I agree with your comments and yes if your server/client can stream data
fast enough to the tape drive you may be able to out-pace a disk drive.
If you can not keep up with the tape drive then you can take a serious
performance hit when the tape stops, repositions and restarts.  This
performance hit is not as great with a virtual tape.  

We also looked at the "3494 VTL or Tape Server" and saw that it is
indeed a poor fit for TSM.  Most TSM configurations already have a disk
based stgpool front-end to the tape library so you would not gain much
by adding one inside the library.

Than
H. Milton Johnson
 
-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Richard Sims
Sent: Tuesday, November 30, 2004 9:36 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: size of active vs. inactive?

On Nov 30, 2004, at 9:58 AM, Johnson, Milton wrote:

> ...Needless to say, I disagree with the statement that TSM doesn't 
> appear to be a good fit for a VTL.  Remember a VTL is just a library 
> that can mount/unmount a tape in less than one second and read/write 
> to the tape at disk speeds.  Why wouldn't TSM work well with a library

> on steroids? ...

Thanks for sharing your experience with VTL in a TSM environment:
it's helpful to have the perspective of experience.
As we've seen in past tape vs. disk discussions, though, don't regard
"disk speed" as some kind of ultimate data processing I/O attainment:
high performance tape technologies streaming to sequential media can in
many cases out-pace disk throughput.  It is in mounting and tape
positioning that tape is a poor performer relative to disk, where one
can wait a minute or more before I/O can proceed.  That's were VTL
shines.

    Richard Sims