tar/doc/tar-snapshot-edit.texi

@c This is part of the paxutils manual.
@c Copyright (C) 2007 Free Software Foundation, Inc.
@c This file is distributed under GFDL 1.1 or any later version
@c published by the Free Software Foundation.

@cindex Device numbers, changing
@cindex snapshot files, editing
@cindex snapshot files, fixing device numbers
  Sometimes device numbers can change after upgrading your kernel
version or recofiguring the harvare.  Reportedly this is the case with 
some newer @i{Linux} kernels, when using @acronym{LVM}.  In majority of
cases this change is unnoticed by the users.  However, it influences
@command{tar} incremental backups: the device number is stored in tar
snapshot files (@pxref{Snapshot Files}) and is used to determine whether
the file has changed since the last backup.  If the device numbers
change for some reason, the next backup you run will be a full backup.

@pindex tar-snapshot-edit
  To minimize the impact in these cases, GNU @command{tar} comes with
the @command{tar-snapshot-edit} utility for inspecting and updating
device numbers in snapshot files.  The utility, written by
Dustin J.@: Mitchell, is available from
@uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tar-snapshot-edit.html,
@GNUTAR{} home page}. 

  To obtain the device numbers used in the snapshot file, run 

@smallexample
$ @kbd{tar-snapshot-edit @var{snapfile}}
@end smallexample

@noindent
where @var{snapfile} is the name of the snapshot file (you can supply as many
files as you wish in a single command line ).

To update all occurrences of the given device number in the file, use
@option{-r} option.  It takes a single argument of the form
@samp{@var{olddev}-@var{newdev}},  where @var{olddev} is the device number
used in the snapshot file, and @var{newdev} is the corresponding new device
number.  Both numbers may be specified in hex (e.g., @samp{0xfe01}),
decimal (e.g., @samp{65025}), or as a major:minor number pair (e.g.,
@samp{254:1}).  To change several device numbers at once, specify them
in a single comma-separated list, as in
@option{-r 0x3060-0x4500,0x307-0x4600}.

Before updating the snapshot file, it is a good idea to create a backup
copy of it.  This is accomplished by @samp{-b} option.  The name of the
backup file is obtained by appending @samp{~} to the original file name.

An example session:
@smallexample
$ @kbd{tar-snapshot-edit /var/backup/snap.a}
file version 2
/tmp/snap: Device 0x0306 occurs 634 times.
$ @kbd{tar-snapshot-edit -b -r 0x0306-0x4500 /var/backup/snap.a}
file version 2
@end smallexample