Re: Mac/GS dual boot hard drive?

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <59dki4$889@ccnet3.ccnet.com> fmlin@ccnet.com (Frank/G.speed) writes:
>One of the feature in SCSI (is) was bus arbitration.  Meaning a device
>could take control of the bus.  I think it was implemented on Macs 
>thus your friend had no problem.
>
>This feature was not available on Apple's SCSI card for the II's and
>the HD would probably become corrupt if attached to a Mac and GS and
>both computer are on.

My Apple Rev. C SCSI card implements arbitration.  I rather doubt they took
arbitration support out of the HS SCSI card.

I've never had a RamFast SCSI card to play with, but the comments I've read
about it seem to indicate that it doesn't do arbitration.  Arbitration was
optional under SCSI-1, and the RamFast seems to be a thoroughly SCSI-1
device.  (Arbitration is mandatory in SCSI-2.)


But even if all the devices on your SCSI bus support arbitration, that
doesn't necessarily mean it's safe to connect two computers to the same
hard drive simultaneously.  Arbitration only guarantees that two computers
won't try to access the same drive at the same time.  Most computer
operating systems aren't even secure against the possibility that two
computers will try to access the same drive at *different* times, and will
merrily overwrite each other's data in spite of arbitration.

So if you have two computers connected to the same disk drive, you
shouldn't turn on both computers simultaneously unless their operating
systems are specifically designed to cope with this situation.  (You might
be able to get away with having both computers turned on at the same time
if each computer has its own dedicated partition on the common disk, but
don't push your luck too far...even in this case, all heck will break loose
if one of the computers repartitions the disk.)


Note that file-serving systems like NFS and Appleshare aren't affected by
this limitation, since only one computer is actually connected to the
physical drive...other computers access the device by issuing requests to
the server software.

               - Neil Parker
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker
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