Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!samsung!umich!ox.com!math.fu-berlin.de!unido!fauern!forwiss.uni-passau.de!unipas.fmi.uni-passau.de!baier From: baier@unipas.fmi.uni-passau.de (Joern Baier) Newsgroups: comp.unix.admin Subject: SUMMARY: Backup while in multi-user mode Message-ID: <1991May20.123129.14433@forwiss.uni-passau.de> Date: 20 May 91 12:31:29 GMT Sender: usenet@forwiss.uni-passau.de (USENET News System) Organization: University of Passau, W-Germany Lines: 42 Nntp-Posting-Host: zaphod.fmi.uni-passau.de 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 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@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@unipas.fmi.uni-passau.de) Jesuitengasse 9 D-W8390 Passau Tel.: +49/851/35239