[PATCH] Add dm-snapshot tutorial in Documentation

Device-mapper snapshot support

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Device-mapper allows you, without massive data copying:

*) To create snapshots of any block device i.e. mountable, saved states of

the block device which are also writable without interfering with the

original content;

*) To create device "forks", i.e. multiple different versions of the

same data stream.

In both cases, dm copies only the chunks of data that get changed and

uses a separate copy-on-write (COW) block device for storage.

There are two dm targets available: snapshot and snapshot-origin.

*) snapshot-origin <origin>

which will normally have one or more snapshots based on it.

You must create the snapshot-origin device before you can create snapshots.

Reads will be mapped directly to the backing device. For each write, the

original data will be saved in the <COW device> of each snapshot to keep

its visible content unchanged, at least until the <COW device> fills up.

*) snapshot <origin> <COW device> <persistent?> <chunksize>

A snapshot is created of the <origin> block device. Changed chunks of

<chunksize> sectors will be stored on the <COW device>.  Writes will

only go to the <COW device>.  Reads will come from the <COW device> or

from <origin> for unchanged data.  <COW device> will often be

smaller than the origin and if it fills up the snapshot will become

useless and be disabled, returning errors.  So it is important to monitor

the amount of free space and expand the <COW device> before it fills up.

<persistent?> is P (Persistent) or N (Not persistent - will not survive

after reboot).

How this is used by LVM2

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When you create the first LVM2 snapshot of a volume, four dm devices are used:

1) a device containing the original mapping table of the source volume;

2) a device used as the <COW device>;

3) a "snapshot" device, combining #1 and #2, which is the visible snapshot

   volume;

4) the "original" volume (which uses the device number used by the original

   source volume), whose table is replaced by a "snapshot-origin" mapping

   from device #1.

A fixed naming scheme is used, so with the following commands:

lvcreate -L 1G -n base volumeGroup

lvcreate -L 100M --snapshot -n snap volumeGroup/base

we'll have this situation (with volumes in above order):

# dmsetup table|grep volumeGroup

volumeGroup-base-real: 0 2097152 linear 8:19 384

volumeGroup-snap-cow: 0 204800 linear 8:19 2097536

volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16

volumeGroup-base: 0 2097152 snapshot-origin 254:11

# ls -lL /dev/mapper/volumeGroup-*

brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real

brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow

brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap

brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base