SUMMARY: Backup while in multi-user mode

[Mark Verber]

In article <43617 at netnews.upenn.edu> rsk at gynko.circ.upenn.edu (Rich
Kulawiec) writes in responce to my question about doing level 0 dumps
in single user mode using BSD 4.3 dump+purdue+brl hacks which are
less likely to have problems with active file system:

    Those mods aren't bulletproof -- but in about five years of using this
    version of dump on many machines (VAX, Sun-3, Sun-4, MIPS, Pmax, etc.)
    I've never encountered a dump that I couldn't restore, i.e. that wasn't
    self-consistent.  That doesn't mean that all such dumps were
    "complete", especially since the meaning of "complete" gets fuzzy when
    we attempt to apply that term to an active filesystem; but it does
    mean that I had what I needed to recover from crashes.

I am glad for you.  On the other hand I have seen restored fail
utterly when the dump was taken on an active file system.  The dump we
used has all the above patches installed!  We all know even with all
those mods that there are failure conditions: Chris Torek and others
have posted them time to time so I am not going to repeat them.
Murphy's Law indicates that when you are in critical need of a clean
dump...  that is when you happened to get an inconsistent one.  I
haven't found a corrupted dump tape often, but it has happened ...
which is enough to keep me doing level 0 in single-user.  I value my
user's data.  [This may come from an incredible three day period of
time when the staff retyped an entire thesis for a graduating PhD
candidate who lost everything after a series of failures.]

Another thing to note is most of us are seeing more and more disk
hanging off our machines.  There are more files and more dumps being
done.  A few years ago most of us had 1-2gb of disk.  These days sites
with 5-10x that are common.  That increases the total number of
possible failures because a lot more dumps are being running.  The
sites I know who have seen inconsistent dumps are also sites that have
10-30 servers and 15-30gb of disk a few years ago.

Mark Verber
Ohio State Physics Dept / Computing Services