On 7 Apr 1998, Tom Ivar Helbekkmo wrote:

> "J. Buck Caldwell" <buck_c@polygon.com> writes:
> 
> > Does NetBSD/vax support dual-porting SDI disks? ie, can you hook the
> > A port to one machine, and the B port to another?
> 
> Only to the "normal" extent of being able to do exactly what you say.
> 
> There is no support for synchronizing the buffer caches of two
> machines working in the same file system.  If you want to hook up an
> SDI disk to two machines, you can, but (as with other operating
> systems) you'll have to keep them from stepping on each other.  The
> three most useful scenarios are a) using the disk from each of the two
> machines at different times, b) using different partitions on each of
> the two machines at the same time, and c) running a failover-scheme to
> make whichever machine is currently the "passive" node mount the file
> systems and take over the running of some service if the other one
> goes down.  I'm not suggesting that VAXen ever do, of course!  :-)
> 
> Oh, yes, I've also heard of raw disk being used as a communications
> area for shared data structures between processes on two machines.

Actually, I'm not sure that dual-ported disks can be accessed on both
ports with NetBSD. I believe it never releases the port, once it has been
acquired, so the second machine who tries to access it will have to wait
for ever. You could, of course, modify the driver to release the port if
it's inactive...

This also means that scenario a and b is impossible today, while c depends
on the machine crashing hard enough that the port is released.

Dual-porting has been an option in almost every DEC drive, but very little
software has ever used it.
DEC used it for the front end RP06 of DEC-20s, which also were accessible
to the PDP-11, each having its own filesystem on the drive simultaneously.
RSX supports dual-porting of a drive, from the same machine, using two
controllers. The idea is that if one controller is busy with another
drive, you can still get the data a bit faster than waiting for the
controller to become free.

The HSCs might also support some sort of sharing dual-ported drives, but
I'm not sure. I would suspect VMS only supports dual-ported drives in the
same manner as RSX.

Please correct me if I'm wrong.

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt@update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol