A few more points:

- It's true that hardware compression is faster than software compression,
and off-loads the server.  Hardware compression often is an extra-cost
item.  This can also be true of encryption, if done in hardware.  The
question is, where do you want to do your encryption?  At the storage
device, on the network, or in the client?  Depends on your requirements.

- Deduplication is going to take processing time wherever it's done.  Be it
by TSM on the TSM server, or by the VTL either in-band or
out-of-band.  Just because it's done out-of-band doesn't mean that you
don't have to worry about performance.  You still only have 24 hours in a
day to get everything done that you need to get done.

- TSM is in the best position (IMHO) to do deduplication, because it knows
most about the data.  To illustrate this, consider the following:  Even
with a VTL, you will still be doing TSM Tape Reclamation.  With a VTL, the
input tape will have to be "rehydrated" as it is read, and the output tape
will have to be "re-dehydrated" as (or after) it is written.  TSM, on the
other hand, could know that the data on the input tape has already been
dehydrated and therefore doesn't have to be re-dehydrated on the output
tape.  It knows what the data is.  (n.b., I don't know how TSM will do this
- this is hypothetical - time will tell).  There can be other examples of
this also, especially thinking forward.  VTLs are more limited in what they
can do.  Their advantage is that they can be dropped in to replace a
physical tape library transparently.

- Given that you're going to have to pay for the extra processing one way
or the other (either by the higher cost of a deduping VTL or by bigger
processors on your TSM server), I'd rather invest my money where the most
intelligence can be - in the TSM server.  Compare the cost of inexpensive
disk to the cost of the VTL solutions you are considering.  Using a
highly-featured disk subsystem for serial-access disk may not be the best
way to spend your money for serial-access-disk.  Look at simple SATA arrays.

My last point - there are a few VTLs that have replicating dedup today,
with more "on the cusp".  If you need something today, look at what is
available today.  If you don't need something until later, re-evaluate the
marketplace later - it's very dynamic right now.

..Paul




--
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