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Sometimes device numbers can change after upgrading your kernel
version or reconfiguring the hardware. Reportedly this is the case with
some newer Linux kernels, when using LVM. In majority of
cases this change is unnoticed by the users. However, it influences
tar incremental backups: the device number is stored in tar
snapshot files (see section Format of the Incremental 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.
To minimize the impact in these cases, GNU
tar comes with
tar-snapshot-edit utility for inspecting and updating
device numbers in snapshot files. The utility, written by
Dustin J. Mitchell, is available from
tar home page.
To obtain the device numbers used in the snapshot file, run
$ tar-snapshot-edit snapfile
where 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 ‘-r’ option. It takes a single argument of the form ‘olddev-newdev’, where olddev is the device number used in the snapshot file, and newdev is the corresponding new device number. Both numbers may be specified in hex (e.g., ‘0xfe01’), decimal (e.g., ‘65025’), or as a major:minor number pair (e.g., ‘254:1’). To change several device numbers at once, specify them in a single comma-separated list, as in ‘-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 ‘-b’ option. The name of the backup file is obtained by appending ‘~’ to the original file name.
An example session:
$ tar-snapshot-edit /var/backup/snap.a file version 2 /tmp/snap: Device 0x0306 occurs 634 times. $ tar-snapshot-edit -b -r 0x0306-0x4500 /var/backup/snap.a file version 2
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