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Documentation / device-mapper / snapshot.txt




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

1	Device-mapper snapshot support
2	==============================
3	
4	Device-mapper allows you, without massive data copying:
5	
6	*) To create snapshots of any block device i.e. mountable, saved states of
7	the block device which are also writable without interfering with the
8	original content;
9	*) To create device "forks", i.e. multiple different versions of the
10	same data stream.
11	*) To merge a snapshot of a block device back into the snapshot's origin
12	device.
13	
14	In the first two cases, dm copies only the chunks of data that get
15	changed and uses a separate copy-on-write (COW) block device for
16	storage.
17	
18	For snapshot merge the contents of the COW storage are merged back into
19	the origin device.
20	
21	
22	There are three dm targets available:
23	snapshot, snapshot-origin, and snapshot-merge.
24	
25	*) snapshot-origin <origin>
26	
27	which will normally have one or more snapshots based on it.
28	Reads will be mapped directly to the backing device. For each write, the
29	original data will be saved in the <COW device> of each snapshot to keep
30	its visible content unchanged, at least until the <COW device> fills up.
31	
32	
33	*) snapshot <origin> <COW device> <persistent?> <chunksize>
34	
35	A snapshot of the <origin> block device is created. Changed chunks of
36	<chunksize> sectors will be stored on the <COW device>.  Writes will
37	only go to the <COW device>.  Reads will come from the <COW device> or
38	from <origin> for unchanged data.  <COW device> will often be
39	smaller than the origin and if it fills up the snapshot will become
40	useless and be disabled, returning errors.  So it is important to monitor
41	the amount of free space and expand the <COW device> before it fills up.
42	
43	<persistent?> is P (Persistent) or N (Not persistent - will not survive
44	after reboot).
45	The difference is that for transient snapshots less metadata must be
46	saved on disk - they can be kept in memory by the kernel.
47	
48	
49	* snapshot-merge <origin> <COW device> <persistent> <chunksize>
50	
51	takes the same table arguments as the snapshot target except it only
52	works with persistent snapshots.  This target assumes the role of the
53	"snapshot-origin" target and must not be loaded if the "snapshot-origin"
54	is still present for <origin>.
55	
56	Creates a merging snapshot that takes control of the changed chunks
57	stored in the <COW device> of an existing snapshot, through a handover
58	procedure, and merges these chunks back into the <origin>.  Once merging
59	has started (in the background) the <origin> may be opened and the merge
60	will continue while I/O is flowing to it.  Changes to the <origin> are
61	deferred until the merging snapshot's corresponding chunk(s) have been
62	merged.  Once merging has started the snapshot device, associated with
63	the "snapshot" target, will return -EIO when accessed.
64	
65	
66	How snapshot is used by LVM2
67	============================
68	When you create the first LVM2 snapshot of a volume, four dm devices are used:
69	
70	1) a device containing the original mapping table of the source volume;
71	2) a device used as the <COW device>;
72	3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
73	   volume;
74	4) the "original" volume (which uses the device number used by the original
75	   source volume), whose table is replaced by a "snapshot-origin" mapping
76	   from device #1.
77	
78	A fixed naming scheme is used, so with the following commands:
79	
80	lvcreate -L 1G -n base volumeGroup
81	lvcreate -L 100M --snapshot -n snap volumeGroup/base
82	
83	we'll have this situation (with volumes in above order):
84	
85	# dmsetup table|grep volumeGroup
86	
87	volumeGroup-base-real: 0 2097152 linear 8:19 384
88	volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
89	volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
90	volumeGroup-base: 0 2097152 snapshot-origin 254:11
91	
92	# ls -lL /dev/mapper/volumeGroup-*
93	brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
94	brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
95	brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
96	brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
97	
98	
99	How snapshot-merge is used by LVM2
100	==================================
101	A merging snapshot assumes the role of the "snapshot-origin" while
102	merging.  As such the "snapshot-origin" is replaced with
103	"snapshot-merge".  The "-real" device is not changed and the "-cow"
104	device is renamed to <origin name>-cow to aid LVM2's cleanup of the
105	merging snapshot after it completes.  The "snapshot" that hands over its
106	COW device to the "snapshot-merge" is deactivated (unless using lvchange
107	--refresh); but if it is left active it will simply return I/O errors.
108	
109	A snapshot will merge into its origin with the following command:
110	
111	lvconvert --merge volumeGroup/snap
112	
113	we'll now have this situation:
114	
115	# dmsetup table|grep volumeGroup
116	
117	volumeGroup-base-real: 0 2097152 linear 8:19 384
118	volumeGroup-base-cow: 0 204800 linear 8:19 2097536
119	volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
120	
121	# ls -lL /dev/mapper/volumeGroup-*
122	brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
123	brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
124	brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
125	
126	
127	How to determine when a merging is complete
128	===========================================
129	The snapshot-merge and snapshot status lines end with:
130	  <sectors_allocated>/<total_sectors> <metadata_sectors>
131	
132	Both <sectors_allocated> and <total_sectors> include both data and metadata.
133	During merging, the number of sectors allocated gets smaller and
134	smaller.  Merging has finished when the number of sectors holding data
135	is zero, in other words <sectors_allocated> == <metadata_sectors>.
136	
137	Here is a practical example (using a hybrid of lvm and dmsetup commands):
138	
139	# lvs
140	  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
141	  base    volumeGroup owi-a- 4.00g
142	  snap    volumeGroup swi-a- 1.00g base  18.97
143	
144	# dmsetup status volumeGroup-snap
145	0 8388608 snapshot 397896/2097152 1560
146	                                  ^^^^ metadata sectors
147	
148	# lvconvert --merge -b volumeGroup/snap
149	  Merging of volume snap started.
150	
151	# lvs volumeGroup/snap
152	  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
153	  base    volumeGroup Owi-a- 4.00g          17.23
154	
155	# dmsetup status volumeGroup-base
156	0 8388608 snapshot-merge 281688/2097152 1104
157	
158	# dmsetup status volumeGroup-base
159	0 8388608 snapshot-merge 180480/2097152 712
160	
161	# dmsetup status volumeGroup-base
162	0 8388608 snapshot-merge 16/2097152 16
163	
164	Merging has finished.
165	
166	# lvs
167	  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
168	  base    volumeGroup owi-a- 4.00g
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