Documentation / device-mapper / snapshot.txt

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Based on kernel version 4.3. Page generated on 2015-11-02 12:44 EST.

	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.
	
	
	* 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

• [ device-mapper ]
• cache-policies.txt
• cache.txt
• delay.txt
• dm-crypt.txt
• dm-flakey.txt
• dm-io.txt
• dm-log.txt
• dm-queue-length.txt
• dm-raid.txt
• dm-service-time.txt
• dm-uevent.txt
• era.txt
• kcopyd.txt
• linear.txt
• log-writes.txt
• persistent-data.txt
• snapshot.txt
• statistics.txt
• striped.txt
• switch.txt
• thin-provisioning.txt
• verity.txt
• zero.txt
•