On Thu, Apr 24, 2003 at 12:31:03PM -0400, Mitch Collinsworth wrote:
> 
> On Thu, 24 Apr 2003, Jon LaBadie wrote:
> 
> > On Thu, Apr 24, 2003 at 11:45:02AM -0400, Mitch Collinsworth wrote:
> 
> > > Yes, I agree.  This sounds suspicious.  The slowdown described sounds
> > > like exactly what happens with DLT4000 and DLT7000 when you can't feed
> > > them data fast enough to keep them streaming.  Supposedly this was fixed
> > > somewhat in DLT8000 but I never bought one to verify that claim.  I've
> > > never heard of anyone claiming this also happens with compression.
> >
> >
> > I mentioned earlier today attending a tape technology talk.
> > This sounds like the "adaptive speed" the presenter mentioned saying
> > that if the source could not keep up with the drive, rather than dropping
> > out of streaming mode it would switch to writing at a slower rate.
> 
> Something like that.  If I remember correctly the DLT8000 implementation
> is to continue streaming the tape at the same physical speed, but write
> the data with varying density depending on data speed.  This implies that
> if you don't keep the tape input queue fed you'll no longer know how much
> data will fit on a tape.  But that's already somewhat true when using
> h/w compression...

That rings a bell.  I remember thinking "Great its a 100GB tape if I feed
it at 15MB/sec, but maybe only a 50GB tape if I feed it at 5MB/sec".


BTW and off topic.

In the charts presented at the talk I was struck by the relationship between
tape capacity and data rate.  Assuming the maximum capacity tapes for the
format, the data rate always seemed to be 1/10000th of the capacity (a small
fudge factor is needed).

For example, a Super DLT-220 has 110GB capacity, 11MB data rate.
An Ultrium 1 with 100GB capacity has 10MB data rate (7.5 - 15 depending on 
manufac).
A DDS-3 with 12GB has 1.0MB rate and a 5GB 8mm tape has a 0.5MB rate.

An upshot of this is that no matter what format, writting a complete tape
takes approximately 10000 seconds, nearly 3 hours.  The technology improvements
have not improved the time it takes to write a tape.  Just how much you write
in that time.


-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
 4455 Province Line Road        (609) 252-0159
 Princeton, NJ  08540-4322      (609) 683-7220 (fax)