| Device-mapper snapshot support |
| ============================== |
| |
| Device-mapper allows you, without massive data copying: |
| |
| *) To create snapshots of any block device i.e. mountable, saved states of |
| the block device which are also writable without interfering with the |
| original content; |
| *) To create device "forks", i.e. multiple different versions of the |
| same data stream. |
| *) To merge a snapshot of a block device back into the snapshot's origin |
| device. |
| |
| In the first two cases, dm copies only the chunks of data that get |
| changed and uses a separate copy-on-write (COW) block device for |
| storage. |
| |
| For snapshot merge the contents of the COW storage are merged back into |
| the origin device. |
| |
| |
| There are three dm targets available: |
| snapshot, snapshot-origin, and snapshot-merge. |
| |
| *) snapshot-origin <origin> |
| |
| which will normally have one or more snapshots based on it. |
| Reads will be mapped directly to the backing device. For each write, the |
| original data will be saved in the <COW device> of each snapshot to keep |
| its visible content unchanged, at least until the <COW device> fills up. |
| |
| |
| *) snapshot <origin> <COW device> <persistent?> <chunksize> |
| |
| A snapshot of the <origin> block device is created. Changed chunks of |
| <chunksize> sectors will be stored on the <COW device>. Writes will |
| only go to the <COW device>. Reads will come from the <COW device> or |
| from <origin> for unchanged data. <COW device> will often be |
| smaller than the origin and if it fills up the snapshot will become |
| useless and be disabled, returning errors. So it is important to monitor |
| the amount of free space and expand the <COW device> before it fills up. |
| |
| <persistent?> is P (Persistent) or N (Not persistent - will not survive |
| after reboot). O (Overflow) can be added as a persistent store option |
| to allow userspace to advertise its support for seeing "Overflow" in the |
| snapshot status. So supported store types are "P", "PO" and "N". |
| |
| The difference between persistent and transient is with transient |
| snapshots less metadata must be saved on disk - they can be kept in |
| memory by the kernel. |
| |
| When loading or unloading the snapshot target, the corresponding |
| snapshot-origin or snapshot-merge target must be suspended. A failure to |
| suspend the origin target could result in data corruption. |
| |
| |
| * snapshot-merge <origin> <COW device> <persistent> <chunksize> |
| |
| takes the same table arguments as the snapshot target except it only |
| works with persistent snapshots. This target assumes the role of the |
| "snapshot-origin" target and must not be loaded if the "snapshot-origin" |
| is still present for <origin>. |
| |
| Creates a merging snapshot that takes control of the changed chunks |
| stored in the <COW device> of an existing snapshot, through a handover |
| procedure, and merges these chunks back into the <origin>. Once merging |
| has started (in the background) the <origin> may be opened and the merge |
| will continue while I/O is flowing to it. Changes to the <origin> are |
| deferred until the merging snapshot's corresponding chunk(s) have been |
| merged. Once merging has started the snapshot device, associated with |
| the "snapshot" target, will return -EIO when accessed. |
| |
| |
| How snapshot is used by LVM2 |
| ============================ |
| When you create the first LVM2 snapshot of a volume, four dm devices are used: |
| |
| 1) a device containing the original mapping table of the source volume; |
| 2) a device used as the <COW device>; |
| 3) a "snapshot" device, combining #1 and #2, which is the visible snapshot |
| volume; |
| 4) the "original" volume (which uses the device number used by the original |
| source volume), whose table is replaced by a "snapshot-origin" mapping |
| from device #1. |
| |
| A fixed naming scheme is used, so with the following commands: |
| |
| lvcreate -L 1G -n base volumeGroup |
| lvcreate -L 100M --snapshot -n snap volumeGroup/base |
| |
| we'll have this situation (with volumes in above order): |
| |
| # dmsetup table|grep volumeGroup |
| |
| volumeGroup-base-real: 0 2097152 linear 8:19 384 |
| volumeGroup-snap-cow: 0 204800 linear 8:19 2097536 |
| volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16 |
| volumeGroup-base: 0 2097152 snapshot-origin 254:11 |
| |
| # ls -lL /dev/mapper/volumeGroup-* |
| brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real |
| brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow |
| brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap |
| brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base |
| |
| |
| How snapshot-merge is used by LVM2 |
| ================================== |
| A merging snapshot assumes the role of the "snapshot-origin" while |
| merging. As such the "snapshot-origin" is replaced with |
| "snapshot-merge". The "-real" device is not changed and the "-cow" |
| device is renamed to <origin name>-cow to aid LVM2's cleanup of the |
| merging snapshot after it completes. The "snapshot" that hands over its |
| COW device to the "snapshot-merge" is deactivated (unless using lvchange |
| --refresh); but if it is left active it will simply return I/O errors. |
| |
| A snapshot will merge into its origin with the following command: |
| |
| lvconvert --merge volumeGroup/snap |
| |
| we'll now have this situation: |
| |
| # dmsetup table|grep volumeGroup |
| |
| volumeGroup-base-real: 0 2097152 linear 8:19 384 |
| volumeGroup-base-cow: 0 204800 linear 8:19 2097536 |
| volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16 |
| |
| # ls -lL /dev/mapper/volumeGroup-* |
| brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real |
| brw------- 1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow |
| brw------- 1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base |
| |
| |
| How to determine when a merging is complete |
| =========================================== |
| The snapshot-merge and snapshot status lines end with: |
| <sectors_allocated>/<total_sectors> <metadata_sectors> |
| |
| Both <sectors_allocated> and <total_sectors> include both data and metadata. |
| During merging, the number of sectors allocated gets smaller and |
| smaller. Merging has finished when the number of sectors holding data |
| is zero, in other words <sectors_allocated> == <metadata_sectors>. |
| |
| Here is a practical example (using a hybrid of lvm and dmsetup commands): |
| |
| # lvs |
| LV VG Attr LSize Origin Snap% Move Log Copy% Convert |
| base volumeGroup owi-a- 4.00g |
| snap volumeGroup swi-a- 1.00g base 18.97 |
| |
| # dmsetup status volumeGroup-snap |
| 0 8388608 snapshot 397896/2097152 1560 |
| ^^^^ metadata sectors |
| |
| # lvconvert --merge -b volumeGroup/snap |
| Merging of volume snap started. |
| |
| # lvs volumeGroup/snap |
| LV VG Attr LSize Origin Snap% Move Log Copy% Convert |
| base volumeGroup Owi-a- 4.00g 17.23 |
| |
| # dmsetup status volumeGroup-base |
| 0 8388608 snapshot-merge 281688/2097152 1104 |
| |
| # dmsetup status volumeGroup-base |
| 0 8388608 snapshot-merge 180480/2097152 712 |
| |
| # dmsetup status volumeGroup-base |
| 0 8388608 snapshot-merge 16/2097152 16 |
| |
| Merging has finished. |
| |
| # lvs |
| LV VG Attr LSize Origin Snap% Move Log Copy% Convert |
| base volumeGroup owi-a- 4.00g |
