linux/Documentation/device-mapper/snapshot.txt
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   1Device-mapper snapshot support
   2==============================
   3
   4Device-mapper allows you, without massive data copying:
   5
   6*) To create snapshots of any block device i.e. mountable, saved states of
   7the block device which are also writable without interfering with the
   8original content;
   9*) To create device "forks", i.e. multiple different versions of the
  10same data stream.
  11*) To merge a snapshot of a block device back into the snapshot's origin
  12device.
  13
  14In the first two cases, dm copies only the chunks of data that get
  15changed and uses a separate copy-on-write (COW) block device for
  16storage.
  17
  18For snapshot merge the contents of the COW storage are merged back into
  19the origin device.
  20
  21
  22There are three dm targets available:
  23snapshot, snapshot-origin, and snapshot-merge.
  24
  25*) snapshot-origin <origin>
  26
  27which will normally have one or more snapshots based on it.
  28Reads will be mapped directly to the backing device. For each write, the
  29original data will be saved in the <COW device> of each snapshot to keep
  30its visible content unchanged, at least until the <COW device> fills up.
  31
  32
  33*) snapshot <origin> <COW device> <persistent?> <chunksize>
  34
  35A snapshot of the <origin> block device is created. Changed chunks of
  36<chunksize> sectors will be stored on the <COW device>.  Writes will
  37only go to the <COW device>.  Reads will come from the <COW device> or
  38from <origin> for unchanged data.  <COW device> will often be
  39smaller than the origin and if it fills up the snapshot will become
  40useless and be disabled, returning errors.  So it is important to monitor
  41the 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
  44after reboot).
  45The difference is that for transient snapshots less metadata must be
  46saved on disk - they can be kept in memory by the kernel.
  47
  48
  49* snapshot-merge <origin> <COW device> <persistent> <chunksize>
  50
  51takes the same table arguments as the snapshot target except it only
  52works with persistent snapshots.  This target assumes the role of the
  53"snapshot-origin" target and must not be loaded if the "snapshot-origin"
  54is still present for <origin>.
  55
  56Creates a merging snapshot that takes control of the changed chunks
  57stored in the <COW device> of an existing snapshot, through a handover
  58procedure, and merges these chunks back into the <origin>.  Once merging
  59has started (in the background) the <origin> may be opened and the merge
  60will continue while I/O is flowing to it.  Changes to the <origin> are
  61deferred until the merging snapshot's corresponding chunk(s) have been
  62merged.  Once merging has started the snapshot device, associated with
  63the "snapshot" target, will return -EIO when accessed.
  64
  65
  66How snapshot is used by LVM2
  67============================
  68When you create the first LVM2 snapshot of a volume, four dm devices are used:
  69
  701) a device containing the original mapping table of the source volume;
  712) a device used as the <COW device>;
  723) a "snapshot" device, combining #1 and #2, which is the visible snapshot
  73   volume;
  744) the "original" volume (which uses the device number used by the original
  75   source volume), whose table is replaced by a "snapshot-origin" mapping
  76   from device #1.
  77
  78A fixed naming scheme is used, so with the following commands:
  79
  80lvcreate -L 1G -n base volumeGroup
  81lvcreate -L 100M --snapshot -n snap volumeGroup/base
  82
  83we'll have this situation (with volumes in above order):
  84
  85# dmsetup table|grep volumeGroup
  86
  87volumeGroup-base-real: 0 2097152 linear 8:19 384
  88volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
  89volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
  90volumeGroup-base: 0 2097152 snapshot-origin 254:11
  91
  92# ls -lL /dev/mapper/volumeGroup-*
  93brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
  94brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
  95brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
  96brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
  97
  98
  99How snapshot-merge is used by LVM2
 100==================================
 101A merging snapshot assumes the role of the "snapshot-origin" while
 102merging.  As such the "snapshot-origin" is replaced with
 103"snapshot-merge".  The "-real" device is not changed and the "-cow"
 104device is renamed to <origin name>-cow to aid LVM2's cleanup of the
 105merging snapshot after it completes.  The "snapshot" that hands over its
 106COW device to the "snapshot-merge" is deactivated (unless using lvchange
 107--refresh); but if it is left active it will simply return I/O errors.
 108
 109A snapshot will merge into its origin with the following command:
 110
 111lvconvert --merge volumeGroup/snap
 112
 113we'll now have this situation:
 114
 115# dmsetup table|grep volumeGroup
 116
 117volumeGroup-base-real: 0 2097152 linear 8:19 384
 118volumeGroup-base-cow: 0 204800 linear 8:19 2097536
 119volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
 120
 121# ls -lL /dev/mapper/volumeGroup-*
 122brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
 123brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
 124brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
 125
 126
 127How to determine when a merging is complete
 128===========================================
 129The snapshot-merge and snapshot status lines end with:
 130  <sectors_allocated>/<total_sectors> <metadata_sectors>
 131
 132Both <sectors_allocated> and <total_sectors> include both data and metadata.
 133During merging, the number of sectors allocated gets smaller and
 134smaller.  Merging has finished when the number of sectors holding data
 135is zero, in other words <sectors_allocated> == <metadata_sectors>.
 136
 137Here is a practical example (using a hybrid of lvm and dmsetup commands):
 138
 139# lvs
 140  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
 141  base    volumeGroup owi-a- 4.00g
 142  snap    volumeGroup swi-a- 1.00g base  18.97
 143
 144# dmsetup status volumeGroup-snap
 1450 8388608 snapshot 397896/2097152 1560
 146                                  ^^^^ metadata sectors
 147
 148# lvconvert --merge -b volumeGroup/snap
 149  Merging of volume snap started.
 150
 151# lvs volumeGroup/snap
 152  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
 153  base    volumeGroup Owi-a- 4.00g          17.23
 154
 155# dmsetup status volumeGroup-base
 1560 8388608 snapshot-merge 281688/2097152 1104
 157
 158# dmsetup status volumeGroup-base
 1590 8388608 snapshot-merge 180480/2097152 712
 160
 161# dmsetup status volumeGroup-base
 1620 8388608 snapshot-merge 16/2097152 16
 163
 164Merging has finished.
 165
 166# lvs
 167  LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
 168  base    volumeGroup owi-a- 4.00g
 169
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