I can't speak for everybody's product out there, but the EMC CDL (EDL)
releases the used pages from the virtual volume as soon as you begin to
overwrite the virtual volume from the beginning.  One thing that does
this is a Label Libvolume.

It would be a simple script to look at all scratch tapes in the virtual
library once a day, and issue a Label Libvolume on each scratch.  Even
in a large VTL this script would run quickly and have no impact on
overall performance. 

Having said that, I personally don't think it is necessarily a wise
thing to oversubscribe a VTL.  By oversubscribe I mean to define, say,
150 virtual tapes when there would only be space for 100 virtual tapes
if they were actually full.  I had a customer do this once, and when the
CDL became completely full TSM started getting I/O errors on all their
virtual tapes, and everything went belly up, and there was no way to
tell from a TSM perspective beforehand that they were getting close to
full.  If you are going to do this, you better set up CDL alerts and
monitor the product, because TSM cannot help you monitor capacity. 

If you define 100 virtual tapes to a CDL that has 100 virtual tapes
worth of capacity, then 'Q STGPOOL' will tell you how close to full the
storage pool is, and be accurate about it.  If you don't collocate this
storage pool, then you ought to get virtual tapes completely filling up
before you mount other virtual tapes, and thus get a fairly accurate
picture of your resource consumption.  Then if your reclamation is
keeping up, you should be getting back scratch tapes often enough to
keep up with demand.  If you see the number of virtual scratch tapes
deminishing over time, you are either not reclaming agressively enough,
or you are outgrowing the CDL capacity and need to upgrade.    

Best Regards,

John D. Schneider
Sr. System Administrator - Storage
Sisters of Mercy Health System
3637 South Geyer Road
St. Louis, MO.  63127
Email:  schnjd AT stlo.mercy DOT net
Office: 314-364-3150, Cell:  314-486-2359


-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Richard Rhodes
Sent: Tuesday, June 12, 2007 10:11 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] How to Incorporate a CDL into a TSM environment?


"ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU> wrote on 06/12/2007
09:30:58 AM:

> >> On Mon, 11 Jun 2007 23:50:20 +0100, Neil Schofield
<neil.schofield AT YORKSHIREWATER.CO DOT UK> said:

>
> > - Since the conversion of a pending delete volume to a scratch tape
takes
> > place purely in the TSM database, a virtual scratch tape will also
occupy
> > the full disk space on the VTL. until it is re-used.

When DataDomain was in for a dog_and_pony_show the other day I asked
about this. They claimed to be working with IBM to become application
aware, so that they could release internal VTL storage when a tape goes
scratch.

I can see how this could be done, but I'll believe it when I see it.

Rick


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