From: janet@cs.uwa.oz.au (Janet Jackson)
Subject: Help! Worrying behaviour of Exabyte 8500C
Summary: retries on dumps; strange behaviour of LEDs; write errors
Organization: Dept. Computer Science, University of Western Australia.
Date: Tue, 26 Jan 1993 07:09:58 GMT

(Please excuse the crosspost.  I'm not sure whether it's a SunOS software
or Exabyte hardware problem.)

Environment:  Third-party Exabyte 8500C (5Gb 8mm drive with hardware
compression interface) connected to Sun 4/470 server running SunOS 4.1.1.
I mostly use it for dumps.

The Exabyte has been acting up lately.  There are three main symptoms,
which may or may not be related.  They all seemed to start happening
at about the same time, though.

The first symptom is strange.  The second symptom is REALLY strange.

FIRST SYMPTOM:

The first symptom I noticed is error messages like:

Jan 26 14:17:44 wambenger vmunix: st2:  warning, the tape may be wearing out or
Jan 26 14:17:44 wambenger vmunix:       the head may need cleaning.
Jan 26 14:17:44 wambenger vmunix: st2:  write retries= 685 (4.1%), file= 3, block

on just about every dump.  The dumps are written OK.  I get read retries on
restores, too.

Each night my automatic backups cat a short ascii label to the front of the
tape, then dump 29 filesystems.  The cat doesn't give any error messages,
but each dump after that gives two:  one each for the first and last block of
the file.  The number of retries increases from near-zero to 32000-odd, at
about the 25th file; after that it's large negative numbers and ridiculous
percentages, like 700% -- presumably this is overflow.  This seems to imply
that the errors are not real, but that some error counter is being incremented
incorrectly.

Dumps I've done manually only seem to give one error message -- perhaps the
double messages are an artifact of the automatic backup software.

Once the error has started happening to dumps on a given tape, it continues
to do so until the tape is ejected.  After that, if you use the same tape
again, it may work OK for a while, but once the error occurs once it
will happen again on subsequent dumps, again, until you eject the tape.
It's as if there's some sort of status in the drive that gets reset when you
eject.

Cleaning the heads does not help.
Replacing the tape does not help either, although in one of my tests,
a new tape did not exhibit the problem the first time I dumped on it.
This has not generally been the case, though.

Before you say "FAQ":  I installed patch 100134-03, which is supposed to
stop spurious messages of this sort, but it hasn't helped.

As far as I can tell it is only dump that produces these messages.
cat, dd, cpio and tar all work normally, on both new and "second-hand" tapes.
(Maybe because of different blocksizes?)

These messages are the least annoying of the symptoms, since the dumps
do work.  However:


SECOND SYMPTOM:

This is the most worrying of all.

Normally when the Exabyte is writing to the tape, the amber LED flashes a
lot, indicating SCSI activity (according to the manual), and the green
LED occasionally flashes slowly, indicating normal tape motion.

However, I have lately noticed that during the occasional dump (no more than
one in 30) the green LED flashes slowly most of the time, and the amber
LED only occasionally.  When this is happening the dump runs about 30 times
as slow as usual.  It's as if it's looking for a good place to write on
the tape!  However, just because one dump on a given tape does it, doesn't mean
that another one will.

I have no idea under what conditions this occurs.  I certainly can't _make_
it happen!  It's happened on various tapes (new and old; different brands);
it's happened just after I cleaned the heads as well as when I hadn't cleaned
them for ages;  other activity on the SCSI bus doesn't seem to make any
difference.

I've only ever seen this happen during dump, but then I hardly ever use
tar, cpio, etc on this tape drive.


THIRD SYMPTOM:

We have had an unusual amount of write errors (hard errors, causing dumps
to fail) lately.  By "unusual", I don't mean it fails constantly -- I mean
three in the last month.  One of them seems to show that it is not the tape
that's at fault:  on 23/01 a write error occurred 17 feet into the first dump
on a tape.  I tried the SAME TAPE again for last night's dumps, and no error
occurred.


WHAT'S CHANGED?

I first noticed these problems in late December.  What have I changed
shortly before that time?

- I've installed patch 100570-03, an ethernet controller patch (surely
  irrelevant!)

- I used some new tapes of a different brand: Verbatim DL112M, rather
  than our usual Sony QG-112M.


NOTES

The Sun 4/470 doesn't have an 8500 driver.  It thinks the drive is an 8200,
but that's never seemed to make any difference to its operation.
Should I install a proper driver?  If so, where can I get one?

I've only ever used data-quality tapes in it, never video tapes.
I clean the heads regularly with an Exatape cleaning cartridge from Exabyte.


SO WHAT'S WRONG?

Has anyone come across this before (especially the second symptom, the LEDs)?

Do you think the heads are damaged?  The compression interface, perhaps?
Some other internal component?  Could changes in humidity or in the power
supply affect it?  Could having a heavy Sun Desktop Storage Module on top
of it be getting to it, even though it has worked perfectly in this
condition for almost a year?

If you have any thoughts about this, please email them to me.
I will summarise if there's interest.

Janet Jackson
<janet@cs.uwa.edu.au>
Systems Administrator
Department of Computer Science
The University of Western Australia