Here is my response to an earlier question on sizing a VTL.  I have been
using a 10GB tape volume size for a couple of years and have not seen a
reason to change.  I originally chose 10GB because ISM recommended that
size when using sequential file devices.

Yes that is an approach that will work. I used:
(Total_Bytes_in_Primary_STG + ((Max_Daily_Amt_Backed-up X (ReuseDelay +
2))) X Growth_factor

I then let the compression factor be my fudge factor.

However give some thought to the size (native capacity) of your virtual
tape volumes.  In the physical world you have little control over this
(we generally use the largest size cartridge we can get to have the
greatest capacity [volume size times fixed number of cartridge slots =
total capacity online]).  In the VTL world things are different, you
total capacity is fixed (determined by the disk space you purchased) but
YOU can determine the volume size and the number cartridge slots equals
the total capacity divided by the size of your virtual tape volumes.

This said, what can you do with it?  I went with a small tape size
(10GB) and have the following benefits:
A) If I need a file at the end of the tape I spin through 10GB not 100GB
of data (ignoring compression).
B) No matter what reclamation threshold you use, you will have tapes
that are 1% short of being reclaimed.  Small tapes in this state "waste"
less space then large tapes in that state.

One gotcha is deciding which physical library you are emulating.  When I
first brought my VTL up, TSM choked.  Why?  Because TSM "knew" that the
library I was emulating could not have 1,415 physical tape slots.  I
changed the emulation to an "ADIC Scalar 10K" and the problem was
solved.  Sorry I don't remember the original library emulation.

Just a thought, think outside the physical box, it's all virtual.

Thanks,
H. Milton Johnson 

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
David E Ehresman
Sent: Thursday, November 29, 2007 8:01 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: [ADSM-L] Sizing virtual tapes

How large are you making the max size of your virtual tapes?  Why?

I let my TS7520 emulating 3592E tapes default to a max size of 460GB but
I'm starting to think a smaller size would make more sense.  The median
size of occupancy for my nodes is around 40GB so I'm thinking that might
be a better max size to facilicate reclamination.

David