This has been discussed pretty thoroughly on the list in the past; go to
www.adsm.org and search on VTS.
Here is the information I posted last time; in general, yes there are
performance issues to consider. Whether or not you can live with them
depends on your environment:
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You should be VERY VERY CAREFUL about your requirements planning before you
do this.
TSM is a worst-case application for a VTS. Not because TSM is doing
anything wrong, but because ANY application that writes single-file tape
images that FILL THE TAPE VOLUME is non-optimal for a VTS. This applies to
DFHSM tapes on a mainframe as well.
The whole point of a VTS is to accept small files that were originally
directed to tape, store them on disk as "virtual" tape images, then let the
virtual tape images get migrated off and stacked together on a large real
tape. When you want to access something on a "Virtual" tape volume again,
the VTS has to stage the data from the tape back to the disk before you can
use it.
If you are talking small application files, the VTS works wonderfully. But
because TSM (and any similar application) writes ONE large file that fills
the "virtual" tape, the WHOLE TSM "volume" gets written to disk, then staged
out to tape. So far so good. But when you want to do a restore of a
particular file, or when TSM wants to append to the volume as it would a
real tape, the WHOLE volume gets staged back in, not just the piece
you want.
So it's a performance issue. I know people who use a VTS with TSM and are
happy with it. Should work just fine if you have a light load.
I also know someone who backs up many GB of data a night with TSM that get
sent to a VTS, then migrated out to tape. Then when they want to copy those
"virtual" tapes to create their offsite copy pool ---- takes a lifetime,
cause each volume has to be staged back to disk first.
So THINK TWICE about your requirements before making TSM (or DFHSM, or
anything that writes single-file tapes) depend on a VTS. Make sure you
understand the performance issues, and that the performance limitations will
be acceptable in your environment.
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