Re: Hard Drive Questions

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <1994Dec13.225221.1921@news.stolaf.edu>,
Peter F Handel <handel@lars.acc.stolaf.edu> wrote:
> That reminds me; I want to buy another IIgs, and when (if) I ever get my 
> original one fixed, I'd like to put a High Speed SCSI card in the second
> one too (run a BBS off of it) and hook both up to a giant external SCSI.  
> Could this work?

Yes, but with some restrictions.

The main one is that it is not safe to allow both computers to access
the same partitions.  Even reading from one computer while writing on
the other computer will cause problems (the reading computer may try
to read a partially written file or directory, and claim the file is
damaged).

If both computers are only reading from the same partition, there
should be no problem (e.g. a read only file archive).

Both cards must be Apple SCSI cards.  I _think_ the original Apple
card supports SCSI arbitration, and I'm sure the High-speed one does.

> What if one was a RamFAST/both were RamFASTs?

Definitely not.  The RamFAST doesn't support SCSI arbitration at all -
it assumes it is the only bus master, so it could clobber a data
transfer to or from the other computer.  Both cards MUST be Apple.

The CMS SCSI card was designed to work in this sort of configuration.
It uses jumpers to set up the partitions and control which computer
accesses which partitions (including read/write enable).

> I could set it up so that the BBS GS would only have access to 2 or
> so 32MB volumes (under ProDOS 8-> METAL!), so there could be no
> chance that both GSes would be trying to read/write the same file.

Sounds fine, as long as you are careful not to write to the partitions
being used by the BBS computer from the other one.

If you are running ProDOS-8 on the second computer, it should be safe
to write files to it (from the main computer) at times when you can
guarantee that there are no open files (e.g. quit from the BBS
software and leave the computer in a simple program selector).

If the second computer is using GS/OS, I would not recommend trying
this, because GS/OS may have cached data blocks on the drive which are
updated by the other computer; GS/OS would then use the old versions
of the blocks instead of re-reading them.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand