Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 8/7/84; site ucbvax.ARPA Path: utzoo!linus!decvax!decwrl!ucbvax!info-vax From: info-vax@ucbvax.ARPA Newsgroups: fa.info-vax Subject: Re: Unibus disks Message-ID: <3218@ucbvax.ARPA> Date: Mon, 12-Nov-84 17:14:17 EST Article-I.D.: ucbvax.3218 Posted: Mon Nov 12 17:14:17 1984 Date-Received: Tue, 13-Nov-84 06:35:13 EST Sender: daemon@ucbvax.ARPA Organization: University of California at Berkeley Lines: 27 From: Jerry Wolf Here, 2nd hand, is the answer I got when I asked a Deccie from their Storage Systems Group "why is DEC pushing the Unibus disks now? Why aren't the Massbus disks better?" When the VAX was designed, they needed a way to accomodate the instantaneous (intermittent) bursts from a disk (as high as 2.2 MB/s from an RP07) -- hence the Massbus, a big fast dumb pipe that's idle most of the time, between disk blocks. Also, they're so dumb that if multiple disks are ready, only one at a time can be active. Technology marches on, specifically enter cheap fast static RAMs and 2901 bitslice microprocessors, which made it feasible to build the UDA50 controller that can buffer the data off the disk at disk rates and send it to the CPU at Unibus rates, and handle 4 drives at once. The average throughput is held to be comparable to the Massbus (in most cases) -- the saving comes from not having to accomodate the high burst rate. Also, the UDA50 is supposed to be smart enough to reorder requests to multiple drives in order to minimize latency and maximize throughput, etc. This claim of better functionality through more advanced technology seems borne out by the fact that Unibus disks are a fair bit cheaper than Massbus, and actual thruput measurements (not just maximum transfer rates) are comparable in most cases, tho there are pathological applications where a Massbus disk can pump data faster.