The approach I used worked very well.  I counted all of the tapes that
would be kept on the VTL and multiplied that by the capacity of the
tapes.  I then calculated the growth rate of the tape pool and added
that to the total, in my case 6 months out.  That is the number I used.
The assumption is that compression will be similar on both.

If you calculate off of occupancy there is a greater uncertainty because
you do not really know how much compression you will get.  Calculating
off of tape counts will be inaccurate because of filling tapes, but this
error is in the admins favor.  Filling tapes can be factored in to
reduce the error.

Andy Huebner

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
William Boyer
Sent: Thursday, June 21, 2007 1:03 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: [ADSM-L] Somewhat OT: Sizing a VTL solution

Has anyone just gone through sizing a VTL solution for a library
replacement? Is it as simple as taking your current
occupancy/retention, applying for some compression and using that figure
for the amount of storage behind the VTL? Or maybe I'm just
trying to make something harder than it is. Wouldn't be the first time!
:-)

TIA.

Bill Boyer
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