About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / device-mapper / snapshot.txt




Custom Search

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

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
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.