> > There are now VTLs that compress the data going to disk; they always
> have 
> > a bit of a gamble as to how well they can mimic the way the real tape 
> > compresses data; I'm told they just use very conservative 
> > estimates, to make sure the disk image will fit on the tape.
>              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> This makes no sense to me.  No tape software ever, AFAIK, guarantees
> that a medium is a certain length or holds a certain amount of data.
> That's just not the way tape has ever worked.

The idea is if you are using your VTL to produce not just random
storage, but map directly onto physical tape.  If your local compression
is greater than the drive's, then when you go to realize the virtual
tape onto physical storage, Nebackup may be tracking a single tape
(TAP001), but that data will not fit onto a physical device.  Very
annoying.  You now have to either get the application involved and
duplicate the data, or you have to track two pieces of physical media as
a single unit and try to keep the application from knowing that....
Good luck with restores that don't involve the VTL.

I've run some production VTLs and we completely disabled compression in
the version we were using because it couldn't guarantee that the data
would fit otherwise.  I think now they do a better job of emulating the
compression used by the drive and lopping off a bit so that you're
reasonably certain of the data fitting onto good media even with local
compression.

-- 
Darren Dunham                                           ddunham at taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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