Documentation/device-mapper/snapshot.txt

   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
  53 * snapshot-merge <origin> <COW device> <persistent> <chunksize>
  54
  55 takes the same table arguments as the snapshot target except it only
  56 works with persistent snapshots.  This target assumes the role of the
  57 "snapshot-origin" target and must not be loaded if the "snapshot-origin"
  58 is still present for <origin>.
  59
  60 Creates a merging snapshot that takes control of the changed chunks
  61 stored in the <COW device> of an existing snapshot, through a handover
  62 procedure, and merges these chunks back into the <origin>.  Once merging
  63 has started (in the background) the <origin> may be opened and the merge
  64 will continue while I/O is flowing to it.  Changes to the <origin> are
  65 deferred until the merging snapshot's corresponding chunk(s) have been
  66 merged.  Once merging has started the snapshot device, associated with
  67 the "snapshot" target, will return -EIO when accessed.
  68
  69
  70 How snapshot is used by LVM2
  71 ============================
  72 When you create the first LVM2 snapshot of a volume, four dm devices are used:
  73
  74 1) a device containing the original mapping table of the source volume;
  75 2) a device used as the <COW device>;
  76 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
  77    volume;
  78 4) the "original" volume (which uses the device number used by the original
  79    source volume), whose table is replaced by a "snapshot-origin" mapping
  80    from device #1.
  81
  82 A fixed naming scheme is used, so with the following commands:
  83
  84 lvcreate -L 1G -n base volumeGroup
  85 lvcreate -L 100M --snapshot -n snap volumeGroup/base
  86
  87 we'll have this situation (with volumes in above order):
  88
  89 # dmsetup table|grep volumeGroup
  90
  91 volumeGroup-base-real: 0 2097152 linear 8:19 384
  92 volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
  93 volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
  94 volumeGroup-base: 0 2097152 snapshot-origin 254:11
  95
  96 # ls -lL /dev/mapper/volumeGroup-*
  97 brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
  98 brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
  99 brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
 100 brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
 101
 102
 103 How snapshot-merge is used by LVM2
 104 ==================================
 105 A merging snapshot assumes the role of the "snapshot-origin" while
 106 merging.  As such the "snapshot-origin" is replaced with
 107 "snapshot-merge".  The "-real" device is not changed and the "-cow"
 108 device is renamed to <origin name>-cow to aid LVM2's cleanup of the
 109 merging snapshot after it completes.  The "snapshot" that hands over its
 110 COW device to the "snapshot-merge" is deactivated (unless using lvchange
 111 --refresh); but if it is left active it will simply return I/O errors.
 112
 113 A snapshot will merge into its origin with the following command:
 114
 115 lvconvert --merge volumeGroup/snap
 116
 117 we'll now have this situation:
 118
 119 # dmsetup table|grep volumeGroup
 120
 121 volumeGroup-base-real: 0 2097152 linear 8:19 384
 122 volumeGroup-base-cow: 0 204800 linear 8:19 2097536
 123 volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
 124
 125 # ls -lL /dev/mapper/volumeGroup-*
 126 brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
 127 brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
 128 brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
 129
 130
 131 How to determine when a merging is complete
 132 ===========================================
 133 The snapshot-merge and snapshot status lines end with:
 134   <sectors_allocated>/<total_sectors> <metadata_sectors>
 135
 136 Both <sectors_allocated> and <total_sectors> include both data and metadata.
 137 During merging, the number of sectors allocated gets smaller and
 138 smaller.  Merging has finished when the number of sectors holding data
 139 is zero, in other words <sectors_allocated> == <metadata_sectors>.
 140
 141 Here is a practical example (using a hybrid of lvm and dmsetup commands):
 142
 143 # lvs
 144   LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
 145   base    volumeGroup owi-a- 4.00g
 146   snap    volumeGroup swi-a- 1.00g base  18.97
 147
 148 # dmsetup status volumeGroup-snap
 149 0 8388608 snapshot 397896/2097152 1560
 150                                   ^^^^ metadata sectors
 151
 152 # lvconvert --merge -b volumeGroup/snap
 153   Merging of volume snap started.
 154
 155 # lvs volumeGroup/snap
 156   LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
 157   base    volumeGroup Owi-a- 4.00g          17.23
 158
 159 # dmsetup status volumeGroup-base
 160 0 8388608 snapshot-merge 281688/2097152 1104
 161
 162 # dmsetup status volumeGroup-base
 163 0 8388608 snapshot-merge 180480/2097152 712
 164
 165 # dmsetup status volumeGroup-base
 166 0 8388608 snapshot-merge 16/2097152 16
 167
 168 Merging has finished.
 169
 170 # lvs
 171   LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
 172   base    volumeGroup owi-a- 4.00g