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| -rw-r--r-- | Documentation/device-mapper/snapshot.txt | 73 |
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diff --git a/Documentation/device-mapper/snapshot.txt b/Documentation/device-mapper/snapshot.txt new file mode 100644 index 00000000000..dca274ff400 --- /dev/null +++ b/Documentation/device-mapper/snapshot.txt @@ -0,0 +1,73 @@ +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 + |