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1 | .TH snap.lvm 8 "6 November 2011" "distorted.org.uk backup" |
2 | .SH NAME | |
3 | snap.lvm \- snapshot handler for LVM logical volumes | |
4 | .SH SYNOPSIS | |
5 | .B snap.lvm | |
6 | .IB vg-name / lv-name | |
7 | .IB key = value | |
8 | \&... | |
9 | .SH DESCRIPTION | |
10 | This is a snapshot handler for LVM volumes: see | |
11 | .BR snap (8) | |
12 | for more information about how snapshot handlers fit into the overall | |
13 | system. It creates or removes a snapshot of the logical volume named | |
14 | .I lv-name | |
15 | on the volume group | |
16 | .IR vg-name . | |
17 | .PP | |
18 | The following options keys are recognized, either with a | |
19 | .RB ` .lvm ' | |
20 | suffix or without. Other keys are reported as errors, unless the key | |
21 | contains a | |
22 | .RB ` . ' | |
23 | character. | |
24 | .TP | |
25 | .BI op= op | |
26 | The | |
27 | .I op | |
28 | must be either | |
29 | .B snap | |
30 | (the default) to create a snapshot, or | |
31 | .B unsnap | |
32 | to remove it. | |
33 | .TP | |
34 | .BI tag= tag | |
35 | The snapshot volume will be named | |
36 | .IB lv-name . tag \fR. | |
37 | The default tag is | |
38 | .BR snap . | |
39 | It is recommended that scripts always provide a tag identifying their | |
40 | purpose. | |
41 | .TP | |
42 | .BI snapsz= sz | |
43 | The size to allocate for the snapshot's backing store. This may be an | |
44 | absolute size followed by an optional unit suffix | |
45 | .RB ` K ', | |
46 | .RB ` M ', | |
47 | .RB ` T ', | |
48 | .RB ` P ', | |
49 | or | |
50 | .RB ` E ' | |
51 | for kilobytes, megabytes (the default), terabytes, petabytes, or | |
52 | exabytes, respectively; or a relative size of the form | |
53 | .IB n % what \fR, | |
54 | where | |
55 | .I what | |
56 | is | |
57 | .BR VG , | |
58 | .BR FREE , | |
59 | or | |
60 | .BR ORIGIN , | |
61 | to request | |
62 | .IR n % | |
63 | of the total space in the volume group, the free space remaining in the | |
64 | volume group, or the size of the origin volume, respectively. (The | |
65 | .I what | |
66 | suffix may be abbreviated to just the first letter; the suffixes are not | |
67 | case-sensitive in either case.) | |
68 | .SH BUGS | |
69 | The LVM tools are rather buggy, and exhibit nasty races with | |
70 | .B udev (8). | |
71 | The | |
72 | .BR lvremove (8) | |
73 | tool is particularly awful. | |
74 | The | |
75 | .B snap.lvm | |
76 | handler uses | |
77 | .BR lvm-rmsnap (8) | |
78 | to work around these bugs. | |
79 | .PP | |
80 | If you make a snapshot of a logical volume while a filesystem is | |
81 | directly mounted from the logical volume on the same machine, and the | |
82 | filesystem kernel driver implements the | |
83 | .B freeze_fs | |
84 | and | |
85 | .B unfreeze_fs | |
86 | superblock operations, then the kernel will arrange for the snapshot to | |
87 | contain a consistent and clean snapshot of the filesystem \(en in | |
88 | particular, it shouldn't need the ministrations of | |
89 | .BR fsck (8). | |
90 | If the filesystem is mounted by a different machine, e.g., a guest | |
91 | running on the same host, or via a network block-device access protocol, | |
92 | you will need to negotiate with the remote machine in order to obtain a | |
93 | clean snapshot. See | |
94 | .BR snap.rfreezefs (8) | |
95 | for a snapshot handler which copes with this, and | |
96 | .BR rfreezefs (8) | |
97 | for the actual machinery. | |
98 | .SH SEE ALSO | |
99 | .BR snap (8), | |
100 | .BR lvm (8), | |
101 | .BR lvm-rmsnap (8), | |
102 | .BR rfreezefs (8). | |
103 | .SH AUTHOR | |
104 | Mark Wooding, <mdw@distorted.org.uk> |