1 ==============================
2 Device-mapper snapshot support
3 ==============================
4
5 Device-mapper allows you, without massive data
6
7 - To create snapshots of any block device i.e
8 the block device which are also writable wi
9 original content;
10 - To create device "forks", i.e. multiple dif
11 same data stream.
12 - To merge a snapshot of a block device back
13 device.
14
15 In the first two cases, dm copies only the chu
16 changed and uses a separate copy-on-write (COW
17 storage.
18
19 For snapshot merge the contents of the COW sto
20 the origin device.
21
22
23 There are three dm targets available:
24 snapshot, snapshot-origin, and snapshot-merge.
25
26 - snapshot-origin <origin>
27
28 which will normally have one or more snapshots
29 Reads will be mapped directly to the backing d
30 original data will be saved in the <COW device
31 its visible content unchanged, at least until
32
33
34 - snapshot <origin> <COW device> <persistent?
35 [<# feature args> [<arg>]*]
36
37 A snapshot of the <origin> block device is cre
38 <chunksize> sectors will be stored on the <COW
39 only go to the <COW device>. Reads will come
40 from <origin> for unchanged data. <COW device
41 smaller than the origin and if it fills up the
42 useless and be disabled, returning errors. So
43 the amount of free space and expand the <COW d
44
45 <persistent?> is P (Persistent) or N (Not pers
46 after reboot). O (Overflow) can be added as a
47 to allow userspace to advertise its support fo
48 snapshot status. So supported store types are
49
50 The difference between persistent and transien
51 snapshots less metadata must be saved on disk
52 memory by the kernel.
53
54 When loading or unloading the snapshot target,
55 snapshot-origin or snapshot-merge target must
56 suspend the origin target could result in data
57
58 Optional features:
59
60 discard_zeroes_cow - a discard issued to th
61 maps to entire chunks to will zero the corr
62 the snapshot's exception store.
63
64 discard_passdown_origin - a discard to the
65 down to the snapshot-origin's underlying de
66 copy-out to the snapshot exception store be
67 target is bypassed.
68
69 The discard_passdown_origin feature depends
70 feature being enabled.
71
72
73 - snapshot-merge <origin> <COW device> <persi
74 [<# feature args> [<arg>]*]
75
76 takes the same table arguments as the snapshot
77 works with persistent snapshots. This target
78 "snapshot-origin" target and must not be loade
79 is still present for <origin>.
80
81 Creates a merging snapshot that takes control
82 stored in the <COW device> of an existing snap
83 procedure, and merges these chunks back into t
84 has started (in the background) the <origin> m
85 will continue while I/O is flowing to it. Cha
86 deferred until the merging snapshot's correspo
87 merged. Once merging has started the snapshot
88 the "snapshot" target, will return -EIO when a
89
90
91 How snapshot is used by LVM2
92 ============================
93 When you create the first LVM2 snapshot of a v
94
95 1) a device containing the original mapping ta
96 2) a device used as the <COW device>;
97 3) a "snapshot" device, combining #1 and #2, w
98 volume;
99 4) the "original" volume (which uses the devic
100 source volume), whose table is replaced by
101 from device #1.
102
103 A fixed naming scheme is used, so with the fol
104
105 lvcreate -L 1G -n base volumeGroup
106 lvcreate -L 100M --snapshot -n snap volumeGr
107
108 we'll have this situation (with volumes in abo
109
110 # dmsetup table|grep volumeGroup
111
112 volumeGroup-base-real: 0 2097152 linear 8:19
113 volumeGroup-snap-cow: 0 204800 linear 8:19 2
114 volumeGroup-snap: 0 2097152 snapshot 254:11
115 volumeGroup-base: 0 2097152 snapshot-origin
116
117 # ls -lL /dev/mapper/volumeGroup-*
118 brw------- 1 root root 254, 11 29 ago 18:15
119 brw------- 1 root root 254, 12 29 ago 18:15
120 brw------- 1 root root 254, 13 29 ago 18:15
121 brw------- 1 root root 254, 10 29 ago 18:14
122
123
124 How snapshot-merge is used by LVM2
125 ==================================
126 A merging snapshot assumes the role of the "sn
127 merging. As such the "snapshot-origin" is rep
128 "snapshot-merge". The "-real" device is not c
129 device is renamed to <origin name>-cow to aid
130 merging snapshot after it completes. The "sna
131 COW device to the "snapshot-merge" is deactiva
132 --refresh); but if it is left active it will s
133
134 A snapshot will merge into its origin with the
135
136 lvconvert --merge volumeGroup/snap
137
138 we'll now have this situation::
139
140 # dmsetup table|grep volumeGroup
141
142 volumeGroup-base-real: 0 2097152 linear 8:19
143 volumeGroup-base-cow: 0 204800 linear 8:19 2
144 volumeGroup-base: 0 2097152 snapshot-merge 2
145
146 # ls -lL /dev/mapper/volumeGroup-*
147 brw------- 1 root root 254, 11 29 ago 18:15
148 brw------- 1 root root 254, 12 29 ago 18:16
149 brw------- 1 root root 254, 10 29 ago 18:16
150
151
152 How to determine when a merging is complete
153 ===========================================
154 The snapshot-merge and snapshot status lines e
155
156 <sectors_allocated>/<total_sectors> <metadat
157
158 Both <sectors_allocated> and <total_sectors> i
159 During merging, the number of sectors allocate
160 smaller. Merging has finished when the number
161 is zero, in other words <sectors_allocated> ==
162
163 Here is a practical example (using a hybrid of
164
165 # lvs
166 LV VG Attr LSize Origin S
167 base volumeGroup owi-a- 4.00g
168 snap volumeGroup swi-a- 1.00g base 18.
169
170 # dmsetup status volumeGroup-snap
171 0 8388608 snapshot 397896/2097152 1560
172 ^^^^ metad
173
174 # lvconvert --merge -b volumeGroup/snap
175 Merging of volume snap started.
176
177 # lvs volumeGroup/snap
178 LV VG Attr LSize Origin S
179 base volumeGroup Owi-a- 4.00g
180
181 # dmsetup status volumeGroup-base
182 0 8388608 snapshot-merge 281688/2097152 1104
183
184 # dmsetup status volumeGroup-base
185 0 8388608 snapshot-merge 180480/2097152 712
186
187 # dmsetup status volumeGroup-base
188 0 8388608 snapshot-merge 16/2097152 16
189
190 Merging has finished.
191
192 ::
193
194 # lvs
195 LV VG Attr LSize Origin S
196 base volumeGroup owi-a- 4.00g
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