At 09:27 AM 6-30-98 -0400, Jim Phillips of Therm-O-Link wrote:
>I beg to disagree with you.  We have a DDS-2 drive (1583) that absolutely
>will not write to 60M tapes, regardless of what type of markings (four
>vertical bars or other MRS-type markings) are on them, or whether
>compression is enabled or disabled.  Believe me, I have tried.  And tried.
>And tried.

Well, being human, it's entirely possible that I am misinformed, making
invalid assumptions, or just plain wrong.  Also, I don't recall working
with the 1583 model of DDS drive.  However, unless the specifications for
the 1583 explicitly indicate that 60m tapes are not supported, I would
consider the failure to write to MRS compliant 60m tapes as a defect in the
drive.  Those DDS standards documents which I have seen were very clear
that backward compatibility is expected in the DDS-2 and DDS-3 drives.

If I were trying to get a (model 1583) DDS-2 drive to write to 60m tapes
and it failed to do so on a regular basis (with multiple demonstrably good
tapes), I would closely review whatever specifications for the drive which
I could locate to check for explicit references to support for shorter
media and if I didn't find it explained there, I would report it to HP (or
whomever provides hardware support for it) as defective.

All that being said, there appears to be some problems with switching media
lengths on DDS drives.  The thickness of the tape is dependant on the
length of the tape.  (Note that a 120m tape and a 60m tape coil up into
roughly the same area; hence the thickness of the 60m tape must be roughly
twice that of a 120m tape.)  Although I don't actually understand why the
circular head of a helical scan mechanism like a DDS drive would care, it's
my understanding that the thickness of the tape, and hence the flexibility
of the tape around the heads, determines the precise shape the heads should
be, which varies depending on that flexibility.  It's also my understanding
that wear on the head by the tape will change that shape, depending on the
flexibility, due to the thickness which in turn is based on the length of
the media.  So... if you write 120m tapes most all the time, then the drive
may have problems with short media; and conversely, if you use 60m tapes
most of the time, the drive may have problems with long media.  However, I
would hope that the problems would be limited to higher error rates rather
than an inability to function with the "unusual" media.

P.S.  As usual, this is my personal opinion as I am not an official
spokesman for anyone and perhaps shouldn't even be allowed to speak on my
own behalf.
--
Jeff Woods
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