@c This is part of the paxutils manual.
@c Copyright (C) 2007, 2014 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
  Various situations can cause device numbers to change: upgrading your
kernel version, reconfiguring your hardware, loading kernel modules in a
different order, using virtual volumes that are assembled dynamically
(such as with @acronym{LVM} or @acronym{RAID}), hot-plugging drives
(e.g. external USB or Firewire drives), etc.  In the 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, by default 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 also available from the
@uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tar-snapshot-edit.html,
@GNUTAR{} home page}.)

  To obtain a summary of the device numbers found 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).  You can then compare the 
numbers across snapshot files, or against those currently in use on the
live filesystem (using @command{ls -l} or @command{stat}).

  Assuming the device numbers have indeed changed, it's often possible
to simply tell @GNUTAR{} to ignore the device number when processing the
incremental snapshot files for these backups, using the
@option{--no-check-device} option (@pxref{device numbers}).

  Alternatively, you can use the @command{tar-edit-snapshot} script's 
@option{-r} option to update all occurrences of the given device
number in the snapshot file(s).  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 root_snap.0 boot_snap.0}
File: root_snap.0
  Detected snapshot file version: 2

  Device 0x0000 occurs 1 times.
  Device 0x0003 occurs 1 times.
  Device 0x0005 occurs 1 times.
  Device 0x0013 occurs 1 times.
  Device 0x6801 occurs 1 times.
  Device 0x6803 occurs 6626 times.
  Device 0xfb00 occurs 1 times.

File: boot_snap.0
  Detected snapshot file version: 2

  Device 0x6801 occurs 3 times.
$ @kbd{tar-snapshot-edit -b -r 0x6801-0x6901,0x6803-0x6903 root_snap.0 boot_snap.0}
File: root_snap.0
  Detected snapshot file version: 2

  Updated 6627 records.

File: boot_snap.0
  Detected snapshot file version: 2

  Updated 3 records.
@end smallexample
