On Tue, 23 May 2006, bob944 wrote:
> > > > 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.
[...]
> There is no way to guarantee that the contents of tape A fit onto tape
> B. Because of that, as I said earlier, no tape software ever made,
> AFAIK, makes such an assumption.
[...]
Then you are obviously not as old as me :-(
Way back, ICL had something called FMS (Filestore Management System,
IIRC) which did exactly this. It used 'fixed capacity tapes' - which
everyone knew weren't actually fixed capacity tapes, since we used the
same physical media for other, variable capacity, purposes as well.
My understanding was that the s/w deliberately underestimated the capacity
of the tapes, for precisely the reasons you outlined. So it routinely
copied tape A onto tape B to make duplicates. I don't believe we ever had
a problem with a tape filling when it "shouldn't".
I hate to think how much tape capacity was being wasted. But it _can_ be
done, and has been.
Richard
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