I have been pondering this for some time.  There are pros and cons
either way (vtl vs SAD (serial-access disk).

VTL Pros:
- Works with LAN-free backups.  SAD does not.
- Most of the newer VTLs either have data reduction technology, or
have it on their roadmap.  By "data reduction" I refer to
deduplication, or elimination of duplicate data.  Different vendors
deduplicate very differently, so be aware of these
differences.  There are good articles in "Storage" magazine (among
others) about the differences.  TSM does not do deduplication with SAD.
- Some VTL products do very well at scaling I/O performance.  With
SAD, this is something you're responsible for tuning.

SAD Pros:
- Possibly lower expense, tho with deduplication this could
change.  Many deduplication vendors talk about fairly high reduction
ratios which could actually make them less expensive, tho I have no
analysis of this.  TSM's use of compression, encryption, and
aggregation could impact reduction ratios, perhaps significantly.
- TSM development is enhancing SAD support to provide things like
active copy pools, et al.  It is not clear to me which, if any, of
these new features will eventually be supported or usable with
VTLs.  If TSM development will be putting more effort into enhancing
SAD, which appears to be the case, then is it a smart move to buy
into VTLs?  If you can't wait, you'll need to base your decision on
what is available today.

Frankly, if TSM deployed some sort of de-duplication technology, this
would sway my decision in favor of SAD.  We don't use LAN-free, and
have no immediate requirement for it, so using VTLs for that is not
an advantage to us.  Unfortunately, we may not be able to wait for that day.

Hope this analysis is of some use to you.
..Paul


At 12:57 AM 2/7/2007, Steven Harris wrote:

Perhaps a little harsh Roger?  Don't some VTLs implement tape hardware
functionality that TSM can find difficult, I'm thinking of hardware
compression in particular, but maybe encryption would be another.

Regards

Steve

Steven Harris
AIX and TSM Admin Brisbane, Australia


Roger Deschner wrote:

In general, discouraged. What you want instead is big, native-mode RAID
arrays, configured as disks using devtype=file, not as virtual tapes.
You get a lot more flexibility and a lot less complexity, compared to a
VTL. TSM even does the backup-to-tape function better. (copy pools) The
VTL software layer is unnecessary overhead, and unnecessary
administrator effort, in the case of TSM. You'll save money, too.

If the VTL salesman tries to convince you that there are limitations in
TSM that make a VTL necessary, he's probably talking about the
one-filesystem limitation for devtype=file, which was removed in TSM
5.3. Since 5.3, the last reason to prefer a VTL is gone. Took me 45
minutes with a very determined EMC salesman who thought he knew all
about TSM, and filled my email inbox with PDF white papers to prove it,
to convince him that I wanted to see a plain ordinary EMC disk box, not
one dressed up as a VTL.

Roger Deschner      University of Illinois at Chicago     rogerd AT uic DOT edu
= "Things are more like they are now than they have ever been before." =
======================== --Dwight D. Eisenhower ========================

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Gill, Geoffrey L.
Sent: 06 February 2007 02:15
To: ADSM-L AT VM.MARIST DOT EDU
Subject: [ADSM-L] VTL's (looking for info)

Does anyone have a doc or a link to anything that discusses the pro's
and con's of a VTL?


Geoff Gill
TSM Administrator
PeopleSoft Sr. Systems Administrator
SAIC M/S-G1b
(858)826-4062
Email: geoffrey.l.gill AT saic DOT com

--
Paul Zarnowski                            Ph: 607-255-4757
Manager, Storage Services                 Fx: 607-255-8521
719 Rhodes Hall, Ithaca, NY 14853-3801    Em: psz1 AT cornell DOT edu