Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 Device-mapper snapshot support 2 ============================== 3 4 Device-mapper allows you, without massive data copying: 5 6 *) To create snapshots of any block device i.e. mountable, saved states of 7 the block device which are also writable without interfering with the 8 original content; 9 *) To create device "forks", i.e. multiple different versions of the 10 same data stream. 11 12 13 In both cases, dm copies only the chunks of data that get changed and 14 uses a separate copy-on-write (COW) block device for storage. 15 16 17 There are two dm targets available: snapshot and snapshot-origin. 18 19 *) snapshot-origin <origin> 20 21 which will normally have one or more snapshots based on it. 22 Reads will be mapped directly to the backing device. For each write, the 23 original data will be saved in the <COW device> of each snapshot to keep 24 its visible content unchanged, at least until the <COW device> fills up. 25 26 27 *) snapshot <origin> <COW device> <persistent?> <chunksize> 28 29 A snapshot of the <origin> block device is created. Changed chunks of 30 <chunksize> sectors will be stored on the <COW device>. Writes will 31 only go to the <COW device>. Reads will come from the <COW device> or 32 from <origin> for unchanged data. <COW device> will often be 33 smaller than the origin and if it fills up the snapshot will become 34 useless and be disabled, returning errors. So it is important to monitor 35 the amount of free space and expand the <COW device> before it fills up. 36 37 <persistent?> is P (Persistent) or N (Not persistent - will not survive 38 after reboot). 39 The difference is that for transient snapshots less metadata must be 40 saved on disk - they can be kept in memory by the kernel. 41 42 43 How this is used by LVM2 44 ======================== 45 When you create the first LVM2 snapshot of a volume, four dm devices are used: 46 47 1) a device containing the original mapping table of the source volume; 48 2) a device used as the <COW device>; 49 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot 50 volume; 51 4) the "original" volume (which uses the device number used by the original 52 source volume), whose table is replaced by a "snapshot-origin" mapping 53 from device #1. 54 55 A fixed naming scheme is used, so with the following commands: 56 57 lvcreate -L 1G -n base volumeGroup 58 lvcreate -L 100M --snapshot -n snap volumeGroup/base 59 60 we'll have this situation (with volumes in above order): 61 62 # dmsetup table|grep volumeGroup 63 64 volumeGroup-base-real: 0 2097152 linear 8:19 384 65 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536 66 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16 67 volumeGroup-base: 0 2097152 snapshot-origin 254:11 68 69 # ls -lL /dev/mapper/volumeGroup-* 70 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real 71 brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow 72 brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap 73 brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base