tape problems with SCO UNIX

[Eric Scott Deese]

I am experiencing a problem for which I quickly need
an explanation/solution.  We are running SCO UNIX 3.2
on IBM PS/2 model 80 machines.  On seperate machines,
we have an Irwin 80 MByte mini-cartridge tape and an
Archive 60 MByte tape (DC600 type cartridges).  On
each system, at various times, we attempt a backup to
tape (via cpio or tar) and receive a message from the
tape driver :  "not enough memory".  A call by our
sister company to SCO received the solution of "add
more RAM".  That system had 4 megabytes and received
the error (almost) always.  That system was upgraded
to 8 megabytes and has not received the error since
(for the last week or two).  A machine here has 6
megabytes and receives the error sporadically for as
long as I can remember.  The problem can be alleviated
by logging off users and/or terminating running
processes.

Current theory is:  Since the tape controllers utilize
DMA (we assume), the tape controller requests a chunk
(how big I really want to know) of CONTIGUOUS memory,
which only gets satisfied for a large RAM size or sparse
utilization by other processes.

  1)  Is this theory correct?
  1a) If correct, two possible solutions :
      -  Build a buffer into the tape driver that it
         gets upon system boot and keeps forever.
         What a pig, BUT a solution I am willing to
         live with to get reliable tape access.
      -  Make the tape driver force the kernel to give
         it a contiguous buffer.  If that means swapping
         other processes, then fine, just do it.
  1b) If correct, is there a non-DMA tape controller
      (such as SCSI for Archive tapes) that would solve
      the whole problem?

  2)  If the above theory is incorrect, what is the real
      explanation?  Our experiences indicate this occurs
      sporadically based upon memory utilization by all
      processes in the system.  How do you know when you
      have enough memory?

All of this is a REAL PROBLEM for our two companies.  We
are prepared to begin shipping a turnkey application based
upon SCO UNIX and some tape drive.  We MUST have a system
that can perform a tar or cpio at any time.  The application
is not such that we can simply say "Kill off some processes
and try the tape again".  We need a real solution, and we
need it fast.

Any ideas/soultions/answers greatly appreciated.  Input is
welcome from SCO, Irwin, Archive, other tape vendors and
other operating system vendors.  But don't let this stop
you.  If you know the answer, PLEASE let us know.

Thanks,
Eric Deese (osu-cis!liebert!esd)   (614) 841-5508