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authorMikulas Patocka <mpatocka@redhat.com>2009-12-10 18:52:30 -0500
committerAlasdair G Kergon <agk@redhat.com>2009-12-10 18:52:30 -0500
commitd698aa4500aa3ca9559142060caf0f79da998744 (patch)
tree175577ce2c8ae0918e6f38a2a27ad2b124bb2466 /Documentation/device-mapper
parent4454a6216f75a9ef8c4bd0a65e34b101f725ef1e (diff)
dm snapshot: add merge target
The snapshot-merge target allows a snapshot to be merged back into the snapshot's origin device. One anticipated use of snapshot merging is the rollback of filesystems to back out problematic system upgrades. This patch adds snapshot-merge target management to both dm_snapshot_init() and dm_snapshot_exit(). As an initial place-holder, snapshot-merge is identical to the snapshot target. Documentation is provided. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Diffstat (limited to 'Documentation/device-mapper')
-rw-r--r--Documentation/device-mapper/snapshot.txt60
1 files changed, 55 insertions, 5 deletions
diff --git a/Documentation/device-mapper/snapshot.txt b/Documentation/device-mapper/snapshot.txt
index a5009c8300f3..e3a77b215135 100644
--- a/Documentation/device-mapper/snapshot.txt
+++ b/Documentation/device-mapper/snapshot.txt
@@ -8,13 +8,19 @@ the block device which are also writable without interfering with the
8original content; 8original content;
9*) To create device "forks", i.e. multiple different versions of the 9*) To create device "forks", i.e. multiple different versions of the
10same data stream. 10same data stream.
11*) To merge a snapshot of a block device back into the snapshot's origin
12device.
11 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.
12 17
13In both cases, dm copies only the chunks of data that get changed and 18For snapshot merge the contents of the COW storage are merged back into
14uses a separate copy-on-write (COW) block device for storage. 19the origin device.
15 20
16 21
17There are two dm targets available: snapshot and snapshot-origin. 22There are three dm targets available:
23snapshot, snapshot-origin, and snapshot-merge.
18 24
19*) snapshot-origin <origin> 25*) snapshot-origin <origin>
20 26
@@ -40,8 +46,25 @@ The difference is that for transient snapshots less metadata must be
40saved on disk - they can be kept in memory by the kernel. 46saved on disk - they can be kept in memory by the kernel.
41 47
42 48
43How this is used by LVM2 49* snapshot-merge <origin> <COW device> <persistent> <chunksize>
44======================== 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============================
45When you create the first LVM2 snapshot of a volume, four dm devices are used: 68When you create the first LVM2 snapshot of a volume, four dm devices are used:
46 69
471) a device containing the original mapping table of the source volume; 701) a device containing the original mapping table of the source volume;
@@ -72,3 +95,30 @@ brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
72brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap 95brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
73brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base 96brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
74 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