SUMMARY: Backup while in multi-user mode

[Joern Baier]

Two weeks ago I posted an article concerning the problems which can arise
doing a dump in multi-user mode.

Nearly everyone who answered pointed out that at his site the backups are
run while in multi-user mode  but nobody has already observed a serious error
as a result of this policy.

Problems may occur when a file or a directory has been deleted between two
passes of the dump.

Peter Renzland <peter at ontmoh.uucp> explained this in great detail so I will
cite him here in full length:
>...
>One thing that can happen is that a file is deleted, and the space is
>re-allocted to a new file between the time the Inode for the old file
>is written to tape and the data for what used to be the old file is written.
>This data now belongs to another file.  This is no problem until you try
>to restore.  Now the old inode points to data blocks which not only contain
>data that could be none of the business of the owner of the old file, but
>there is now a file-system inconsistency, because two files (inodes) now
>point to the same data, and the old one could have had indirect blocks,
>which may result in filesystem corruption that extends beyond just the old
>file, and the (few) new file(s) that were created during the backup window
>of vulnerability.
>
>All this is possible because dump bypasses the filesystem, for added
>speed (and diminished integrity).
>...

According to Alain Brossard (brossard at sasun1.epfl.ch) it is also possible
that the entire dump will become unreadable if the freed inode had been
a directory and is now a file (or vice versa?) but this seems to be very
unlikely.

Thanks to all who answered.

Joern.
--
Joern Baier     (baier at unipas.fmi.uni-passau.de) 
Jesuitengasse 9 
D-W8390 Passau 
Tel.:   +49/851/35239