Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!nuchat!moray!siswat!buck
From: buck@siswat.UUCP (A. Lester Buck)
Newsgroups: comp.periphs
Subject: Re: Synchronous SCSI *Disks*
Message-ID: <432@siswat.UUCP>
Date: 28 Jul 89 02:59:41 GMT
References: <29254@ames.arc.nasa.gov> <257@odin.SGI.COM> <29289@ames.arc.nasa.gov>
Organization: Photon Graphics,  Houston
Lines: 37

In article <29289@ames.arc.nasa.gov>, lamaster@ames.arc.nasa.gov (Hugh LaMaster) writes:
> >that are 20 Mhz drives and can deliver sustained 2.1Mb/sec
> >on read, and 1.9Mb/sec on writes, when doing i/o on contiguous
> 
> >Some of these drives actually can deliver bursts from/to their
> >buffer at 5Mb/sec as measured with a logic analyzer, for 32K to 64K
> 
> I realize that the 4-5 MB/sec. is the SCSI bus rate, but, I guess what I am
> asking is this: IBM, CDC, and more recently, SMD-E drives have been around,
> (some of these for years) with speeds >= 3.0 MB/sec.  CDC and
> Ibis have produced drives with data rates of 10-12 MB/sec 
> (I know, parallel heads).  The need and technology are clearly there, 
> though not necessarily in the same place (price/performance-wise).

If you really want to, just take an SMD drive at 3.0 MB/sec and run it
through the NCR ADP-48 SCSI to SMD synchronous controller for a top speed of
4 MB/sec across the bus (minimum synchronous period 250 ns).  This is the
fastest SCSI disk subsystem which I have heard of - please let me know if
anyone can configure something faster.

> I believe that there would be probably be a market for a dual-parallel-head
> drive which would effectively operate at 48MHz (2x24) and which could keep
> a 5 MByte/sec SCSI bus busy.  About 4 of these would turn one of the new
> 20 MIPS workstations from various companies into a real "CDC 7600 on a desk".

But if you keep the SCSI bus busy (continuously), you have removed a major
reason to use SCSI in the first place.  You now cannot add any more drives
on this bus without a disastrous performance hit.  Although the programming
would not be as easy as with SCSI, you might as well have a host bus to SMD
controller directly for each drive.  To effectively and continuously use
drives that have sustained transfer rates of several MB/sec will require
either the "wide" SCSI or "fast" SCSI (or both) parts of SCSI-2.  Of course,
few host buses can sustain such data rates (and especially not four at
once!), so this is all rather acedemic.

-- 
A. Lester Buck		...!texbell!moray!siswat!buck