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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. | |
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11 | *) To merge a snapshot of a block device back into the snapshot's origin |
12 | device. | |
e484585e | 13 | |
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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. | |
e484585e | 17 | |
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18 | For snapshot merge the contents of the COW storage are merged back into |
19 | the origin device. | |
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20 | |
21 | ||
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22 | There are three dm targets available: |
23 | snapshot, snapshot-origin, and snapshot-merge. | |
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24 | |
25 | *) snapshot-origin <origin> | |
26 | ||
27 | which will normally have one or more snapshots based on it. | |
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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 | ||
411f1140 | 35 | A snapshot of the <origin> block device is created. Changed chunks of |
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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). | |
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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. | |
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47 | |
48 | ||
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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 | ============================ | |
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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 | ||
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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 |