I beleive there is a reserve flag on SCSI devices to restrict access to
one initiator at a time.  Initiators which don't hold the reservation
can only execute a limited number of the SCSI commands.  It can be
disabled on some RAID boxen that support concurrent access but I don't
think this will work on your generic run-of-the-mill SCSI disks.

Disclaimer, I'm not a SCSI guru.

>> i would like to connect a number of Pentiums running NetBSD together
>> using a single SCSI disk.

>> ie.

>> machine
>> with disk   diskless    diskless    diskless
>> --------    --------    --------    --------
>>     |           |          |           |
>>     +-----------+----------+-----------+

>> provided i kept the SCSI cable length within a reasonable length, is such
>> a thing possible?

>This connection should work just fine, assuming that you've set
>all of the controller cards to different SCSI IDs. The machine with
>the highest ID (7, usually) will have priority on the bus, priority
>descends with the SCSI ID.

>> could i have each machine boot and run from a seperate
>> partition on the single SCSI disk?

>This is rather more difficult. You can't have more than one disklabel
>on a disk, and all of the machines will want to mount root from
>sd0a and swap from sd0b, unless you modify the kernel. You might
>be able to do three machines with root/swap on sd0a/sd0b, sd0e/sd0f,
>and sd0g/sd0h if you compile separate kernels for each. You can't
>use sd0c/sd0d because these are generally used for the NetBSD
>portion of the disk and the whole disk respectively (or is it the
>other way around?). You also *must* make sure that no machine writes
>to a partition in use by another machine, or disaster will certainly
>result!

-michael
-- 
Michael Galassi   nerd@percy.rain.com   http://percy.rain.com/~nerd/