Multiple archives on one tape [When is tape full?]

[Peter Jaspers-Fayer]

We too keep multiple incremental backups on one exabyte tape.  The code
outline below has not been tested as a whole, but may be the way to go:

    ...
1   mt -t /dev/tps0d6nr feom
2   bru -vvvvetc etc >brulog
3   set Kbytes = `tail -1 brulog | awk '{print $4}'`
4   set prevtot = `tail -1 totals`
5   set now= `echo "$prevtot + $Kbytes" | sed -e "s/k//" | bc`
6   if ($now >= $safe ) then ...

Notes:
1. "feom" writes to end of last bru 'save-set'
2. 'vvvv' is very verbose (including phrase '<n>k of <m>k ...' where m is a
   running total for this `bru`.
3. Get the last value of the running total for this backup.
4. Retrieve the previous total
5. Get the tape total for all 'save-sets' so far (strip the 'k').
6. Test to see if tape is now full.  [safe = MAX - (average_size + saftey)]

   If it's full, do "echo 0 > prevtot", and ask for a new tape.

That brings up a point.  In bru format, just how much data fits on a 2.3 Gb
tape?  Past experience with other tapes tells me it will be somewhat less
than 2.3 Gb, but noone seems to know.  What percentage of the tape will be
used by IBGs and bru overhead. (We've never filled a tape yet ;-)