I would add to this an amount of tapes for 'cycling'
We do not recall the offsite tapes that have "just' been reclaimed
but put it off for another week so that the tapes they are replacing are
already in place at the offsite location.  For me this number is 20-50.
Current view of offsite pendings are (roughly):

200047                 2001-01-29     COPYPOOL
... continue for  29 more tapes to ...
200447                 2001-01-29     COPYPOOL
200023                 2001-02-05     COPYPOOL
 .... continue for 30 ... more tapes to ...
200490                 2001-02-05     COPYPOOL
200046                 2001-02-12     COPYPOOL
  ... continue for 35 more tapes (someone with 220 Gigs canceled the
       service after getting a bill for $4000./month.
200094                 2001-02-14     COPYPOOL

Plus there is always 9 DB backup tapes 3 to go along with each weekly
cycle.
            .. joe.f.

I don't think compress enters into this equation because I have one or two
clients that 'got away' with not using compress and their tapes show 30Gig
in use but the tapes are 20Gig 3590s.  All other tapes 'hover' around 20
gigs when they go to full so I figure their data is already compressed.

 Joseph A Faracchio,  Systems Programmer, UC Berkeley


On Thu, 15 Feb 2001, Paul Zarnowski wrote:

> At 04:50 PM 2/15/2001 -0500, Prather, Wanda wrote:
> >The formula I use is:
> >
> >Need to back up xxx TB
> >Divid by the number of GB that will fit on a tape.
> >Adjust as you have suggested for extra versions
> >Divide by .65 on the assumption that the not-yet-reclaimed tapes will be, on
> >average, 65% full
> >Multiply by 2 for the offsite copies
>
> This is pretty close to what I use, too, but make sure you factor in
> compression somewhere.  Also, if you are using collocation, then you also
> need to consider how rapidly your data is changing.  If you have high data
> turnover, then you will be reclaiming data more frequently.  If you don't
> have enough tape drives or wall clock time to handle this, you will need
> more tapes, plan on a lower average tape utilization, and raise your
> reclaim threshold to relieve the pressure on reclaim.
>
> This reclaim issue will not surface at first in a new robot.  You will only
> run into it over time, after a bunch of your tapes have filled up and you
> start doing more reclaims.  This issue is also dependent on the tape
> technology you are using, as some perform reclaims faster than others.
>
> ..Paul
>