11/70 <-> uVax II link

[Dan Ts'o]

In article <1486 at uwmacc.UUCP> jwp at uwmacc.UUCP (Jeffrey W Percival) writes:
>We have a PDP 11/70 running 2.8BSD.  We are thinking of getting
>a MicroVax II, and getting Berkeley Vax Unix to run on that.  Instead
>of trying to get networking going on our 11/70, we thought we'd
>get a Systems Industries disk and controller that will be shared
>by the two machines.  Is this a reasonable approach?  Our
>expectation is that file systems on the shared disk will be
>accessible to users of both Unixes.  Are the 2.8/4.2 file systems
>identical?  Can we just "mount" a shared file system on each of
>the systems, so that the directories and files in that file
>system appear in the directory paths of users on both machines?

	In general, not a very feasible idea.

	- Both 4.2BSD and Ultrix use a very different filesystem layout from
older UNIXs, including 2.8BSD. One cannot just mount a 2.8BSD filesystem on
a 4.2BSD UNIX and vice-versa. It might be reasonable, if you really wanted to,
to put another filesystem type within 4.2BSD to allow it to read the 2.8BSD
filesystem. Another alternative is to try to run 4.1BSD on the MicrovaxII,
which has a very similar filesystem structure to 2.8BSD. Even with these two
possibilities, one runs into another problem (at this level): the PDP-11 C
compiler stores longs differently than the VAX C compiler (gotcha!). This
can be fixed by altering the PDP-11 C compiler. I believe Donn Seeley when
through this entire exercise to allow his 2.8BSD PDP-11's to access his 4.1BSD
VAX filesystems and vice-versa. You may ask him if he thinks it is worth it.
Moving 4.1BSD to the MicrovaxII sounds pretty stupid itself. Perhaps you should
consider System V on the MicrovaxII (which DEC may well offer). Well, this is
ridiculous enough, but...

	- You are also screwed because even if they were two identical machines
or filesystems, you could not mount them both read-write (and get away with it).
As soon as one UNIX wrote the filesystem, modifying inodes and the freelist, the
other would get mighty confused. Indeed, even mounting only one read-write is
not a good idea, though probably not fatal. You could devise some kind of
locking scheme to allow multiple read-write access. A lot of work. Just
having both read only would be okay.

	- Since the MicrovaxII is a Qbus machine, you will need a Qbus
controller from System Industries that is format compatible with and can dual
access with your Unibus SI controller. Most Qbus SMD controllers do heavy
interleaving (3:1), more so than Unibus controllers, thus making the two
not format compatible. Perhaps SI does, I don't know.

	All in all, not very feasible in my mind. A more reasonble tack IS to
network the two machines, using either 2.9BSD or Ultrix-11 V3 (which seems to
work quite well), on your PDP-11/70.

					Cheers,
					Dan Ts'o
					Dept. Neurobiology
					Rockefeller Univ.
					1230 York Ave.
					NY, NY 10021
					212-570-7671
					...cmcl2!rna!dan
					rna!dan at cmcl2.arpa