that is
78	cached will go only to the cache and the block will be marked dirty in
79	the metadata.
81	If writethrough is selected then a write to a cached block will not
82	complete until it has hit both the origin and cache devices.  Clean
83	blocks should remain clean.
85	If passthrough is selected, useful when the cache contents are not known
86	to be coherent with the origin device, then all reads are served from
87	the origin device (all reads miss the cache) and all writes are
88	forwarded to the origin device; additionally, write hits cause cache
89	block invalidates.  To enable passthrough mode the cache must be clean.
90	Passthrough mode allows a cache device to be activated without having to
91	worry about coherency.  Coherency that exists is maintained, although
92	the cache will gradually cool as writes take place.  If the coherency of
93	the cache can later be verified, or established through use of the
94	"invalidate_cblocks" message, the cache device can be transitioned to
95	writethrough or writeback mode while still warm.  Otherwise, the cache
96	contents can be discarded prior to transitioning to the desired
97	operating mode.
99	A simple cleaner policy is provided, which will clean (write back) all
100	dirty blocks in a cache.  Useful for decommissioning a cache or when
101	shrinking a cache.  Shrinking the cache's fast device requires all cache
102	blocks, in the area of the cache being removed, to be clean.  If the
103	area being removed from the cache still contains dirty blocks the resize
104	will fail.  Care must be taken to never reduce the volume used for the
105	cache's fast device until the cache is clean.  This is of particular
106	importance if writeback mode is used.  Writethrough and passthrough
107	modes already maintain a clean cache.  Future support to partially clean
108	the cache, above a specified threshold, will allow for keeping the cache
109	warm and in writeback mode during resize.
111	Migration throttling
112	--------------------
114	Migrating data between the origin and cache device uses bandwidth.
115	The user can set a throttle to prevent more than a certain amount of
116	migration occurring at any one time.  Currently we're not taking any
117	account of normal io traffic going to the devices.  More work needs
118	doing here to avoid migrating during those peak io moments.
120	For the time being, a message "migration_threshold <#sectors>"
121	can be used to set the maximum number of sectors being migrated,
122	the default being 204800 sectors (or 100MB).
124	Updating on-disk metadata
125	-------------------------
127	On-disk metadata is committed every time a REQ_SYNC or REQ_FUA bio is
128	written.  If no such requests are made then commits will occur every
129	second.  This means the cache behaves like a physical disk that has a
130	write cache (the same is true of the thin-provisioning target).  If
131	power is lost you may lose some recent writes.  The metadata should
132	always be consistent in spite of any crash.
134	The 'dirty' state for a cache block changes far too frequently for us
135	to keep updating it on the fly.  So we treat it as a hint.  In normal
136	operation it will be written when the dm device is suspended.  If the
137	system crashes all cache blocks will be assumed dirty when restarted.
139	Per-block policy hints
140	----------------------
142	Policy plug-ins can store a chunk of data per cache block.  It's up to
143	the policy how big this chunk is, but it should be kept small.  Like the
144	dirty flags this data is lost if there's a crash so a safe fallback
145	value should always be possible.
147	For instance, the 'mq' policy, which is currently the default policy,
148	uses this facility to store the hit count of the cache blocks.  If
149	there's a crash this information will be lost, which means the cache
150	may be less efficient until those hit counts are regenerated.
152	Policy hints affect performance, not correctness.
154	Policy messaging
155	----------------
157	Policies will have different tunables, specific to each one, so we
158	need a generic way of getting and setting these.  Device-mapper
159	messages are used.  Refer to cache-policies.txt.
161	Discard bitset resolution
162	-------------------------
164	We can avoid copying data during migration if we know the block has
165	been discarded.  A prime example of this is when mkfs discards the
166	whole block device.  We store a bitset tracking the discard state of
167	blocks.  However, we allow this bitset to have a different block size
168	from the cache blocks.  This is because we need to track the discard
169	state for all of the origin device (compare with the dirty bitset
170	which is just for the smaller cache device).
172	Target interface
173	================
175	Constructor
176	-----------
178	 cache <metadata dev> <cache dev> <origin dev> <block size>
179	       <#feature args> [<feature arg>]*
180	       <policy> <#policy args> [policy args]*
182	 metadata dev    : fast device holding the persistent metadata
183	 cache dev	 : fast device holding cached data blocks
184	 origin dev	 : slow device holding original data blocks
185	 block size      : cache unit size in sectors
187	 #feature args   : number of feature arguments passed
188	 feature args    : writethrough or passthrough (The default is writeback.)
190	 policy          : the replacement policy to use
191	 #policy args    : an even number of arguments corresponding to
192	                   key/value pairs passed to the policy
193	 policy args     : key/value pairs passed to the policy
194			   E.g. 'sequential_threshold 1024'
195			   See cache-policies.txt for details.
197	Optional feature arguments are:
198	   writethrough  : write through caching that prohibits cache block
199			   content from being different from origin block content.
200			   Without this argument, the default behaviour is to write
201			   back cache block contents later for performance reasons,
202			   so they may differ from the corresponding origin blocks.
204	   passthrough	 : a degraded mode useful for various cache coherency
205			   situations (e.g., rolling back snapshots of
206			   underlying storage).	 Reads and writes always go to
207			   the origin.	If a write goes to a cached origin
208			   block, then the cache block is invalidated.
209			   To enable passthrough mode the cache must be clean.
211	A policy called 'default' is always registered.  This is an alias for
212	the policy we currently think is giving best all round performance.
214	As the default policy could vary between kernels, if you are relying on
215	the characteristics of a specific policy, always request it by name.
217	Status
218	------
220	<#used metadata blocks>/<#total metadata blocks> <#read hits> <#read misses>
221	<#write hits> <#write misses> <#demotions> <#promotions> <#blocks in cache>
222	<#dirty> <#features> <features>* <#core args> <core args>* <#policy args>
223	<policy args>*
225	#used metadata blocks    : Number of metadata blocks used
226	#total metadata blocks   : Total number of metadata blocks
227	#read hits               : Number of times a READ bio has been mapped
228				     to the cache
229	#read misses             : Number of times a READ bio has been mapped
230				     to the origin
231	#write hits              : Number of times a WRITE bio has been mapped
232				     to the cache
233	#write misses            : Number of times a WRITE bio has been
234				     mapped to the origin
235	#demotions               : Number of times a block has been removed
236				     from the cache
237	#promotions              : Number of times a block has been moved to
238				     the cache
239	#blocks in cache         : Number of blocks resident in the cache
240	#dirty                   : Number of blocks in the cache that differ
241				     from the origin
242	#feature args            : Number of feature args to follow
243	feature args             : 'writethrough' (optional)
244	#core args               : Number of core arguments (must be even)
245	core args                : Key/value pairs for tuning the core
246				     e.g. migration_threshold
247	#policy args             : Number of policy arguments to follow (must be even)
248	policy args              : Key/value pairs
249				     e.g. 'sequential_threshold 1024
251	Messages
252	--------
254	Policies will have different tunables, specific to each one, so we
255	need a generic way of getting and setting these.  Device-mapper
256	messages are used.  (A sysfs interface would also be possible.)
258	The message format is:
260	   <key> <value>
262	E.g.
263	   dmsetup message my_cache 0 sequential_threshold 1024
266	Invalidation is removing an entry from the cache without writing it
267	back.  Cache blocks can be invalidated via the invalidate_cblocks
268	message, which takes an arbitrary number of cblock ranges.  Each cblock
269	range's end value is "one past the end", meaning 5-10 expresses a range
270	of values from 5 to 9.  Each cblock must be expressed as a decimal
271	value, in the future a variant message that takes cblock ranges
272	expressed in hexidecimal may be needed to better support efficient
273	invalidation of larger caches.  The cache must be in passthrough mode
274	when invalidate_cblocks is used.
276	   invalidate_cblocks [<cblock>|<cblock begin>-<cblock end>]*
278	E.g.
279	   dmsetup message my_cache 0 invalidate_cblocks 2345 3456-4567 5678-6789
281	Examples
282	========
284	The test suite can be found here:
286	https://github.com/jthornber/device-mapper-test-suite
288	dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
289		/dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0'
290	dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
291		/dev/mapper/ssd /dev/mapper/origin 1024 1 writeback \
292		mq 4 sequential_threshold 1024 random_threshold 8'
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