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From: info-vax@ucbvax.ARPA
Newsgroups: fa.info-vax
Subject: Re: UNIBUS disks...
Message-ID: <3253@ucbvax.ARPA>
Date: Tue, 13-Nov-84 14:09:41 EST
Article-I.D.: ucbvax.3253
Posted: Tue Nov 13 14:09:41 1984
Date-Received: Thu, 15-Nov-84 02:36:47 EST
Sender: daemon@ucbvax.ARPA
Organization: University of California at Berkeley
Lines: 42

From: engvax!KVC@cit-vax

I have to say I agree completely with Richard Garland.  The DSA
disks on the UDA-50 are a low-end alternative to what DEC really
wants to provide.  The hypothetical 785 that was posed in an earlier
message could be outfitted with a UDA and DSA disks now and later
upgraded quite easily to a CI based system.  I would porobably buy
the CI now, however, and skip the UDA-50.  I've watched system loads
grow too many times in the past to underestimate the usefulness
of the cluster concept.   I also agree that VMS manages the UNIBUS
adequately enough for a low-end system to survive with a UDA.  I
would argue, however, that someone trying to get the best performance
possible out of the 785 should be concerned about the growth of the
system in the future.  The CI gives you an easy upgrade by allowing
you to add more processors as you need them.  Of course, the system
posed hypothetically is going to run UNIX, in which case you have no
cluster alternative.  I think with UNIX I would stick to the MASSBUS
since UNIX tends to favor proven hardware.

My system right now has MASSBUS Eagles on an SI 9900 controller and an
RP07.  All very fast MASSBUS disks.  I find, however, that my system is
completely out of CPU power, while only using something like 10% of
it's disk bandwidth.  I would love to be able to plug in another VAX
and split my user load without having to purchase any more disks
controllers.  (To be fair, a cluster WOULD allow me to use MASSBUS
disks on any system in the cluster from any processor in the cluster
via the MSCP server, but I know I would have to buy at least a system
disk dedicated each additional CPU if I did not have an HSC).

One final note: for a very thorough discussion of UDA, HSC, and MASSBUS
performance, take a look at the article:

	Disk System Lantency in VAX/VMS

by Richard Wrenn and Mark Freeman in the Spring '84 proceedings of the
Digital Computer Users Society (DECUS).  Although the HSC and cluster
performance would be meaningless to a UNIX shop, you should take a look
at the UDA vs MASSBUS figures.  This is probably the best discussion of
disk performance I have seen so far.

	/Kevin Carosso                engvax!kvc @ CIT-VAX.ARPA
	 Hughes Aircraft Co,