As I see it, there are two areas where you get performance hits when
restoring from non-collocated volumes:
1) Tapes Mounts:  In my experience my VTL makes this problem
insignificant.

2) Spinning Sequential Media:  Yes, VTL volumes are sequential and if
you define your tapes as 50GB native and then with compression get 100GB
written to the tape, you may have to spin through 99.9GB of data to
retrieve a 0.1Gb file.  However if you define 10GB volumes you only have
to spin through 1/5 of the data to reach your 0.1GB file.  Also with
smaller volumes you are more likely to get "natural collocation" because
a client that writes directly to tape is more likely to fill up a tape.
Obviously if you define smaller and smaller volumes at some point you
will have a "tape mount bottle neck".

Just one way to manage the trade offs.  

H. Milton Johnson
-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Nicholas Cassimatis
Sent: Monday, June 11, 2007 12:45 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: [ADSM-L] Fw: How to Incorporate a CDL into TSM environment?

A couple of comments about what Wanda said about collocation and VTL's:

At some point, you do have a finite number of mount points defined for
your VTL.  Even if virtual tape "mounts" are near instant, there is
still some overhead.  A large number of clients "mounting" virtual tape
after virtual tape after virtual tape will have some sort of negative
effect on the overall throughput of their sessions.  I'm not saying it
will be significant, but it could get there, depending on the VTL
technology.  A few milliseconds here, a few there, and a controller that
gets bogged down under the mount request queue, you could cause yourself
some issues.

And don't forget that virtual tapes are the same as physical tapes in
one major factor - they're sequential!  So non-collocated storage pools
could have multiple clients asking for the same virtual tape, so there
would be a wait queue for the virtual tape.  A VTL doesn't resolve this
type of contention, as it's at the TSM level.

I would argue that the cost of creating collocated volumes in a VTL is
negligible, and still has benefits on the restore side.

To echo a number of others comments in the thread - if you don't plan it
out right, it's not going to work.  That goes for just about anything,
from vacations to VTL's!

Nick Cassimatis

----- Forwarded by Nicholas Cassimatis/Raleigh/IBM on 06/11/2007 01:33
PM
-----

> And you don't have to collocate in a VTL, since there is zero 
> effective tape mount time.