Backup Question (dump)

[George Robbins]

In article <1991May22.145513.15048 at honte.uleth.ca> senetza at honte.uleth.ca (Len Senetza) writes:
> I'm wondering how dump handles open files.  Does it back them up?  Does
> it skip them?  Does it trash them in the process of backing them up?  I
> realize that having a file open while dumping (or any other type of
> access) can have unpredictable results, but I wanna know how dump
> handles it.  

Basically, dump "reads" the files and puts what it reads on the tape.  If
the size of what it reads doesn't agree with what the i-node said initially,
then you get a warning message that the file size changed.  If the file is
simply "growing" than this is probably reasonably competent behavior.

If some other nature of updating is going on, then you'll have to assume that
the image of that file on the backup tape is not neccessarily consistent, but
but perhaps better than no dump.

The real risks come from files that are added / deleted or suffer radical size
changes during the dump.  Traditionally, it was easy to generate a dump date
that would blow off the restore because the data on the tape was confused.

Supposedly, the current Ultrix dump has been ruggedized and this problem is
minimized.

> See, when we do our backups we don't shut-down to single user mode.  We
> have a DECserver 3100 serving 2 DECstation 3100's and 22 VAXstation
> 3100's in a computing science lab.  This stuff has to be running almost
> continually (hardware failure is practically the only excuse).  So, we
> have to do live backups.  

There is no big problem with doing incremental dumps on live filesystems, but
you are taking a major risk if your level 0 dumps aren't done single user
or with the filesystems unmounted.  If you do frequent full dumps to then
you can probably live with the possibility of one of them being unrestorable.

-- 
George Robbins - now working for,     uucp:   {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing:   domain: grr at cbmvax.commodore.com
Commodore, Engineering Department     phone:  215-431-9349 (only by moonlite)