when is it safe to run dump(8)?

[John Jorgensen]

We just bought an Exabyte 8200 8mm drive to do backups for our
network of Suns, and I'm trying to figure out when we can run our
full dumps, now that we don't need someone around to change
tapes.  

The man entry for dump says that you should bring the system down
to single-user mode to do level 0 dumps.  Until now, we have been
doing full backups to 1/2" tape by halting all the client machines,
dumping the first few filesystems with only the file servers
running, then bringing the system up with the last filesystem's
mode changed to allow root access only, and dumping that
filesystem while the others are available to users.  We have been
running nightly incremental dumps at 1 a.m.  with the system
fully available (we do have a handful of users who log on at such
hours).  I have not seen restore fail on any of the nightly
tapes.

Now that we have the Exabyte, we would like to do unattended full
backups at night, so that we don't have to bring the system down
every week.  I think that the system is pretty quiet between 0200
and 0600, but I don't see any way to prohibit access then.  If I
do full dumps while there is a user or two active, how big is
the risk that the dump won't work correctly?  Am I only risking
messing up the files that the user happens to be using at the
time, or could the entire dump be compromised?  If it's just a
matter of missing the occasional file, I think I'd be willing to
trade that off on the increased redundancy we would get from
running full dumps more often.

I'm especially interested in hearing how people who already have
Exabytes or similar drives are handling unattended full dumps.
Has anyone thought of a method for making a filesystem
inaccessible during the dump (if you just change permissions,
what happens to the person who started editting a file in the
filesystem just before you took away his ability to write to
it?).

You can send replies to me, or post to the net--this issue
might actually prompt some discussion, though it's hard to
imagine much controversy arising from the mundane question of
running backups.

John Jorgensen
jorgnsn at qucis.bitnet 
jorgnsn at qucis.queensu.ca