Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!mit-eddie!uw-beaver!apollo!wanginst!wang!ephraim
From: ephraim@wang.UUCP
Newsgroups: comp.sys.mac
Subject: Re: Connecting two MacPlusses to one SCSI drive!
Message-ID: <438@wang.UUCP>
Date: Fri, 10-Apr-87 07:41:00 EST
Article-I.D.: wang.438
Posted: Fri Apr 10 07:41:00 1987
Date-Received: Wed, 15-Apr-87 04:42:13 EST
References: <243@sering.mcvax.cwi.nl>
Organization: Wang Labs, Lowell MA
Lines: 30
Summary: I shudder at the thought...

In article <243@sering.mcvax.cwi.nl>, frankk@mcvax.cwi.nl (Frank Kuiper) writes:
> How reliable is it, to connect two Mac's to the same SCSI hard disk?
> 
> I just tried it, and is *seems* to work, that is, both Mac's boot
> from the drive, and they can read from and write to it.
> 
> The big question is: what will happen if both of the Mac's want to
> write to the drive?

Bad things!

> I know that SCSI devices need different numbers to perform ok when
> connected, so now there are two devices (i.e. the Mac's) with the
> same device number (=7) connected!
> 
Since nobody addresses the Mac on the SCSI bus, the fact that both Macs
respond at the same address is not a problem.  There are other problems:

	(1) Most SCSI drivers for the Mac are written on the assumption
	    that there's no other host out there, so there's no bus
	    activity that they didn't initiate.  If they find the bus
	    or drive busy, they simply return an error.

	(2) The Mac file system is written on the assumption that 
	    native volumes are the sole property of the local Mac.
	    Important data structures, such as the volume header,
	    volume allocation map, and directories are cached in
	    memory and flushed out to disk at various times.  With
	    two Macs mounting the same volume, you're headed for
	    big trouble *unless* the volume is read-only.