On Thu, Oct 16, 2003 at 04:25:48PM +0100, Tom Brown wrote:
> > > tapecycle 20 tapes:
> > > number of tapes to use per dumpcycle of 2 weeks. 10 tapes X 2 dumpcycles
> =
> > > 20 tapes.
> >
> > Yes, but you really should have an extra tape or two in there to lessen
> > the chance of a failed backup overwriting your last full backup.  You
> > should really consider doubling that number so you always have two copies
> > in case one should error on a restore (in which case you would lose your
> > newest data but you could at least still recover older files.).
> 
> really? in my case i allways have tapecycle as dumpcycle*runspercycle
> 
> is that bad?
> 
> e.g ...
> 
> dumpcycle 1 weeks       # the number of days in the normal dump cycle
> runspercycle 5          # the number of amdump runs in dumpcycle days
>                         # (4 weeks * 5 amdump runs per week -- just
> weekdays)
> tapecycle 5 tapes      # the number of tapes in rotation
> 
> or another config....
> 
> dumpcycle 2 weeks       # the number of days in the normal dump cycle
> runspercycle 10         # the number of amdump runs in dumpcycle days
>                         # (4 weeks * 5 amdump runs per week -- just
> weekdays)
> tapecycle 10 tapes      # the number of tapes in rotation

Tom,
you don't have it as dumpcycle*runspercycle.
In your two examples you have it as "runspercycle" period.

To demostrate the problem consider the simplest situation.
A dumpcycle of 1 day, a runspercycle of 1, and a tapecycle of 1.

Each amdump uses the ONLY tape containing the last level 0's.
Each amdump overwrites, and destroys your only level 0's.

Thus, if anything happens to that tape, and particularly if
anything happens during the amdump run, you have trashed the
only tape containing your level 0.

The same thing can happen whenever tapecycle == runspercycle.
The tape being used by the current amdump "could" contain the
one and only level 0 of a DLE.  If something happens to it,
you have nothing left to restore/recover from.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
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