About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / device-mapper / thin-provisioning.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

1	Introduction
2	============
3	
4	This document describes a collection of device-mapper targets that
5	between them implement thin-provisioning and snapshots.
6	
7	The main highlight of this implementation, compared to the previous
8	implementation of snapshots, is that it allows many virtual devices to
9	be stored on the same data volume.  This simplifies administration and
10	allows the sharing of data between volumes, thus reducing disk usage.
11	
12	Another significant feature is support for an arbitrary depth of
13	recursive snapshots (snapshots of snapshots of snapshots ...).  The
14	previous implementation of snapshots did this by chaining together
15	lookup tables, and so performance was O(depth).  This new
16	implementation uses a single data structure to avoid this degradation
17	with depth.  Fragmentation may still be an issue, however, in some
18	scenarios.
19	
20	Metadata is stored on a separate device from data, giving the
21	administrator some freedom, for example to:
22	
23	- Improve metadata resilience by storing metadata on a mirrored volume
24	  but data on a non-mirrored one.
25	
26	- Improve performance by storing the metadata on SSD.
27	
28	Status
29	======
30	
31	These targets are very much still in the EXPERIMENTAL state.  Please
32	do not yet rely on them in production.  But do experiment and offer us
33	feedback.  Different use cases will have different performance
34	characteristics, for example due to fragmentation of the data volume.
35	
36	If you find this software is not performing as expected please mail
37	dm-devel@redhat.com with details and we'll try our best to improve
38	things for you.
39	
40	Userspace tools for checking and repairing the metadata are under
41	development.
42	
43	Cookbook
44	========
45	
46	This section describes some quick recipes for using thin provisioning.
47	They use the dmsetup program to control the device-mapper driver
48	directly.  End users will be advised to use a higher-level volume
49	manager such as LVM2 once support has been added.
50	
51	Pool device
52	-----------
53	
54	The pool device ties together the metadata volume and the data volume.
55	It maps I/O linearly to the data volume and updates the metadata via
56	two mechanisms:
57	
58	- Function calls from the thin targets
59	
60	- Device-mapper 'messages' from userspace which control the creation of new
61	  virtual devices amongst other things.
62	
63	Setting up a fresh pool device
64	------------------------------
65	
66	Setting up a pool device requires a valid metadata device, and a
67	data device.  If you do not have an existing metadata device you can
68	make one by zeroing the first 4k to indicate empty metadata.
69	
70	    dd if=/dev/zero of=$metadata_dev bs=4096 count=1
71	
72	The amount of metadata you need will vary according to how many blocks
73	are shared between thin devices (i.e. through snapshots).  If you have
74	less sharing than average you'll need a larger-than-average metadata device.
75	
76	As a guide, we suggest you calculate the number of bytes to use in the
77	metadata device as 48 * $data_dev_size / $data_block_size but round it up
78	to 2MB if the answer is smaller.  If you're creating large numbers of
79	snapshots which are recording large amounts of change, you may find you
80	need to increase this.
81	
82	The largest size supported is 16GB: If the device is larger,
83	a warning will be issued and the excess space will not be used.
84	
85	Reloading a pool table
86	----------------------
87	
88	You may reload a pool's table, indeed this is how the pool is resized
89	if it runs out of space.  (N.B. While specifying a different metadata
90	device when reloading is not forbidden at the moment, things will go
91	wrong if it does not route I/O to exactly the same on-disk location as
92	previously.)
93	
94	Using an existing pool device
95	-----------------------------
96	
97	    dmsetup create pool \
98		--table "0 20971520 thin-pool $metadata_dev $data_dev \
99			 $data_block_size $low_water_mark"
100	
101	$data_block_size gives the smallest unit of disk space that can be
102	allocated at a time expressed in units of 512-byte sectors.
103	$data_block_size must be between 128 (64KB) and 2097152 (1GB) and a
104	multiple of 128 (64KB).  $data_block_size cannot be changed after the
105	thin-pool is created.  People primarily interested in thin provisioning
106	may want to use a value such as 1024 (512KB).  People doing lots of
107	snapshotting may want a smaller value such as 128 (64KB).  If you are
108	not zeroing newly-allocated data, a larger $data_block_size in the
109	region of 256000 (128MB) is suggested.
110	
111	$low_water_mark is expressed in blocks of size $data_block_size.  If
112	free space on the data device drops below this level then a dm event
113	will be triggered which a userspace daemon should catch allowing it to
114	extend the pool device.  Only one such event will be sent.
115	Resuming a device with a new table itself triggers an event so the
116	userspace daemon can use this to detect a situation where a new table
117	already exceeds the threshold.
118	
119	A low water mark for the metadata device is maintained in the kernel and
120	will trigger a dm event if free space on the metadata device drops below
121	it.
122	
123	Updating on-disk metadata
124	-------------------------
125	
126	On-disk metadata is committed every time a FLUSH or FUA bio is written.
127	If no such requests are made then commits will occur every second.  This
128	means the thin-provisioning target behaves like a physical disk that has
129	a volatile write cache.  If power is lost you may lose some recent
130	writes.  The metadata should always be consistent in spite of any crash.
131	
132	If data space is exhausted the pool will either error or queue IO
133	according to the configuration (see: error_if_no_space).  If metadata
134	space is exhausted or a metadata operation fails: the pool will error IO
135	until the pool is taken offline and repair is performed to 1) fix any
136	potential inconsistencies and 2) clear the flag that imposes repair.
137	Once the pool's metadata device is repaired it may be resized, which
138	will allow the pool to return to normal operation.  Note that if a pool
139	is flagged as needing repair, the pool's data and metadata devices
140	cannot be resized until repair is performed.  It should also be noted
141	that when the pool's metadata space is exhausted the current metadata
142	transaction is aborted.  Given that the pool will cache IO whose
143	completion may have already been acknowledged to upper IO layers
144	(e.g. filesystem) it is strongly suggested that consistency checks
145	(e.g. fsck) be performed on those layers when repair of the pool is
146	required.
147	
148	Thin provisioning
149	-----------------
150	
151	i) Creating a new thinly-provisioned volume.
152	
153	  To create a new thinly- provisioned volume you must send a message to an
154	  active pool device, /dev/mapper/pool in this example.
155	
156	    dmsetup message /dev/mapper/pool 0 "create_thin 0"
157	
158	  Here '0' is an identifier for the volume, a 24-bit number.  It's up
159	  to the caller to allocate and manage these identifiers.  If the
160	  identifier is already in use, the message will fail with -EEXIST.
161	
162	ii) Using a thinly-provisioned volume.
163	
164	  Thinly-provisioned volumes are activated using the 'thin' target:
165	
166	    dmsetup create thin --table "0 2097152 thin /dev/mapper/pool 0"
167	
168	  The last parameter is the identifier for the thinp device.
169	
170	Internal snapshots
171	------------------
172	
173	i) Creating an internal snapshot.
174	
175	  Snapshots are created with another message to the pool.
176	
177	  N.B.  If the origin device that you wish to snapshot is active, you
178	  must suspend it before creating the snapshot to avoid corruption.
179	  This is NOT enforced at the moment, so please be careful!
180	
181	    dmsetup suspend /dev/mapper/thin
182	    dmsetup message /dev/mapper/pool 0 "create_snap 1 0"
183	    dmsetup resume /dev/mapper/thin
184	
185	  Here '1' is the identifier for the volume, a 24-bit number.  '0' is the
186	  identifier for the origin device.
187	
188	ii) Using an internal snapshot.
189	
190	  Once created, the user doesn't have to worry about any connection
191	  between the origin and the snapshot.  Indeed the snapshot is no
192	  different from any other thinly-provisioned device and can be
193	  snapshotted itself via the same method.  It's perfectly legal to
194	  have only one of them active, and there's no ordering requirement on
195	  activating or removing them both.  (This differs from conventional
196	  device-mapper snapshots.)
197	
198	  Activate it exactly the same way as any other thinly-provisioned volume:
199	
200	    dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 1"
201	
202	External snapshots
203	------------------
204	
205	You can use an external _read only_ device as an origin for a
206	thinly-provisioned volume.  Any read to an unprovisioned area of the
207	thin device will be passed through to the origin.  Writes trigger
208	the allocation of new blocks as usual.
209	
210	One use case for this is VM hosts that want to run guests on
211	thinly-provisioned volumes but have the base image on another device
212	(possibly shared between many VMs).
213	
214	You must not write to the origin device if you use this technique!
215	Of course, you may write to the thin device and take internal snapshots
216	of the thin volume.
217	
218	i) Creating a snapshot of an external device
219	
220	  This is the same as creating a thin device.
221	  You don't mention the origin at this stage.
222	
223	    dmsetup message /dev/mapper/pool 0 "create_thin 0"
224	
225	ii) Using a snapshot of an external device.
226	
227	  Append an extra parameter to the thin target specifying the origin:
228	
229	    dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 0 /dev/image"
230	
231	  N.B. All descendants (internal snapshots) of this snapshot require the
232	  same extra origin parameter.
233	
234	Deactivation
235	------------
236	
237	All devices using a pool must be deactivated before the pool itself
238	can be.
239	
240	    dmsetup remove thin
241	    dmsetup remove snap
242	    dmsetup remove pool
243	
244	Reference
245	=========
246	
247	'thin-pool' target
248	------------------
249	
250	i) Constructor
251	
252	    thin-pool <metadata dev> <data dev> <data block size (sectors)> \
253		      <low water mark (blocks)> [<number of feature args> [<arg>]*]
254	
255	    Optional feature arguments:
256	
257	      skip_block_zeroing: Skip the zeroing of newly-provisioned blocks.
258	
259	      ignore_discard: Disable discard support.
260	
261	      no_discard_passdown: Don't pass discards down to the underlying
262				   data device, but just remove the mapping.
263	
264	      read_only: Don't allow any changes to be made to the pool
265			 metadata.
266	
267	      error_if_no_space: Error IOs, instead of queueing, if no space.
268	
269	    Data block size must be between 64KB (128 sectors) and 1GB
270	    (2097152 sectors) inclusive.
271	
272	
273	ii) Status
274	
275	    <transaction id> <used metadata blocks>/<total metadata blocks>
276	    <used data blocks>/<total data blocks> <held metadata root>
277	    [no_]discard_passdown ro|rw
278	
279	    transaction id:
280		A 64-bit number used by userspace to help synchronise with metadata
281		from volume managers.
282	
283	    used data blocks / total data blocks
284		If the number of free blocks drops below the pool's low water mark a
285		dm event will be sent to userspace.  This event is edge-triggered and
286		it will occur only once after each resume so volume manager writers
287		should register for the event and then check the target's status.
288	
289	    held metadata root:
290		The location, in blocks, of the metadata root that has been
291		'held' for userspace read access.  '-' indicates there is no
292		held root.
293	
294	    discard_passdown|no_discard_passdown
295		Whether or not discards are actually being passed down to the
296		underlying device.  When this is enabled when loading the table,
297		it can get disabled if the underlying device doesn't support it.
298	
299	    ro|rw
300		If the pool encounters certain types of device failures it will
301		drop into a read-only metadata mode in which no changes to
302		the pool metadata (like allocating new blocks) are permitted.
303	
304		In serious cases where even a read-only mode is deemed unsafe
305		no further I/O will be permitted and the status will just
306		contain the string 'Fail'.  The userspace recovery tools
307		should then be used.
308	
309	    error_if_no_space|queue_if_no_space
310		If the pool runs out of data or metadata space, the pool will
311		either queue or error the IO destined to the data device.  The
312		default is to queue the IO until more space is added or the
313		'no_space_timeout' expires.  The 'no_space_timeout' dm-thin-pool
314		module parameter can be used to change this timeout -- it
315		defaults to 60 seconds but may be disabled using a value of 0.
316	
317	iii) Messages
318	
319	    create_thin <dev id>
320	
321		Create a new thinly-provisioned device.
322		<dev id> is an arbitrary unique 24-bit identifier chosen by
323		the caller.
324	
325	    create_snap <dev id> <origin id>
326	
327		Create a new snapshot of another thinly-provisioned device.
328		<dev id> is an arbitrary unique 24-bit identifier chosen by
329		the caller.
330		<origin id> is the identifier of the thinly-provisioned device
331		of which the new device will be a snapshot.
332	
333	    delete <dev id>
334	
335		Deletes a thin device.  Irreversible.
336	
337	    set_transaction_id <current id> <new id>
338	
339		Userland volume managers, such as LVM, need a way to
340		synchronise their external metadata with the internal metadata of the
341		pool target.  The thin-pool target offers to store an
342		arbitrary 64-bit transaction id and return it on the target's
343		status line.  To avoid races you must provide what you think
344		the current transaction id is when you change it with this
345		compare-and-swap message.
346	
347	    reserve_metadata_snap
348	
349	        Reserve a copy of the data mapping btree for use by userland.
350	        This allows userland to inspect the mappings as they were when
351	        this message was executed.  Use the pool's status command to
352	        get the root block associated with the metadata snapshot.
353	
354	    release_metadata_snap
355	
356	        Release a previously reserved copy of the data mapping btree.
357	
358	'thin' target
359	-------------
360	
361	i) Constructor
362	
363	    thin <pool dev> <dev id> [<external origin dev>]
364	
365	    pool dev:
366		the thin-pool device, e.g. /dev/mapper/my_pool or 253:0
367	
368	    dev id:
369		the internal device identifier of the device to be
370		activated.
371	
372	    external origin dev:
373		an optional block device outside the pool to be treated as a
374		read-only snapshot origin: reads to unprovisioned areas of the
375		thin target will be mapped to this device.
376	
377	The pool doesn't store any size against the thin devices.  If you
378	load a thin target that is smaller than you've been using previously,
379	then you'll have no access to blocks mapped beyond the end.  If you
380	load a target that is bigger than before, then extra blocks will be
381	provisioned as and when needed.
382	
383	If you wish to reduce the size of your thin device and potentially
384	regain some space then send the 'trim' message to the pool.
385	
386	ii) Status
387	
388	     <nr mapped sectors> <highest mapped sector>
389	
390		If the pool has encountered device errors and failed, the status
391		will just contain the string 'Fail'.  The userspace recovery
392		tools should then be used.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.