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1==============================2Device-mapper snapshot support3==============================4 5Device-mapper allows you, without massive data copying:6 7- To create snapshots of any block device i.e. mountable, saved states of8 the block device which are also writable without interfering with the9 original content;10- To create device "forks", i.e. multiple different versions of the11 same data stream.12- To merge a snapshot of a block device back into the snapshot's origin13 device.14 15In the first two cases, dm copies only the chunks of data that get16changed and uses a separate copy-on-write (COW) block device for17storage.18 19For snapshot merge the contents of the COW storage are merged back into20the 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, the30original data will be saved in the <COW device> of each snapshot to keep31its 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 of38<chunksize> sectors will be stored on the <COW device>. Writes will39only go to the <COW device>. Reads will come from the <COW device> or40from <origin> for unchanged data. <COW device> will often be41smaller than the origin and if it fills up the snapshot will become42useless and be disabled, returning errors. So it is important to monitor43the 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 survive46after reboot). O (Overflow) can be added as a persistent store option47to allow userspace to advertise its support for seeing "Overflow" in the48snapshot status. So supported store types are "P", "PO" and "N".49 50The difference between persistent and transient is with transient51snapshots less metadata must be saved on disk - they can be kept in52memory by the kernel.53 54When loading or unloading the snapshot target, the corresponding55snapshot-origin or snapshot-merge target must be suspended. A failure to56suspend the origin target could result in data corruption.57 58Optional features:59 60 discard_zeroes_cow - a discard issued to the snapshot device that61 maps to entire chunks to will zero the corresponding exception(s) in62 the snapshot's exception store.63 64 discard_passdown_origin - a discard to the snapshot device is passed65 down to the snapshot-origin's underlying device. This doesn't cause66 copy-out to the snapshot exception store because the snapshot-origin67 target is bypassed.68 69 The discard_passdown_origin feature depends on the discard_zeroes_cow70 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 only77works with persistent snapshots. This target assumes the role of the78"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 chunks82stored in the <COW device> of an existing snapshot, through a handover83procedure, and merges these chunks back into the <origin>. Once merging84has started (in the background) the <origin> may be opened and the merge85will continue while I/O is flowing to it. Changes to the <origin> are86deferred until the merging snapshot's corresponding chunk(s) have been87merged. Once merging has started the snapshot device, associated with88the "snapshot" target, will return -EIO when accessed.89 90 91How snapshot is used by LVM292============================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 snapshot98 volume;994) the "original" volume (which uses the device number used by the original100 source volume), whose table is replaced by a "snapshot-origin" mapping101 from device #1.102 103A fixed naming scheme is used, so with the following commands::104 105 lvcreate -L 1G -n base volumeGroup106 lvcreate -L 100M --snapshot -n snap volumeGroup/base107 108we'll have this situation (with volumes in above order)::109 110 # dmsetup table|grep volumeGroup111 112 volumeGroup-base-real: 0 2097152 linear 8:19 384113 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536114 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16115 volumeGroup-base: 0 2097152 snapshot-origin 254:11116 117 # ls -lL /dev/mapper/volumeGroup-*118 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real119 brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow120 brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap121 brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base122 123 124How snapshot-merge is used by LVM2125==================================126A merging snapshot assumes the role of the "snapshot-origin" while127merging. As such the "snapshot-origin" is replaced with128"snapshot-merge". The "-real" device is not changed and the "-cow"129device is renamed to <origin name>-cow to aid LVM2's cleanup of the130merging snapshot after it completes. The "snapshot" that hands over its131COW device to the "snapshot-merge" is deactivated (unless using lvchange132--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/snap137 138we'll now have this situation::139 140 # dmsetup table|grep volumeGroup141 142 volumeGroup-base-real: 0 2097152 linear 8:19 384143 volumeGroup-base-cow: 0 204800 linear 8:19 2097536144 volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16145 146 # ls -lL /dev/mapper/volumeGroup-*147 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real148 brw------- 1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow149 brw------- 1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base150 151 152How to determine when a merging is complete153===========================================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 and160smaller. Merging has finished when the number of sectors holding data161is zero, in other words <sectors_allocated> == <metadata_sectors>.162 163Here is a practical example (using a hybrid of lvm and dmsetup commands)::164 165 # lvs166 LV VG Attr LSize Origin Snap% Move Log Copy% Convert167 base volumeGroup owi-a- 4.00g168 snap volumeGroup swi-a- 1.00g base 18.97169 170 # dmsetup status volumeGroup-snap171 0 8388608 snapshot 397896/2097152 1560172 ^^^^ metadata sectors173 174 # lvconvert --merge -b volumeGroup/snap175 Merging of volume snap started.176 177 # lvs volumeGroup/snap178 LV VG Attr LSize Origin Snap% Move Log Copy% Convert179 base volumeGroup Owi-a- 4.00g 17.23180 181 # dmsetup status volumeGroup-base182 0 8388608 snapshot-merge 281688/2097152 1104183 184 # dmsetup status volumeGroup-base185 0 8388608 snapshot-merge 180480/2097152 712186 187 # dmsetup status volumeGroup-base188 0 8388608 snapshot-merge 16/2097152 16189 190Merging has finished.191 192::193 194 # lvs195 LV VG Attr LSize Origin Snap% Move Log Copy% Convert196 base volumeGroup owi-a- 4.00g197