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Documentation / device-mapper / cache.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 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33	may be out of date or kept in sync with the copy on the cache device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	sectors (32KB) and 2097152 sectors (1GB) and a multiple of 64 sectors (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 2048 sectors (1MB).
124	Updating on-disk metadata
125	-------------------------
127	On-disk metadata is committed every time a FLUSH or FUA bio is written.
128	If no such requests are made then commits will occur every second.  This
129	means the cache behaves like a physical disk that has a volatile write
130	cache.  If power is lost you may lose some recent writes.  The metadata
131	should always be consistent in spite of any crash.
133	The 'dirty' state for a cache block changes far too frequently for us
134	to keep updating it on the fly.  So we treat it as a hint.  In normal
135	operation it will be written when the dm device is suspended.  If the
136	system crashes all cache blocks will be assumed dirty when restarted.
138	Per-block policy hints
139	----------------------
141	Policy plug-ins can store a chunk of data per cache block.  It's up to
142	the policy how big this chunk is, but it should be kept small.  Like the
143	dirty flags this data is lost if there's a crash so a safe fallback
144	value should always be possible.
146	Policy hints affect performance, not correctness.
148	Policy messaging
149	----------------
151	Policies will have different tunables, specific to each one, so we
152	need a generic way of getting and setting these.  Device-mapper
153	messages are used.  Refer to cache-policies.txt.
155	Discard bitset resolution
156	-------------------------
158	We can avoid copying data during migration if we know the block has
159	been discarded.  A prime example of this is when mkfs discards the
160	whole block device.  We store a bitset tracking the discard state of
161	blocks.  However, we allow this bitset to have a different block size
162	from the cache blocks.  This is because we need to track the discard
163	state for all of the origin device (compare with the dirty bitset
164	which is just for the smaller cache device).
166	Target interface
167	================
169	Constructor
170	-----------
172	 cache <metadata dev> <cache dev> <origin dev> <block size>
173	       <#feature args> [<feature arg>]*
174	       <policy> <#policy args> [policy args]*
176	 metadata dev    : fast device holding the persistent metadata
177	 cache dev	 : fast device holding cached data blocks
178	 origin dev	 : slow device holding original data blocks
179	 block size      : cache unit size in sectors
181	 #feature args   : number of feature arguments passed
182	 feature args    : writethrough or passthrough (The default is writeback.)
184	 policy          : the replacement policy to use
185	 #policy args    : an even number of arguments corresponding to
186	                   key/value pairs passed to the policy
187	 policy args     : key/value pairs passed to the policy
188			   E.g. 'sequential_threshold 1024'
189			   See cache-policies.txt for details.
191	Optional feature arguments are:
192	   writethrough  : write through caching that prohibits cache block
193			   content from being different from origin block content.
194			   Without this argument, the default behaviour is to write
195			   back cache block contents later for performance reasons,
196			   so they may differ from the corresponding origin blocks.
198	   passthrough	 : a degraded mode useful for various cache coherency
199			   situations (e.g., rolling back snapshots of
200			   underlying storage).	 Reads and writes always go to
201			   the origin.	If a write goes to a cached origin
202			   block, then the cache block is invalidated.
203			   To enable passthrough mode the cache must be clean.
205	   metadata2	: use version 2 of the metadata.  This stores the dirty bits
206	                  in a separate btree, which improves speed of shutting
207			  down the cache.
209	A policy called 'default' is always registered.  This is an alias for
210	the policy we currently think is giving best all round performance.
212	As the default policy could vary between kernels, if you are relying on
213	the characteristics of a specific policy, always request it by name.
215	Status
216	------
218	<metadata block size> <#used metadata blocks>/<#total metadata blocks>
219	<cache block size> <#used cache blocks>/<#total cache blocks>
220	<#read hits> <#read misses> <#write hits> <#write misses>
221	<#demotions> <#promotions> <#dirty> <#features> <features>*
222	<#core args> <core args>* <policy name> <#policy args> <policy args>*
223	<cache metadata mode>
225	metadata block size	 : Fixed block size for each metadata block in
226				     sectors
227	#used metadata blocks	 : Number of metadata blocks used
228	#total metadata blocks	 : Total number of metadata blocks
229	cache block size	 : Configurable block size for the cache device
230				     in sectors
231	#used cache blocks	 : Number of blocks resident in the cache
232	#total cache blocks	 : Total number of cache blocks
233	#read hits		 : Number of times a READ bio has been mapped
234				     to the cache
235	#read misses		 : Number of times a READ bio has been mapped
236				     to the origin
237	#write hits		 : Number of times a WRITE bio has been mapped
238				     to the cache
239	#write misses		 : Number of times a WRITE bio has been
240				     mapped to the origin
241	#demotions		 : Number of times a block has been removed
242				     from the cache
243	#promotions		 : Number of times a block has been moved to
244				     the cache
245	#dirty			 : Number of blocks in the cache that differ
246				     from the origin
247	#feature args		 : Number of feature args to follow
248	feature args		 : 'writethrough' (optional)
249	#core args		 : Number of core arguments (must be even)
250	core args		 : Key/value pairs for tuning the core
251				     e.g. migration_threshold
252	policy name		 : Name of the policy
253	#policy args		 : Number of policy arguments to follow (must be even)
254	policy args		 : Key/value pairs e.g. sequential_threshold
255	cache metadata mode      : ro if read-only, rw if read-write
256		In serious cases where even a read-only mode is deemed unsafe
257		no further I/O will be permitted and the status will just
258		contain the string 'Fail'.  The userspace recovery tools
259		should then be used.
260	needs_check		 : 'needs_check' if set, '-' if not set
261		A metadata operation has failed, resulting in the needs_check
262		flag being set in the metadata's superblock.  The metadata
263		device must be deactivated and checked/repaired before the
264		cache can be made fully operational again.  '-' indicates
265		needs_check is not set.
267	Messages
268	--------
270	Policies will have different tunables, specific to each one, so we
271	need a generic way of getting and setting these.  Device-mapper
272	messages are used.  (A sysfs interface would also be possible.)
274	The message format is:
276	   <key> <value>
278	E.g.
279	   dmsetup message my_cache 0 sequential_threshold 1024
282	Invalidation is removing an entry from the cache without writing it
283	back.  Cache blocks can be invalidated via the invalidate_cblocks
284	message, which takes an arbitrary number of cblock ranges.  Each cblock
285	range's end value is "one past the end", meaning 5-10 expresses a range
286	of values from 5 to 9.  Each cblock must be expressed as a decimal
287	value, in the future a variant message that takes cblock ranges
288	expressed in hexadecimal may be needed to better support efficient
289	invalidation of larger caches.  The cache must be in passthrough mode
290	when invalidate_cblocks is used.
292	   invalidate_cblocks [<cblock>|<cblock begin>-<cblock end>]*
294	E.g.
295	   dmsetup message my_cache 0 invalidate_cblocks 2345 3456-4567 5678-6789
297	Examples
298	========
300	The test suite can be found here:
302	https://github.com/jthornber/device-mapper-test-suite
304	dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
305		/dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0'
306	dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
307		/dev/mapper/ssd /dev/mapper/origin 1024 1 writeback \
308		mq 4 sequential_threshold 1024 random_threshold 8'
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