VTL and over subscription as I understand it.

Definition: When (tape volume size) X (number of defined volumes) >
native capacity of VTL you have over subscribed.  If you try to fill-up
all your defined volumes to their defined native capacity you will fail
as you will run out of space on your VTL.

Why would one want to over subscribe?  If you define a large tape volume
size (i.e. 100GB), and only want to write 10GB to a tape then yes it
would be neat if the VTL only allocated the actual space written to
virtual tape volume (i.e. 10GB).  When would this be beneficial in the
TSM application?
1) TSM DB backups:  Why waste a 100GB volume for one 20GB backup?

2) Using Collocation:  If you collocate a client with 50GB of space then
why waste a 100GB volume on the client?

But the problem is as you point out, when you move a tape from pending
to scratch the getting the VTL to reclaim the space previously allocated
to the virtual tape volume involves:
A) Checking out the scratch volume from TSM (so it will not attempt to
use it during the following steps)
B) Delete the volume from the VTL (this returns the space to the VTL)
C) Redefining tape volume to the VTL
D) Checking in and labeling the redefined volume into TSM
(I imagine that a VTL could replace steps B & C by truncating a volume,
but you would still have to get TSM to rewrite the truncated label.)

This is not a procedure I wanted to manage in a manual or automated
manner, so I chose the following:
1) Define small virtual tape volumes (i.e. 10GB)
2) Do not use collocation
3) Do not over subscribe

I have found tape mount time to be insignificant and the smaller virtual
tape sizes to makes collocation unnecessary.  This is just my way of
managing the trade offs.

Thanks,
H. Milton Johnson
-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Neil Schofield
Sent: Monday, June 11, 2007 5:50 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] How to Incorporate a CDL into a TSM environment?

Hi there

We too are in the throes of a debate about virtual vs. physical tape
libraries.

On the VTL side, much is made of the ability to over-provision the disk
capacity - eg a 100Gb virtual tape will only occupy as much space on
disk as has been written to it. As a result, so the theory goes, we need
only consider the occupancy when sizing the VTL. In a TSM environment,
this seems to be wrong on a number of counts.

- We still need to take into account the overhead of the reclaimable
space on a virtual tape. This can be managed by varying the reclamation
thresholds, but not eliminated.
- A pending delete volume will still occupy an underlying disk capacity
equivalent to its size.
- 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.

So am I correct in thinking that in the whole scratch, filling, full,
reclaim, pending, delete, scratch lifecycle of a storage pool volume,
the only time that we get the benefit of the over-provisioning is when
it's filling? In our current physical tape environment (with collocation
at the node level), only about 20% of volumes are filling. Ignoring
de-dupe for now, does it seem reasonable to base the sizing for a
replacement on the total physical tape capacity of the existing library
and some estimates of expected growth.

Regards
Neil Schofield
Yorkshire Water Services Ltd.

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