Re: Hard Drives Partition

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

In article <3la88n$15i@wraith.cs.uow.edu.au>,
David E A Wilson <david@wraith.cs.uow.edu.au> wrote:
> nathan@cco.caltech.edu (Nathan Mates) writes:
> >2) SCSI is not normally designed to be accessed by multiple systems at
> >once. It's something called 'bus mastering,' where you have multiple
> >controllers for one drive, and they arbitrate between themselves for
> >access. This really isn't done too much anyhow, so I don't believe
> >that any Apple II SCSI controllers support this. (I could be wrong,
> >though)
> 
> This is not strictly true. In SCSI-1 arbitration was optional. In SCSI-2
> arbitration was made mandatory. Arbitration is how multiple
> initiators (hosts) sort out which one can use the SCSI bus if more
> than one tries at the same time (the one with the higher SCSI ID wins).

The Apple High-Speed SCSI card supports SCSI arbitration.  I think
the original Apple SCSI card does as well (but haven't tried it).

The RamFast does not.  It assumes it is the only Master (it also
doesn't support disconnecting from the drive and reconnecting once the
command is ready).

> What Cirtech did is provide partitioning software that allows different
> portions of the hard drive to be dedicated to different hosts.

The CMS SCSI card can also do this (with a billion jumpers on the card
to set the partition mapping).  It must also support arbitration.

> Having support for arbitration does not automatically allow two
> computers to share a hard disk - if user 1 creates a file at the same
> time as user 2 the two computers will both try and update the same
> directory block - and only the last file created will appear (at
> best). If you only allow read only access then the problems are much
> less. 

A local user wasn't too careful with how he set up his CMS SCSI cards,
and gave all computers write access to all partitions.  He kept
complaining that his AppleWorks spreadsheets kept getting bits of
directories appearing in them.  :-)

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand