1Device-mapper snapshot support
2==============================
3
4Device-mapper allows you, without massive data copying:
5
6*) To create snapshots of any block device i.e. mountable, saved states of
7the block device which are also writable without interfering with the
8original content;
9*) To create device "forks", i.e. multiple different versions of the
10same data stream.
11*) To merge a snapshot of a block device back into the snapshot's origin
12device.
13
14In the first two cases, dm copies only the chunks of data that get
15changed and uses a separate copy-on-write (COW) block device for
16storage.
17
18For snapshot merge the contents of the COW storage are merged back into
19the origin device.
20
21
22There are three dm targets available:
23snapshot, snapshot-origin, and snapshot-merge.
24
25*) snapshot-origin <origin>
26
27which will normally have one or more snapshots based on it.
28Reads will be mapped directly to the backing device. For each write, the
29original data will be saved in the <COW device> of each snapshot to keep
30its visible content unchanged, at least until the <COW device> fills up.
31
32
33*) snapshot <origin> <COW device> <persistent?> <chunksize>
34
35A snapshot of the <origin> block device is created. Changed chunks of
36<chunksize> sectors will be stored on the <COW device>.  Writes will
37only go to the <COW device>.  Reads will come from the <COW device> or
38from <origin> for unchanged data.  <COW device> will often be
39smaller than the origin and if it fills up the snapshot will become
40useless and be disabled, returning errors.  So it is important to monitor
41the 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
44after reboot).
45The difference is that for transient snapshots less metadata must be
46saved on disk - they can be kept in memory by the kernel.
47
48
49* snapshot-merge <origin> <COW device> <persistent> <chunksize>
50
51takes the same table arguments as the snapshot target except it only
52works with persistent snapshots.  This target assumes the role of the
53"snapshot-origin" target and must not be loaded if the "snapshot-origin"
54is still present for <origin>.
55
56Creates a merging snapshot that takes control of the changed chunks
57stored in the <COW device> of an existing snapshot, through a handover
58procedure, and merges these chunks back into the <origin>.  Once merging
59has started (in the background) the <origin> may be opened and the merge
60will continue while I/O is flowing to it.  Changes to the <origin> are
61deferred until the merging snapshot's corresponding chunk(s) have been
62merged.  Once merging has started the snapshot device, associated with
63the "snapshot" target, will return -EIO when accessed.
64
65
66How snapshot is used by LVM2
67============================
68When you create the first LVM2 snapshot of a volume, four dm devices are used:
69
701) a device containing the original mapping table of the source volume;
712) a device used as the <COW device>;
723) a "snapshot" device, combining #1 and #2, which is the visible snapshot
73   volume;
744) 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
78A fixed naming scheme is used, so with the following commands:
79
80lvcreate -L 1G -n base volumeGroup
81lvcreate -L 100M --snapshot -n snap volumeGroup/base
82
83we'll have this situation (with volumes in above order):
84
85# dmsetup table|grep volumeGroup
86
87volumeGroup-base-real: 0 2097152 linear 8:19 384
88volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
89volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
90volumeGroup-base: 0 2097152 snapshot-origin 254:11
91
92# ls -lL /dev/mapper/volumeGroup-*
93brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
94brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
95brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
96brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
97
98
99How snapshot-merge is used by LVM2
100==================================
101A merging snapshot assumes the role of the "snapshot-origin" while
102merging.  As such the "snapshot-origin" is replaced with
103"snapshot-merge".  The "-real" device is not changed and the "-cow"
104device is renamed to <origin name>-cow to aid LVM2's cleanup of the
105merging snapshot after it completes.  The "snapshot" that hands over its
106COW 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
109A snapshot will merge into its origin with the following command:
110
111lvconvert --merge volumeGroup/snap
112
113we'll now have this situation:
114
115# dmsetup table|grep volumeGroup
116
117volumeGroup-base-real: 0 2097152 linear 8:19 384
118volumeGroup-base-cow: 0 204800 linear 8:19 2097536
119volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
120
121# ls -lL /dev/mapper/volumeGroup-*
122brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
123brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
124brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
125
126
127How to determine when a merging is complete
128===========================================
129The snapshot-merge and snapshot status lines end with:
130  <sectors_allocated>/<total_sectors> <metadata_sectors>
131
132Both <sectors_allocated> and <total_sectors> include both data and metadata.
133During merging, the number of sectors allocated gets smaller and
134smaller.  Merging has finished when the number of sectors holding data
135is zero, in other words <sectors_allocated> == <metadata_sectors>.
136
137Here 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
1450 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
1560 8388608 snapshot-merge 281688/2097152 1104
157
158# dmsetup status volumeGroup-base
1590 8388608 snapshot-merge 180480/2097152 712
160
161# dmsetup status volumeGroup-base
1620 8388608 snapshot-merge 16/2097152 16
163
164Merging has finished.
165
166# lvs
167  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
168  base    volumeGroup owi-a- 4.00g
169