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
12
13 In both cases, dm copies only the chunks of data that get changed and
14 uses a separate copy-on-write (COW) block device for storage.
15
16
17 There are two dm targets available: snapshot and snapshot-origin.
18
19 *) snapshot-origin <origin>
20
21 which will normally have one or more snapshots based on it.
22 Reads will be mapped directly to the backing device. For each write, the
23 original data will be saved in the <COW device> of each snapshot to keep
24 its visible content unchanged, at least until the <COW device> fills up.
25
26
27 *) snapshot <origin> <COW device> <persistent?> <chunksize>
28
29 A snapshot of the <origin> block device is created. Changed chunks of
30 <chunksize> sectors will be stored on the <COW device>. Writes will
31 only go to the <COW device>. Reads will come from the <COW device> or
32 from <origin> for unchanged data. <COW device> will often be
33 smaller than the origin and if it fills up the snapshot will become
34 useless and be disabled, returning errors. So it is important to monitor
35 the amount of free space and expand the <COW device> before it fills up.
36
37 <persistent?> is P (Persistent) or N (Not persistent - will not survive
38 after reboot).
39 The difference is that for transient snapshots less metadata must be
40 saved on disk - they can be kept in memory by the kernel.
41
42
43 How this is used by LVM2
44 ========================
45 When you create the first LVM2 snapshot of a volume, four dm devices are used:
46
47 1) a device containing the original mapping table of the source volume;
48 2) a device used as the <COW device>;
49 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
50 volume;
51 4) the "original" volume (which uses the device number used by the original
52 source volume), whose table is replaced by a "snapshot-origin" mapping
53 from device #1.
54
55 A fixed naming scheme is used, so with the following commands:
56
57 lvcreate -L 1G -n base volumeGroup
58 lvcreate -L 100M --snapshot -n snap volumeGroup/base
59
60 we'll have this situation (with volumes in above order):
61
62 # dmsetup table|grep volumeGroup
63
64 volumeGroup-base-real: 0 2097152 linear 8:19 384
65 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
66 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
67 volumeGroup-base: 0 2097152 snapshot-origin 254:11
68
69 # ls -lL /dev/mapper/volumeGroup-*
70 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
71 brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
72 brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
73 brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
74
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