Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 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 *) To merge a snapshot of a block device back into the snapshot's origin 12 device. 13 14 In the first two cases, dm copies only the chunks of data that get 15 changed and uses a separate copy-on-write (COW) block device for 16 storage. 17 18 For snapshot merge the contents of the COW storage are merged back into 19 the origin device. 20 21 22 There are three dm targets available: 23 snapshot, snapshot-origin, and snapshot-merge. 24 25 *) snapshot-origin <origin> 26 27 which will normally have one or more snapshots based on it. 28 Reads will be mapped directly to the backing device. For each write, the 29 original data will be saved in the <COW device> of each snapshot to keep 30 its visible content unchanged, at least until the <COW device> fills up. 31 32 33 *) snapshot <origin> <COW device> <persistent?> <chunksize> 34 35 A snapshot of the <origin> block device is created. Changed chunks of 36 <chunksize> sectors will be stored on the <COW device>. Writes will 37 only go to the <COW device>. Reads will come from the <COW device> or 38 from <origin> for unchanged data. <COW device> will often be 39 smaller than the origin and if it fills up the snapshot will become 40 useless and be disabled, returning errors. So it is important to monitor 41 the amount of free space and expand the <COW device> before it fills up. 42 43 <persistent?> is P (Persistent) or N (Not persistent - will not survive 44 after reboot). O (Overflow) can be added as a persistent store option 45 to allow userspace to advertise its support for seeing "Overflow" in the 46 snapshot status. So supported store types are "P", "PO" and "N". 47 48 The difference between persistent and transient is with transient 49 snapshots less metadata must be saved on disk - they can be kept in 50 memory by the kernel. 51 52 When loading or unloading the snapshot target, the corresponding 53 snapshot-origin or snapshot-merge target must be suspended. A failure to 54 suspend the origin target could result in data corruption. 55 56 57 * snapshot-merge <origin> <COW device> <persistent> <chunksize> 58 59 takes the same table arguments as the snapshot target except it only 60 works with persistent snapshots. This target assumes the role of the 61 "snapshot-origin" target and must not be loaded if the "snapshot-origin" 62 is still present for <origin>. 63 64 Creates a merging snapshot that takes control of the changed chunks 65 stored in the <COW device> of an existing snapshot, through a handover 66 procedure, and merges these chunks back into the <origin>. Once merging 67 has started (in the background) the <origin> may be opened and the merge 68 will continue while I/O is flowing to it. Changes to the <origin> are 69 deferred until the merging snapshot's corresponding chunk(s) have been 70 merged. Once merging has started the snapshot device, associated with 71 the "snapshot" target, will return -EIO when accessed. 72 73 74 How snapshot is used by LVM2 75 ============================ 76 When you create the first LVM2 snapshot of a volume, four dm devices are used: 77 78 1) a device containing the original mapping table of the source volume; 79 2) a device used as the <COW device>; 80 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot 81 volume; 82 4) the "original" volume (which uses the device number used by the original 83 source volume), whose table is replaced by a "snapshot-origin" mapping 84 from device #1. 85 86 A fixed naming scheme is used, so with the following commands: 87 88 lvcreate -L 1G -n base volumeGroup 89 lvcreate -L 100M --snapshot -n snap volumeGroup/base 90 91 we'll have this situation (with volumes in above order): 92 93 # dmsetup table|grep volumeGroup 94 95 volumeGroup-base-real: 0 2097152 linear 8:19 384 96 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536 97 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16 98 volumeGroup-base: 0 2097152 snapshot-origin 254:11 99 100 # ls -lL /dev/mapper/volumeGroup-* 101 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real 102 brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow 103 brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap 104 brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base 105 106 107 How snapshot-merge is used by LVM2 108 ================================== 109 A merging snapshot assumes the role of the "snapshot-origin" while 110 merging. As such the "snapshot-origin" is replaced with 111 "snapshot-merge". The "-real" device is not changed and the "-cow" 112 device is renamed to <origin name>-cow to aid LVM2's cleanup of the 113 merging snapshot after it completes. The "snapshot" that hands over its 114 COW device to the "snapshot-merge" is deactivated (unless using lvchange 115 --refresh); but if it is left active it will simply return I/O errors. 116 117 A snapshot will merge into its origin with the following command: 118 119 lvconvert --merge volumeGroup/snap 120 121 we'll now have this situation: 122 123 # dmsetup table|grep volumeGroup 124 125 volumeGroup-base-real: 0 2097152 linear 8:19 384 126 volumeGroup-base-cow: 0 204800 linear 8:19 2097536 127 volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16 128 129 # ls -lL /dev/mapper/volumeGroup-* 130 brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real 131 brw------- 1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow 132 brw------- 1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base 133 134 135 How to determine when a merging is complete 136 =========================================== 137 The snapshot-merge and snapshot status lines end with: 138 <sectors_allocated>/<total_sectors> <metadata_sectors> 139 140 Both <sectors_allocated> and <total_sectors> include both data and metadata. 141 During merging, the number of sectors allocated gets smaller and 142 smaller. Merging has finished when the number of sectors holding data 143 is zero, in other words <sectors_allocated> == <metadata_sectors>. 144 145 Here is a practical example (using a hybrid of lvm and dmsetup commands): 146 147 # lvs 148 LV VG Attr LSize Origin Snap% Move Log Copy% Convert 149 base volumeGroup owi-a- 4.00g 150 snap volumeGroup swi-a- 1.00g base 18.97 151 152 # dmsetup status volumeGroup-snap 153 0 8388608 snapshot 397896/2097152 1560 154 ^^^^ metadata sectors 155 156 # lvconvert --merge -b volumeGroup/snap 157 Merging of volume snap started. 158 159 # lvs volumeGroup/snap 160 LV VG Attr LSize Origin Snap% Move Log Copy% Convert 161 base volumeGroup Owi-a- 4.00g 17.23 162 163 # dmsetup status volumeGroup-base 164 0 8388608 snapshot-merge 281688/2097152 1104 165 166 # dmsetup status volumeGroup-base 167 0 8388608 snapshot-merge 180480/2097152 712 168 169 # dmsetup status volumeGroup-base 170 0 8388608 snapshot-merge 16/2097152 16 171 172 Merging has finished. 173 174 # lvs 175 LV VG Attr LSize Origin Snap% Move Log Copy% Convert 176 base volumeGroup owi-a- 4.00g