mdw@git.distorted.org.uk Git - distorted-backup/blob - lvm-rmsnap.8

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   7 .TH lvm-rmsnap 8 "6 November 2011" "distorted.org.uk backup"
   8 .SH NAME
   9 lvm-rmsnap \- remove an LVM snapshot, despite LVM bugs
  10 .SH SYNOPSIS
  11 .B lvm-rmsnap
  12 .RB [ \-dn ]
  13 .IB vgname / lvname
  14 .SH DESCRIPTION
  15 The
  16 .B lvm-rmsnap
  17 tool removes an LVM snapshot volume.  The command
  18 .IP
  19 .BI "lvm-rmsnap " vgname / lvname
  20 .PP
  21 does what you'd hope, in an ideal world,
  22 .IP
  23 .BI "lvremove \-f " vgname / lvname
  24 .PP
  25 would do.  (Yes,
  26 .B lvm-rmsnap
  27 doesn't prompt if the snapshot is active.  There are LVM bugs which
  28 prevent you from deactivating the snapshot, and this tool is intended to
  29 be used in noninteractive scripts.)
  30 .PP
  31 Unfortunately, the world is not ideal, and trying to remove snapshots
  32 using current versions of LVM runs into a collection of nasty race
  33 conditions with
  34 .BR udev (7).
  35 The purpose of
  36 .B lvm-rmsnap
  37 is to remove snapshots despite these bugs.  It acts as a `nanny' for
  38 .BR lvremove ,
  39 carefully tracking all of the toys, and putting them back in the pram.
  40 .SS "Command line"
  41 The following command-line options are accepted.
  42 .TP
  43 .B "\-h, \-\-help"
  44 Print a help message to standard output and exit with status zero.
  45 .TP
  46 .B "\-v, \-\-version"
  47 Print the program's version number to standard output and exit with
  48 status zero.
  49 .TP
  50 .B "\-d, \-\-debug"
  51 Print to standard error information about what the program is doing and
  52 what things it's found out.  This may be useful if you're trying to find
  53 out why
  54 .B lvm-rmsnap
  55 is misbehaving.
  56 .TP
  57 .B "\-n, \-\-noact"
  58 Don't actually perform corrective actions.  This is pretty useless
  59 without
  60 .BR \-d .
  61 You probably don't want to use this unless you're testing
  62 .BR lvm-rmsnap .
  63 .SS "Operation"
  64 The basic problem with
  65 .BR lvremove (8)
  66 is that, partway through, it discovers that some device it was about to
  67 fiddle with is currently in use by some other process \(en usually
  68 invoked by a
  69 .BR udev (7)
  70 rule \(en and then gives up, leaving things in a messed-up state.
  71 There's an elaborate and wobbly synchronization protocol which involves
  72 passing System V semaphore set ids through the kernel and is meant to
  73 make stuff like this not go wrong, but it does anyway.
  74 .PP
  75 The job of
  76 .B lvm-rmsnap
  77 is to remove a snapshot despite these bugs.  Usually, repeating the
  78 .B lvremove
  79 attempt will succeed, though there's often debris of various kinds to be
  80 cleared away.  Here's a list of the things that
  81 .B lvm-rmsnap
  82 tries to do.
  83 .hP \*o
  84 If you're very unlucky, then LVM will leave the snapshot origin volume
  85 suspended, which will cause a subsequent
  86 .B lvremove
  87 attempt to wedge itself (and block any other processes trying to do I/O
  88 to that volume).  So
  89 .B lvm-rmsnap
  90 resumes the volume before retrying.
  91 .hP \*o
  92 A failed
  93 .B lvremove
  94 can leak `cookies', which are really System V semaphore sets.  These use
  95 up kernel memory, and can't be automatically garbage-collected (this is
  96 a well-known mistake in the System V IPC design).  So
  97 .B lvm-rmsnap
  98 keeps track of the cookies used, and releases them if
  99 .B lvmremove
 100 failed to do so.
 101 .hP \*o
 102 Setting up a snapshot involves a little juggle: a new
 103 .RB ` real '
 104 device is created, exactly like the origin volume; and then the origin
 105 volume is changed to be a
 106 .B snapshot-origin
 107 volume pointing at the new device.  When
 108 .B lvremove
 109 fails, the
 110 .RB ` real '
 111 device can be left behind.  So
 112 .B lvm-rmsnap
 113 detects this situation and removes it, after checking that it really
 114 isn't needed for anything.
 115 .SH BUGS
 116 If you know of an LVM snapshot-removal bug which this won't work around
 117 then please let the author know.
 118 .SH SEE ALSO
 119 .BR dmsetup (8),
 120 .BR lvm (8),
 121 .BR lvremove (8),
 122 .BR udev (7).
 123 .SH AUTHOR
 124 Mark Wooding, <mdw@distorted.org.uk>