Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!ncar!gatech!mcnc!decvax!mandrill!nitrex!rbl
From: rbl@nitrex.UUCP ( Dr. Robin Lake )
Newsgroups: comp.arch
Subject: Re: Disk drives -- speed of?
Message-ID: <740@nitrex.UUCP>
Date: 6 May 88 14:09:44 GMT
References: <2746@sundc.UUCP> <76700012@uiucdcsp> <2759@sundc.UUCP> <887@gethen.UUCP>
Reply-To: rbl@nitrex.UUCP ( Dr. Robin Lake )
Organization: BP America Research and Development
Lines: 48
Summary: It's not the controller that's slow --- it's the device driver!

In article <887@gethen.UUCP> farren@gethen.UUCP (Michael J. Farren) writes:
>In article <2759@sundc.UUCP> bwong@sundc.UUCP (Brian Wong) writes:
>>
>>OK, here's the $64k question: why is it that a mechanical speed is being
>>bottled up by an electrical speed?  
>
>It's not, really.  The data rate coming off of the disk is a function
>of two things: the rate of speed of rotation of the disk, and the
>density of the bit packing on the media.  You could have the same data
>rates on a disk turning at 1 RPM, if you could pack the bits in there
>tighter.
>
>There's a couple of things operating here.  First, most disk controllers
>are limited in bandwidth.  This has to do, as much as anything, with
>the corresponding limitations in the I/O channel they are working with.
>Second, recording technology is also limited in bandwidth - this has
>to do with the fact that it takes time to cause a flux change in a
>magnetic medium.  While this can be improved, there are limits there.

As the co-designer of a solid-state disk product (Monolithic Systems EMU),
I've got some real data (courtesy of Sugit Kumar's Ph.D. dissertation).

1.  The controllers can keep up with any computer bus.

2.  Because almost all operating systems are designed for rotating disks
(vs. solid-state disks), the device drivers are the system I/O bottleneck
for very high speed devices.  Even tho the "EMU" had 1/17000 the rotational
latency of the DEC RF/RS-11 it replaced, the 1,000 microseconds or so that
each call to the device driver took reduced the effective I/O performance
to about 17 times the rotating equivalent.  Note that both RF/RS-11 and
EMU are Word-Addressable, so blocking wasn't an issue.

3.  There appear to be systems under development that move the I/O
overhead into the memory management unit and device controllers, letting
the CPU use very simple 1 - 5 instruction I/O commands.  This can result
in a uniform "Name Space" for the entire system, from main memory through
disk, tape and networks.  With RISC and cache taking essentially all delays
out of the CPU, and (for the sake of an acryonym  "Uniform Name Space")
taking essentially all delays out of the I/O, performance should be
OUTSTANDING!

DISCLAIMER:  This information has nothing to do with my employer.  In fact,
this work is a decade old and I haven't been here that long!
-----
-- 
Rob Lake
BP America Research and Development
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