Installation guide

Appendix A. The Device Mapper

1. A device with a linear mapping containing the original mapping table of the source volume.

2. A device with a linear mapping used as the copy-on-write (COW) device for the source volume;

for each write, the original data is saved in the COW device of each snapshot to keep its visible

content unchanged (until the COW device fills up).

3. A device with a snapshot mapping 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 to create these devices, For example, you might use the following

commands to create an LVM volume named base and a snapshot volume named snap based on that

volume.

# lvcreate -L 1G -n base volumeGroup

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

This yields four devices, which you can view with the following commands:

# 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

The format for the snapshot-origin target is as follows:

start length snapshot-origin origin

start

starting block in virtual device

length

length of this segment

origin

base volume of snapshot

The snapshot-origin will normally have one or more snapshots based on it. 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 until the COW device fills up.

The format for the snapshot target is as follows:

start length snapshot origin COW-device P|N chunksize