92 lines
3.6 KiB
Plaintext
92 lines
3.6 KiB
Plaintext
@c This is part of the paxutils manual.
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@c Copyright (C) 2007-2019 Free Software Foundation, Inc.
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@c This file is distributed under GFDL 1.1 or any later version
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@c published by the Free Software Foundation.
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@cindex Device numbers, changing
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@cindex snapshot files, editing
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@cindex snapshot files, fixing device numbers
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Various situations can cause device numbers to change: upgrading your
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kernel version, reconfiguring your hardware, loading kernel modules in a
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different order, using virtual volumes that are assembled dynamically
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(such as with @acronym{LVM} or @acronym{RAID}), hot-plugging drives
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(e.g. external USB or Firewire drives), etc. In the majority of
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cases this change is unnoticed by the users. However, it influences
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@command{tar} incremental backups: the device number is stored in tar
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snapshot files (@pxref{Snapshot Files}) and is used to determine whether
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the file has changed since the last backup. If the device numbers
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change for some reason, by default the next backup you run will be a
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full backup.
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@pindex tar-snapshot-edit
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To minimize the impact in these cases, GNU @command{tar} comes with
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the @command{tar-snapshot-edit} utility for inspecting and updating
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device numbers in snapshot files. (The utility, written by
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Dustin J.@: Mitchell, is also available from the
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@uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tar-snapshot-edit.html,
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@GNUTAR{} home page}.)
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To obtain a summary of the device numbers found in the snapshot file, run
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@smallexample
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$ @kbd{tar-snapshot-edit @var{snapfile}}
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@end smallexample
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@noindent
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where @var{snapfile} is the name of the snapshot file (you can supply as many
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files as you wish in a single command line). You can then compare the
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numbers across snapshot files, or against those currently in use on the
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live filesystem (using @command{ls -l} or @command{stat}).
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Assuming the device numbers have indeed changed, it's often possible
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to simply tell @GNUTAR{} to ignore the device number when processing the
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incremental snapshot files for these backups, using the
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@option{--no-check-device} option (@pxref{device numbers}).
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Alternatively, you can use the @command{tar-edit-snapshot} script's
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@option{-r} option to update all occurrences of the given device
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number in the snapshot file(s). It takes a single argument
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of the form
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@samp{@var{olddev}-@var{newdev}}, where @var{olddev} is the device number
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used in the snapshot file, and @var{newdev} is the corresponding new device
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number. Both numbers may be specified in hex (e.g., @samp{0xfe01}),
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decimal (e.g., @samp{65025}), or as a major:minor number pair (e.g.,
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@samp{254:1}). To change several device numbers at once, specify them
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in a single comma-separated list, as in
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@option{-r 0x3060-0x4500,0x307-0x4600}.
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Before updating the snapshot file, it is a good idea to create a backup
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copy of it. This is accomplished by @samp{-b} option. The name of the
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backup file is obtained by appending @samp{~} to the original file name.
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An example session:
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@smallexample
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$ @kbd{tar-snapshot-edit root_snap.0 boot_snap.0}
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File: root_snap.0
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Detected snapshot file version: 2
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Device 0x0000 occurs 1 times.
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Device 0x0003 occurs 1 times.
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Device 0x0005 occurs 1 times.
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Device 0x0013 occurs 1 times.
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Device 0x6801 occurs 1 times.
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Device 0x6803 occurs 6626 times.
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Device 0xfb00 occurs 1 times.
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File: boot_snap.0
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Detected snapshot file version: 2
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Device 0x6801 occurs 3 times.
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$ @kbd{tar-snapshot-edit -b -r 0x6801-0x6901,0x6803-0x6903 root_snap.0 boot_snap.0}
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File: root_snap.0
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Detected snapshot file version: 2
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Updated 6627 records.
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File: boot_snap.0
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Detected snapshot file version: 2
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Updated 3 records.
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@end smallexample
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