Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!usc!samsung!uunet!fciva!dag
From: dag@fciva.FRANKLIN.COM (Daniel A. Graifer)
Newsgroups: comp.unix.i386
Subject: Re: SCSI vs ESDI
Summary: Can two hosts share a SCSI device?  How?
Keywords: SCSI ESDI bus
Message-ID: <520@fciva.FRANKLIN.COM>
Date: 25 May 90 13:53:01 GMT
References: <1990May23.151746.18784@cbnewsm.att.com> <1945@east.East.Sun.COM>
Reply-To: dag@fciva.UUCP (Daniel A. Graifer)
Distribution: na
Organization: Franklin Mortgage Capital Corporation, McLean, Va.
Lines: 37

In article <1945@east.East.Sun.COM> gsteckel@diag2.East.Sun.COM (Geoff Steckel - Sun BOS Software) writes:
>[...list of attributes of ESDI & SCSI systems...]
>SCSI
>	multiple masters (can share devices)
>[...discussion...]
>One SCSI host adapter on that system talks to [...]
>and could be connected to another CPU for a fake network. [...]
>	geoff steckel (gwes@wjh12.harvard.EDU)
>			(...!husc6!wjh12!omnivor!gws)

I didn't know SCSI supported multiple masters.  I've heard of a Macintosh 
product that connected two Mac's via their SCSI ports, but I thought there
were two SCSI buses and a gateway in that system, not one bus.

We have two SCSI bus machines here each with >500MB of disk.  We also have
one Excebyte 2.2GB SCSI tape system.  We can only do unattended backups
overnight on one machine per night.  Is it hard to get a SCSI device like
a tape backup system to talk to two hosts?  How is contention handled?  If
I have a bus that looks like this:

    [SYSA]-[DSKA0]-[DSKA1]-[TAPE]-[DSKB1]-[DSKB0]-[SYSB]

How do I keep SYSA from trying to control the DSKB?s ?  (Actually, guess this
wont work for me anyways.  Each machine has a QIC tape on it as well. That's
too many SCSI addresses (4/system + Exebyte). Maybe someday we'll get SCSI2!).

If this wont work, does anybody make a box that allows one device to sit on
two SCSI buses, and handles the contention between them?  You could just
simulate DEVICE-NOT-READY when the device was in-use by the other system.

Thanks in advance?
Dan

-- 
Daniel A. Graifer			Franklin Mortgage Capital Corporation
uunet!dag@fmccva.franklin.com		7900 Westpark Drive, Suite A130
(703)448-3300				McLean, VA  22102