admin-guide/device-mapper/snapshot.rst

2 Device-mapper snapshot support
5 Device-mapper allows you, without massive data copying:
7 -  To create snapshots of any block device i.e. mountable, saved states of
10 -  To create device "forks", i.e. multiple different versions of the
12 -  To merge a snapshot of a block device back into the snapshot's origin
16 changed and uses a separate copy-on-write (COW) block device for
24 snapshot, snapshot-origin, and snapshot-merge.
26 -  snapshot-origin <origin>
34 -  snapshot <origin> <COW device> <persistent?> <chunksize>
43 the amount of free space and expand the <COW device> before it fills up.
45 <persistent?> is P (Persistent) or N (Not persistent - will not survive
51 snapshots less metadata must be saved on disk - they can be kept in
55 snapshot-origin or snapshot-merge target must be suspended. A failure to
60    discard_zeroes_cow - a discard issued to the snapshot device that
64    discard_passdown_origin - a discard to the snapshot device is passed
65    down to the snapshot-origin's underlying device.  This doesn't cause
66    copy-out to the snapshot exception store because the snapshot-origin
73 -  snapshot-merge <origin> <COW device> <persistent> <chunksize>
78 "snapshot-origin" target and must not be loaded if the "snapshot-origin"
85 will continue while I/O is flowing to it.  Changes to the <origin> are
88 the "snapshot" target, will return -EIO when accessed.
100    source volume), whose table is replaced by a "snapshot-origin" mapping
105   lvcreate -L 1G -n base volumeGroup
106   lvcreate -L 100M --snapshot -n snap volumeGroup/base
112   volumeGroup-base-real: 0 2097152 linear 8:19 384
113   volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
114   volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
115   volumeGroup-base: 0 2097152 snapshot-origin 254:11
117   # ls -lL /dev/mapper/volumeGroup-*
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 /dev/mapper/volumeGroup-snap-cow
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 /dev/mapper/volumeGroup-base
124 How snapshot-merge is used by LVM2
126 A merging snapshot assumes the role of the "snapshot-origin" while
127 merging.  As such the "snapshot-origin" is replaced with
128 "snapshot-merge".  The "-real" device is not changed and the "-cow"
129 device is renamed to <origin name>-cow to aid LVM2's cleanup of the
131 COW device to the "snapshot-merge" is deactivated (unless using lvchange
132 --refresh); but if it is left active it will simply return I/O errors.
136   lvconvert --merge volumeGroup/snap
142   volumeGroup-base-real: 0 2097152 linear 8:19 384
143   volumeGroup-base-cow: 0 204800 linear 8:19 2097536
144   volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
146   # ls -lL /dev/mapper/volumeGroup-*
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 /dev/mapper/volumeGroup-base-cow
149   brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
154 The snapshot-merge and snapshot status lines end with:
167     base    volumeGroup owi-a- 4.00g
168     snap    volumeGroup swi-a- 1.00g base  18.97
170   # dmsetup status volumeGroup-snap
174   # lvconvert --merge -b volumeGroup/snap
179     base    volumeGroup Owi-a- 4.00g          17.23
181   # dmsetup status volumeGroup-base
182   0 8388608 snapshot-merge 281688/2097152 1104
184   # dmsetup status volumeGroup-base
185   0 8388608 snapshot-merge 180480/2097152 712
187   # dmsetup status volumeGroup-base
188   0 8388608 snapshot-merge 16/2097152 16
196     base    volumeGroup owi-a- 4.00g