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