Re: Hard Drives Partition

[david@wraith.cs.uow.edu.au (David E A Wilson)]

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).

Cirtech (a Scottish company) released two SCSI controllers for the Apple II
that support arbitration - the Cirtech SCSI card & the Cirtech InSyder card.

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.

What Cirtech did is provide partitioning software that allows different
portions of the hard drive to be dedicated to different hosts. Thus each user
works happily in his/her own partition and may or may not have access to read
files in other partitions.

Obviously all the computers must be able to boot from the same boot blocks
so different types of hosts are more difficult than all Apple //e for example.
--
David Wilson	Dept CompSci Uni Wollongong Australia	david@cs.uow.edu.au