1============================== 2Device-mapper snapshot support 3============================== 4 5Device-mapper allows you, without massive data copying: 6 7- To create snapshots of any block device i.e. mountable, saved states of 8 the block device which are also writable without interfering with the 9 original content; 10- To create device "forks", i.e. multiple different versions of the 11 same data stream. 12- To merge a snapshot of a block device back into the snapshot's origin 13 device. 14 15In the first two cases, dm copies only the chunks of data that get 16changed and uses a separate copy-on-write (COW) block device for 17storage. 18 19For snapshot merge the contents of the COW storage are merged back into 20the origin device. 21 22 23There are three dm targets available: 24snapshot, snapshot-origin, and snapshot-merge. 25 26- snapshot-origin <origin> 27 28which will normally have one or more snapshots based on it. 29Reads will be mapped directly to the backing device. For each write, the 30original data will be saved in the <COW device> of each snapshot to keep 31its visible content unchanged, at least until the <COW device> fills up. 32 33 34- snapshot <origin> <COW device> <persistent?> <chunksize> 35 [<# feature args> [<arg>]*] 36 37A snapshot of the <origin> block device is created. Changed chunks of 38<chunksize> sectors will be stored on the <COW device>. Writes will 39only go to the <COW device>. Reads will come from the <COW device> or 40from <origin> for unchanged data. <COW device> will often be 41smaller than the origin and if it fills up the snapshot will become 42useless and be disabled, returning errors. So it is important to monitor 43the amount of free space and expand the <COW device> before it fills up. 44 45<persistent?> is P (Persistent) or N (Not persistent - will not survive 46after reboot). O (Overflow) can be added as a persistent store option 47to allow userspace to advertise its support for seeing "Overflow" in the 48snapshot status. So supported store types are "P", "PO" and "N". 49 50The difference between persistent and transient is with transient 51snapshots less metadata must be saved on disk - they can be kept in 52memory by the kernel. 53 54When loading or unloading the snapshot target, the corresponding 55snapshot-origin or snapshot-merge target must be suspended. A failure to 56suspend the origin target could result in data corruption. 57 58Optional features: 59 60 discard_zeroes_cow - a discard issued to the snapshot device that 61 maps to entire chunks to will zero the corresponding exception(s) in 62 the snapshot's exception store. 63 64 discard_passdown_origin - a discard to the snapshot device is passed 65 down to the snapshot-origin's underlying device. This doesn't cause 66 copy-out to the snapshot exception store because the snapshot-origin 67 target is bypassed. 68 69 The discard_passdown_origin feature depends on the discard_zeroes_cow 70 feature being enabled. 71 72 73- snapshot-merge <origin> <COW device> <persistent> <chunksize> 74 [<# feature args> [<arg>]*] 75 76takes the same table arguments as the snapshot target except it only 77works with persistent snapshots. This target assumes the role of the 78"snapshot-origin" target and must not be loaded if the "snapshot-origin" 79is still present for <origin>. 80 81Creates a merging snapshot that takes control of the changed chunks 82stored in the <COW device> of an existing snapshot, through a handover 83procedure, and merges these chunks back into the <origin>. Once merging 84has started (in the background) the <origin> may be opened and the merge 85will continue while I/O is flowing to it. Changes to the <origin> are 86deferred until the merging snapshot's corresponding chunk(s) have been 87merged. Once merging has started the snapshot device, associated with 88the "snapshot" target, will return -EIO when accessed. 89 90 91How snapshot is used by LVM2 92============================ 93When you create the first LVM2 snapshot of a volume, four dm devices are used: 94 951) a device containing the original mapping table of the source volume; 962) a device used as the <COW device>; 973) a "snapshot" device, combining #1 and #2, which is the visible snapshot 98 volume; 994) the "original" volume (which uses the device number used by the original 100 source volume), whose table is replaced by a "snapshot-origin" mapping 101 from device #1. 102 103A fixed naming scheme is used, so with the following commands:: 104 105 lvcreate -L 1G -n base volumeGroup 106 lvcreate -L 100M --snapshot -n snap volumeGroup/base 107 108we'll have this situation (with volumes in above order):: 109 110 # dmsetup table|grep volumeGroup 111 112 volumeGroup-base-real: 0 2097152 linear 8:19 384 113 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536 114 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16 115 volumeGroup-base: 0 2097152 snapshot-origin 254:11 116 117 # ls -lL /dev/mapper/volumeGroup-* 118 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real 119 brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow 120 brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap 121 brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base 122 123 124How snapshot-merge is used by LVM2 125================================== 126A merging snapshot assumes the role of the "snapshot-origin" while 127merging. As such the "snapshot-origin" is replaced with 128"snapshot-merge". The "-real" device is not changed and the "-cow" 129device is renamed to <origin name>-cow to aid LVM2's cleanup of the 130merging snapshot after it completes. The "snapshot" that hands over its 131COW device to the "snapshot-merge" is deactivated (unless using lvchange 132--refresh); but if it is left active it will simply return I/O errors. 133 134A snapshot will merge into its origin with the following command:: 135 136 lvconvert --merge volumeGroup/snap 137 138we'll now have this situation:: 139 140 # dmsetup table|grep volumeGroup 141 142 volumeGroup-base-real: 0 2097152 linear 8:19 384 143 volumeGroup-base-cow: 0 204800 linear 8:19 2097536 144 volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16 145 146 # ls -lL /dev/mapper/volumeGroup-* 147 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real 148 brw------- 1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow 149 brw------- 1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base 150 151 152How to determine when a merging is complete 153=========================================== 154The snapshot-merge and snapshot status lines end with: 155 156 <sectors_allocated>/<total_sectors> <metadata_sectors> 157 158Both <sectors_allocated> and <total_sectors> include both data and metadata. 159During merging, the number of sectors allocated gets smaller and 160smaller. Merging has finished when the number of sectors holding data 161is zero, in other words <sectors_allocated> == <metadata_sectors>. 162 163Here is a practical example (using a hybrid of lvm and dmsetup commands):: 164 165 # lvs 166 LV VG Attr LSize Origin Snap% Move Log Copy% Convert 167 base volumeGroup owi-a- 4.00g 168 snap volumeGroup swi-a- 1.00g base 18.97 169 170 # dmsetup status volumeGroup-snap 171 0 8388608 snapshot 397896/2097152 1560 172 ^^^^ metadata sectors 173 174 # lvconvert --merge -b volumeGroup/snap 175 Merging of volume snap started. 176 177 # lvs volumeGroup/snap 178 LV VG Attr LSize Origin Snap% Move Log Copy% Convert 179 base volumeGroup Owi-a- 4.00g 17.23 180 181 # dmsetup status volumeGroup-base 182 0 8388608 snapshot-merge 281688/2097152 1104 183 184 # dmsetup status volumeGroup-base 185 0 8388608 snapshot-merge 180480/2097152 712 186 187 # dmsetup status volumeGroup-base 188 0 8388608 snapshot-merge 16/2097152 16 189 190Merging has finished. 191 192:: 193 194 # lvs 195 LV VG Attr LSize Origin Snap% Move Log Copy% Convert 196 base volumeGroup owi-a- 4.00g 197