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Documentation / device-mapper / snapshot.txt




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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 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).  O (Overflow) can be added as a persistent store option
45	to allow userspace to advertise its support for seeing "Overflow" in the
46	snapshot status.  So supported store types are "P", "PO" and "N".
47	
48	The difference between persistent and transient is with transient
49	snapshots less metadata must be saved on disk - they can be kept in
50	memory by the kernel.
51	
52	When loading or unloading the snapshot target, the corresponding
53	snapshot-origin or snapshot-merge target must be suspended. A failure to
54	suspend the origin target could result in data corruption.
55	
56	
57	* snapshot-merge <origin> <COW device> <persistent> <chunksize>
58	
59	takes the same table arguments as the snapshot target except it only
60	works with persistent snapshots.  This target assumes the role of the
61	"snapshot-origin" target and must not be loaded if the "snapshot-origin"
62	is still present for <origin>.
63	
64	Creates a merging snapshot that takes control of the changed chunks
65	stored in the <COW device> of an existing snapshot, through a handover
66	procedure, and merges these chunks back into the <origin>.  Once merging
67	has started (in the background) the <origin> may be opened and the merge
68	will continue while I/O is flowing to it.  Changes to the <origin> are
69	deferred until the merging snapshot's corresponding chunk(s) have been
70	merged.  Once merging has started the snapshot device, associated with
71	the "snapshot" target, will return -EIO when accessed.
72	
73	
74	How snapshot is used by LVM2
75	============================
76	When you create the first LVM2 snapshot of a volume, four dm devices are used:
77	
78	1) a device containing the original mapping table of the source volume;
79	2) a device used as the <COW device>;
80	3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
81	   volume;
82	4) the "original" volume (which uses the device number used by the original
83	   source volume), whose table is replaced by a "snapshot-origin" mapping
84	   from device #1.
85	
86	A fixed naming scheme is used, so with the following commands:
87	
88	lvcreate -L 1G -n base volumeGroup
89	lvcreate -L 100M --snapshot -n snap volumeGroup/base
90	
91	we'll have this situation (with volumes in above order):
92	
93	# dmsetup table|grep volumeGroup
94	
95	volumeGroup-base-real: 0 2097152 linear 8:19 384
96	volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
97	volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
98	volumeGroup-base: 0 2097152 snapshot-origin 254:11
99	
100	# ls -lL /dev/mapper/volumeGroup-*
101	brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
102	brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
103	brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
104	brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
105	
106	
107	How snapshot-merge is used by LVM2
108	==================================
109	A merging snapshot assumes the role of the "snapshot-origin" while
110	merging.  As such the "snapshot-origin" is replaced with
111	"snapshot-merge".  The "-real" device is not changed and the "-cow"
112	device is renamed to <origin name>-cow to aid LVM2's cleanup of the
113	merging snapshot after it completes.  The "snapshot" that hands over its
114	COW device to the "snapshot-merge" is deactivated (unless using lvchange
115	--refresh); but if it is left active it will simply return I/O errors.
116	
117	A snapshot will merge into its origin with the following command:
118	
119	lvconvert --merge volumeGroup/snap
120	
121	we'll now have this situation:
122	
123	# dmsetup table|grep volumeGroup
124	
125	volumeGroup-base-real: 0 2097152 linear 8:19 384
126	volumeGroup-base-cow: 0 204800 linear 8:19 2097536
127	volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
128	
129	# ls -lL /dev/mapper/volumeGroup-*
130	brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
131	brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
132	brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
133	
134	
135	How to determine when a merging is complete
136	===========================================
137	The snapshot-merge and snapshot status lines end with:
138	  <sectors_allocated>/<total_sectors> <metadata_sectors>
139	
140	Both <sectors_allocated> and <total_sectors> include both data and metadata.
141	During merging, the number of sectors allocated gets smaller and
142	smaller.  Merging has finished when the number of sectors holding data
143	is zero, in other words <sectors_allocated> == <metadata_sectors>.
144	
145	Here is a practical example (using a hybrid of lvm and dmsetup commands):
146	
147	# lvs
148	  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
149	  base    volumeGroup owi-a- 4.00g
150	  snap    volumeGroup swi-a- 1.00g base  18.97
151	
152	# dmsetup status volumeGroup-snap
153	0 8388608 snapshot 397896/2097152 1560
154	                                  ^^^^ metadata sectors
155	
156	# lvconvert --merge -b volumeGroup/snap
157	  Merging of volume snap started.
158	
159	# lvs volumeGroup/snap
160	  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
161	  base    volumeGroup Owi-a- 4.00g          17.23
162	
163	# dmsetup status volumeGroup-base
164	0 8388608 snapshot-merge 281688/2097152 1104
165	
166	# dmsetup status volumeGroup-base
167	0 8388608 snapshot-merge 180480/2097152 712
168	
169	# dmsetup status volumeGroup-base
170	0 8388608 snapshot-merge 16/2097152 16
171	
172	Merging has finished.
173	
174	# lvs
175	  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
176	  base    volumeGroup owi-a- 4.00g
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