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- Device-mapper snapshot support
- ==============================
- 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
- ========================
- 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
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