Vax Unix tape problem

[Kathryn Smith]

	I've been running into some tape drive problems that I'm hoping some
Unix wizard out there will be able to help me with.  We are running a 4.2BSD
on a Vax 11/750 with a TU77 tape drive unit.  We've been experiencing two 
problems which may or may not be related.

	The first problem is that both the nightly and the level 0 dumps seem
unable to cope with writing a full tape of information.  Dump aborts on a 
tape write error consistently somewhere in the last 400 feet of a 2400 foot
mag tape.  This is apparently a software error, since no hardware diagnostics
are showing up on the console.  We have just recently upgraded to 4.2, but 
were experiencing the same thing on 4.1.  

	I tried instumenting a copy of dump to find out what is going on, and
found that the error is coming from the unix write primitive.  The error code
returned is 'I/O error' (enlightening).  Right now we are functioning by 
running dump with the size option specifying a tape size of 2000 feet, but 
don't want to keep doing this for obvious reasons.

	The other(?) problem is apparently a hardware problem.  DEC Field 
Service was here about two weeks ago to do preventive maintenance on the
Vax, and had problems with their tape diagnostics.  We had three field service
engineers here for two working days (during which we got absolutely nothing
done because the system was down) trying to locate the problem.  Their 
diagnostic consists of writing bit patterns on the tape then reading them 
back and verifying them.  The net result was that the diagnostic failed
a few time when run on a very high quality tape which they use for calibrating
and adjusting drives, significantly more on a standard DEC mag tape, and 
slightly more than on the DEC tape on one of our tapes (a BASF taken new out
of the box that morning).  

	They spent two full days taking the drive apart and replacing various
parts, probing it with an oscilloscope, and then rerunning their diagnostics.
At the end of that time they concluded that the problem was probably in the
controller rather than the tape drive itself, since they had tried replacing
most of the parts in the tape drve, but that they didn't know exactly what it
was, and would have to come back the next day with a controller kit to look
into that.  

	At that point, we decided that we would limp along with the drive as
is, since except for the dumps it is only failing about 10% of the time, mostly
on read operations, since we couldn't afford to be down anymore because of
time constraints on some projects.  I don't know if there is any connection
between the failed diagnostics and the dump problems, but I am inclined to
suspect so because the dumps began failing on 4.1 before there were any changes
to the software, so the only variables involved should be the hardware and the
tapes.  I discount the tapes because the problem appears both with old tapes 
which had been being used for dumps without difficulty, and for new tapes fresh
out of the box.  I didn't get any response from DEC Field Service about the
software problem.  In fact, I'm not at all sure they even paid attention when
I was describing it to them.

	If there is anyone out there who has run into a similar problem, or
even has educated guesses at what our problem might be, I would much appreciate
hearing from you.  If I get anything useful in response to this, I will
summarize for the net.

					Kathryn Smith
					General Systems Group, Inc
					Salem, NH ( ... decvax!gsg!kathy)