1 ============================== 1 ==============================
2 Device-mapper snapshot support 2 Device-mapper snapshot support
3 ============================== 3 ==============================
4 4
5 Device-mapper allows you, without massive data 5 Device-mapper allows you, without massive data copying:
6 6
7 - To create snapshots of any block device i.e 7 - To create snapshots of any block device i.e. mountable, saved states of
8 the block device which are also writable wi 8 the block device which are also writable without interfering with the
9 original content; 9 original content;
10 - To create device "forks", i.e. multiple dif 10 - To create device "forks", i.e. multiple different versions of the
11 same data stream. 11 same data stream.
12 - To merge a snapshot of a block device back 12 - To merge a snapshot of a block device back into the snapshot's origin
13 device. 13 device.
14 14
15 In the first two cases, dm copies only the chu 15 In the first two cases, dm copies only the chunks of data that get
16 changed and uses a separate copy-on-write (COW 16 changed and uses a separate copy-on-write (COW) block device for
17 storage. 17 storage.
18 18
19 For snapshot merge the contents of the COW sto 19 For snapshot merge the contents of the COW storage are merged back into
20 the origin device. 20 the origin device.
21 21
22 22
23 There are three dm targets available: 23 There are three dm targets available:
24 snapshot, snapshot-origin, and snapshot-merge. 24 snapshot, snapshot-origin, and snapshot-merge.
25 25
26 - snapshot-origin <origin> 26 - snapshot-origin <origin>
27 27
28 which will normally have one or more snapshots 28 which will normally have one or more snapshots based on it.
29 Reads will be mapped directly to the backing d 29 Reads will be mapped directly to the backing device. For each write, the
30 original data will be saved in the <COW device 30 original data will be saved in the <COW device> of each snapshot to keep
31 its visible content unchanged, at least until 31 its visible content unchanged, at least until the <COW device> fills up.
32 32
33 33
34 - snapshot <origin> <COW device> <persistent? 34 - snapshot <origin> <COW device> <persistent?> <chunksize>
35 [<# feature args> [<arg>]*] 35 [<# feature args> [<arg>]*]
36 36
37 A snapshot of the <origin> block device is cre 37 A snapshot of the <origin> block device is created. Changed chunks of
38 <chunksize> sectors will be stored on the <COW 38 <chunksize> sectors will be stored on the <COW device>. Writes will
39 only go to the <COW device>. Reads will come 39 only go to the <COW device>. Reads will come from the <COW device> or
40 from <origin> for unchanged data. <COW device 40 from <origin> for unchanged data. <COW device> will often be
41 smaller than the origin and if it fills up the 41 smaller than the origin and if it fills up the snapshot will become
42 useless and be disabled, returning errors. So 42 useless and be disabled, returning errors. So it is important to monitor
43 the amount of free space and expand the <COW d 43 the amount of free space and expand the <COW device> before it fills up.
44 44
45 <persistent?> is P (Persistent) or N (Not pers 45 <persistent?> is P (Persistent) or N (Not persistent - will not survive
46 after reboot). O (Overflow) can be added as a 46 after reboot). O (Overflow) can be added as a persistent store option
47 to allow userspace to advertise its support fo 47 to allow userspace to advertise its support for seeing "Overflow" in the
48 snapshot status. So supported store types are 48 snapshot status. So supported store types are "P", "PO" and "N".
49 49
50 The difference between persistent and transien 50 The difference between persistent and transient is with transient
51 snapshots less metadata must be saved on disk 51 snapshots less metadata must be saved on disk - they can be kept in
52 memory by the kernel. 52 memory by the kernel.
53 53
54 When loading or unloading the snapshot target, 54 When loading or unloading the snapshot target, the corresponding
55 snapshot-origin or snapshot-merge target must 55 snapshot-origin or snapshot-merge target must be suspended. A failure to
56 suspend the origin target could result in data 56 suspend the origin target could result in data corruption.
57 57
58 Optional features: 58 Optional features:
59 59
60 discard_zeroes_cow - a discard issued to th 60 discard_zeroes_cow - a discard issued to the snapshot device that
61 maps to entire chunks to will zero the corr 61 maps to entire chunks to will zero the corresponding exception(s) in
62 the snapshot's exception store. 62 the snapshot's exception store.
63 63
64 discard_passdown_origin - a discard to the 64 discard_passdown_origin - a discard to the snapshot device is passed
65 down to the snapshot-origin's underlying de 65 down to the snapshot-origin's underlying device. This doesn't cause
66 copy-out to the snapshot exception store be 66 copy-out to the snapshot exception store because the snapshot-origin
67 target is bypassed. 67 target is bypassed.
68 68
69 The discard_passdown_origin feature depends 69 The discard_passdown_origin feature depends on the discard_zeroes_cow
70 feature being enabled. 70 feature being enabled.
71 71
72 72
73 - snapshot-merge <origin> <COW device> <persi 73 - snapshot-merge <origin> <COW device> <persistent> <chunksize>
74 [<# feature args> [<arg>]*] 74 [<# feature args> [<arg>]*]
75 75
76 takes the same table arguments as the snapshot 76 takes the same table arguments as the snapshot target except it only
77 works with persistent snapshots. This target 77 works with persistent snapshots. This target assumes the role of the
78 "snapshot-origin" target and must not be loade 78 "snapshot-origin" target and must not be loaded if the "snapshot-origin"
79 is still present for <origin>. 79 is still present for <origin>.
80 80
81 Creates a merging snapshot that takes control 81 Creates a merging snapshot that takes control of the changed chunks
82 stored in the <COW device> of an existing snap 82 stored in the <COW device> of an existing snapshot, through a handover
83 procedure, and merges these chunks back into t 83 procedure, and merges these chunks back into the <origin>. Once merging
84 has started (in the background) the <origin> m 84 has started (in the background) the <origin> may be opened and the merge
85 will continue while I/O is flowing to it. Cha 85 will continue while I/O is flowing to it. Changes to the <origin> are
86 deferred until the merging snapshot's correspo 86 deferred until the merging snapshot's corresponding chunk(s) have been
87 merged. Once merging has started the snapshot 87 merged. Once merging has started the snapshot device, associated with
88 the "snapshot" target, will return -EIO when a 88 the "snapshot" target, will return -EIO when accessed.
89 89
90 90
91 How snapshot is used by LVM2 91 How snapshot is used by LVM2
92 ============================ 92 ============================
93 When you create the first LVM2 snapshot of a v 93 When you create the first LVM2 snapshot of a volume, four dm devices are used:
94 94
95 1) a device containing the original mapping ta 95 1) a device containing the original mapping table of the source volume;
96 2) a device used as the <COW device>; 96 2) a device used as the <COW device>;
97 3) a "snapshot" device, combining #1 and #2, w 97 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
98 volume; 98 volume;
99 4) the "original" volume (which uses the devic 99 4) the "original" volume (which uses the device number used by the original
100 source volume), whose table is replaced by 100 source volume), whose table is replaced by a "snapshot-origin" mapping
101 from device #1. 101 from device #1.
102 102
103 A fixed naming scheme is used, so with the fol 103 A fixed naming scheme is used, so with the following commands::
104 104
105 lvcreate -L 1G -n base volumeGroup 105 lvcreate -L 1G -n base volumeGroup
106 lvcreate -L 100M --snapshot -n snap volumeGr 106 lvcreate -L 100M --snapshot -n snap volumeGroup/base
107 107
108 we'll have this situation (with volumes in abo 108 we'll have this situation (with volumes in above order)::
109 109
110 # dmsetup table|grep volumeGroup 110 # dmsetup table|grep volumeGroup
111 111
112 volumeGroup-base-real: 0 2097152 linear 8:19 112 volumeGroup-base-real: 0 2097152 linear 8:19 384
113 volumeGroup-snap-cow: 0 204800 linear 8:19 2 113 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
114 volumeGroup-snap: 0 2097152 snapshot 254:11 114 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
115 volumeGroup-base: 0 2097152 snapshot-origin 115 volumeGroup-base: 0 2097152 snapshot-origin 254:11
116 116
117 # ls -lL /dev/mapper/volumeGroup-* 117 # ls -lL /dev/mapper/volumeGroup-*
118 brw------- 1 root root 254, 11 29 ago 18:15 118 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
119 brw------- 1 root root 254, 12 29 ago 18:15 119 brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
120 brw------- 1 root root 254, 13 29 ago 18:15 120 brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
121 brw------- 1 root root 254, 10 29 ago 18:14 121 brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
122 122
123 123
124 How snapshot-merge is used by LVM2 124 How snapshot-merge is used by LVM2
125 ================================== 125 ==================================
126 A merging snapshot assumes the role of the "sn 126 A merging snapshot assumes the role of the "snapshot-origin" while
127 merging. As such the "snapshot-origin" is rep 127 merging. As such the "snapshot-origin" is replaced with
128 "snapshot-merge". The "-real" device is not c 128 "snapshot-merge". The "-real" device is not changed and the "-cow"
129 device is renamed to <origin name>-cow to aid 129 device is renamed to <origin name>-cow to aid LVM2's cleanup of the
130 merging snapshot after it completes. The "sna 130 merging snapshot after it completes. The "snapshot" that hands over its
131 COW device to the "snapshot-merge" is deactiva 131 COW device to the "snapshot-merge" is deactivated (unless using lvchange
132 --refresh); but if it is left active it will s 132 --refresh); but if it is left active it will simply return I/O errors.
133 133
134 A snapshot will merge into its origin with the 134 A snapshot will merge into its origin with the following command::
135 135
136 lvconvert --merge volumeGroup/snap 136 lvconvert --merge volumeGroup/snap
137 137
138 we'll now have this situation:: 138 we'll now have this situation::
139 139
140 # dmsetup table|grep volumeGroup 140 # dmsetup table|grep volumeGroup
141 141
142 volumeGroup-base-real: 0 2097152 linear 8:19 142 volumeGroup-base-real: 0 2097152 linear 8:19 384
143 volumeGroup-base-cow: 0 204800 linear 8:19 2 143 volumeGroup-base-cow: 0 204800 linear 8:19 2097536
144 volumeGroup-base: 0 2097152 snapshot-merge 2 144 volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
145 145
146 # ls -lL /dev/mapper/volumeGroup-* 146 # ls -lL /dev/mapper/volumeGroup-*
147 brw------- 1 root root 254, 11 29 ago 18:15 147 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
148 brw------- 1 root root 254, 12 29 ago 18:16 148 brw------- 1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
149 brw------- 1 root root 254, 10 29 ago 18:16 149 brw------- 1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
150 150
151 151
152 How to determine when a merging is complete 152 How to determine when a merging is complete
153 =========================================== 153 ===========================================
154 The snapshot-merge and snapshot status lines e 154 The snapshot-merge and snapshot status lines end with:
155 155
156 <sectors_allocated>/<total_sectors> <metadat 156 <sectors_allocated>/<total_sectors> <metadata_sectors>
157 157
158 Both <sectors_allocated> and <total_sectors> i 158 Both <sectors_allocated> and <total_sectors> include both data and metadata.
159 During merging, the number of sectors allocate 159 During merging, the number of sectors allocated gets smaller and
160 smaller. Merging has finished when the number 160 smaller. Merging has finished when the number of sectors holding data
161 is zero, in other words <sectors_allocated> == 161 is zero, in other words <sectors_allocated> == <metadata_sectors>.
162 162
163 Here is a practical example (using a hybrid of 163 Here is a practical example (using a hybrid of lvm and dmsetup commands)::
164 164
165 # lvs 165 # lvs
166 LV VG Attr LSize Origin S 166 LV VG Attr LSize Origin Snap% Move Log Copy% Convert
167 base volumeGroup owi-a- 4.00g 167 base volumeGroup owi-a- 4.00g
168 snap volumeGroup swi-a- 1.00g base 18. 168 snap volumeGroup swi-a- 1.00g base 18.97
169 169
170 # dmsetup status volumeGroup-snap 170 # dmsetup status volumeGroup-snap
171 0 8388608 snapshot 397896/2097152 1560 171 0 8388608 snapshot 397896/2097152 1560
172 ^^^^ metad 172 ^^^^ metadata sectors
173 173
174 # lvconvert --merge -b volumeGroup/snap 174 # lvconvert --merge -b volumeGroup/snap
175 Merging of volume snap started. 175 Merging of volume snap started.
176 176
177 # lvs volumeGroup/snap 177 # lvs volumeGroup/snap
178 LV VG Attr LSize Origin S 178 LV VG Attr LSize Origin Snap% Move Log Copy% Convert
179 base volumeGroup Owi-a- 4.00g 179 base volumeGroup Owi-a- 4.00g 17.23
180 180
181 # dmsetup status volumeGroup-base 181 # dmsetup status volumeGroup-base
182 0 8388608 snapshot-merge 281688/2097152 1104 182 0 8388608 snapshot-merge 281688/2097152 1104
183 183
184 # dmsetup status volumeGroup-base 184 # dmsetup status volumeGroup-base
185 0 8388608 snapshot-merge 180480/2097152 712 185 0 8388608 snapshot-merge 180480/2097152 712
186 186
187 # dmsetup status volumeGroup-base 187 # dmsetup status volumeGroup-base
188 0 8388608 snapshot-merge 16/2097152 16 188 0 8388608 snapshot-merge 16/2097152 16
189 189
190 Merging has finished. 190 Merging has finished.
191 191
192 :: 192 ::
193 193
194 # lvs 194 # lvs
195 LV VG Attr LSize Origin S 195 LV VG Attr LSize Origin Snap% Move Log Copy% Convert
196 base volumeGroup owi-a- 4.00g 196 base volumeGroup owi-a- 4.00g
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