admin-guide/device-mapper/snapshot.rst

2 Device-mapper snapshot support
12 -  To merge a snapshot of a block device back into the snapshot's origin
19 For snapshot merge the contents of the COW storage are merged back into
24 snapshot, snapshot-origin, and snapshot-merge.
26 -  snapshot-origin <origin>
30 original data will be saved in the <COW device> of each snapshot to keep
34 -  snapshot <origin> <COW device> <persistent?> <chunksize>
37 A snapshot of the <origin> block device is created. Changed chunks of
41 smaller than the origin and if it fills up the snapshot will become
48 snapshot status.  So supported store types are "P", "PO" and "N".
54 When loading or unloading the snapshot target, the corresponding
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
62    the snapshot's exception store.
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>
76 takes the same table arguments as the snapshot target except it only
78 "snapshot-origin" target and must not be loaded if the "snapshot-origin"
81 Creates a merging snapshot that takes control of the changed chunks
82 stored in the <COW device> of an existing snapshot, through a handover
86 deferred until the merging snapshot's corresponding chunk(s) have been
87 merged.  Once merging has started the snapshot device, associated with
88 the "snapshot" target, will return -EIO when accessed.
91 How snapshot is used by LVM2
93 When you create the first LVM2 snapshot of a volume, four dm devices are used:
97 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
100    source volume), whose table is replaced by a "snapshot-origin" mapping
106   lvcreate -L 100M --snapshot -n snap volumeGroup/base
114   volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
115   volumeGroup-base: 0 2097152 snapshot-origin 254:11
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"
130 merging snapshot after it completes.  The "snapshot" that hands over its
131 COW device to the "snapshot-merge" is deactivated (unless using lvchange
134 A snapshot will merge into its origin with the following command::
144   volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
154 The snapshot-merge and snapshot status lines end with:
171   0 8388608 snapshot 397896/2097152 1560
182   0 8388608 snapshot-merge 281688/2097152 1104
185   0 8388608 snapshot-merge 180480/2097152 712
188   0 8388608 snapshot-merge 16/2097152 16