more exabyte questions

[Mark Bartelt]

OK, back to the well again.  Several more questions about exabyte drives
under IRIX.  Several weeks ago I wrote

| I have an exabyte drive (from Dilog) on a 4D25.  When I recently upgraded
| to IRIX 3.3.1, I was pleased to find the /dev/{r}mt/tps*v devices:  It's
| nice to finally be able to read/write arbitrary-length tape records!

to which several people pointed out (more politely than I deserved) that
I didn't RTFM.  In fact, the tps*v devices were there all along.  However,
this brings up question #1:

It turns out that MAKEDEV will create the tps*v devices *only* if there is
an exabyte drive attached to the SCSI bus and powered on.  (This is why I'd
been under the misimpression that the tps*v devices were new:  The exabyte
hadn't yet arrived when I installed 3.2.1, but was connected when 3.3.1 got
installed.)

How come?  Specifically, why is this particular class of special files treated
differently from nearly everything else?  For example, if I do

	MAKEDEV t3270

it just goes ahead and makes the appropriate special files, irrespective of
whether or not I actually have an IBM 3270 interface installed on my IRIS.
This is actually a nice feature, since I can ensure that the special files
I want are there in advance, in anticipation of a device actually getting
installed.  Not so, for some reason, with SCSI tapes, at least in certain
circumstances.  The tps*v files get created only if "mt -t whatever status"
reports that there's an exabyte there.  Why?

Anyway, this brings me to question #2:  How exactly are the tps*v forms of
the device handled?  Various people, in a previous exabyte discussion, made
some comments about the way that data gets stored in physical records on
tape.  For example ...

 [ Doug Thompson ]

 | Physical records on 1kb records grouped 8 records per helical track,
 | 8kb per track.

 [ Dave Olson ]

 | The 'block size' is written as part of the header info.  Larger block
 | sizes gain you no capacity or performance.  You DO want to make sure the
 | block sizes is a multiple of 1024 for maximum capacity  [ ... ]

So question #2 is, given that the actual on-tape physical records are 1k,
how are variable-length records written?  Are logical records somehow packed
into the 1k physical records, incorporating some sort of record structure on
the physical tape records (something similar to the way Fortran unformatted
disk files are implemented under UNIX, for example)?  Or is the information
out-of-band somewhere?  And, if the latter, where?  In the record headers?

I'm not sure knowing the answer to this is really crucial, but it would be
culturally interesting, and possibly useful when understanding portability
problems between IRIX and exabyte drives on other vendors' systems.

Finally, question #3.  Does anyone have a clue as to what's going on here?

  % maketape /dev/rmt/tps0d7nsv		# "nsv" device works fine
  5+0 records in
  5+0 records out
  10+0 records in
  10+0 records out
  15+0 records in
  15+0 records out
  % 
  % maketape /dev/rmt/tps0d7ns		# "ns" device doesn't
  5+0 records in
  5+0 records out
  dd: write error: I/O error
  3+0 records in
  3+0 records out
  dd: write error: I/O error
  3+0 records in
  3+0 records out
  % 

Here's what "maketape" looks like.  Note that the dd's bs is 1k, so I'd
expect that the exabyte should be happy, regardless of whether it's the
"nsv" or "ns" device.  Changing the bs to, say, 8k, doesn't help (in fact,
the error appears after a smaller number of records!).  Also, whenever the
failure occurs, the orange light on the drive comes on for a rather long
time (~30 seconds, though I haven't actually timed it), during which time
all console windows are semi-frozen, in that they echo what's typed, but it
appears that no processes get to run (keyboard input doesn't get processed
until the exabyte stops fussing around).  Is the drive locking up the SCSI
bus?  Is the IRIX SCSI driver sleeping at some high priority, waiting for
the drive to finish chatting with it?  A combination of both of the above?
Or something else?  So, actually, two related questions:  (A) why does the
script fail for the "ns" flavour of the device; (B) when it fails, why does
the whole system go out to lunch for a semi-eternity?

---------------------------------------------------------------------------

  #!/bin/sh

  #  script to demonstrate exabyte weirdness

  case $# in
          1)      tape=$1 ;;
          *)      echo "Usage: `basename $0` tapedev" >&2; exit 1 ;;
  esac

  mt -t $tape rewind

  for x in 5 10 15; do
          dd if=/dev/zero bs=1k of=$tape count=$x
  done

  mt -t $tape rewind

---------------------------------------------------------------------------

As usual, and as many times before, thanks in advance.

Mark Bartelt                                                   416/978-5619
Canadian Institute for                             sysmark at cita.toronto.edu
Theoretical Astrophysics                           sysmark at cita.utoronto.ca