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From: telam@pyrps5.pyramid.com (Thomas Elam)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Hard disk transfer rates
Message-ID: <126198@pyramid.pyramid.com>
Date: 7 Sep 90 23:21:11 GMT
Sender: daemon@pyramid.pyramid.com
Reply-To: telam@pyrps5.pyramid.com (Thomas Elam)
Organization: Pyramid Technology Corp., Mountain View, CA
Lines: 37

In article <1990Sep1.231510.10650@agate.berkeley.edu> c60c-1gd@e260-1c (Joon Song) writes:
>I'm sorry if this has been discussed before.

I hope I'm not violating any rules of n-et-iquette in my follow-up.
Could someone please tell by electronic mail to me if I am.

>I've read several articles
>claiming that hard disk transfer rates through a SCSI port is as high as
>3.5 meg/sec.  Is this the actual throughput of the hard disk system?  

I was confused too, until I remembered seeing disk I/O throughput
measurements on our Pyramid mainframe computers.  They were around 400
Kilobyte per second or higher, measured by user-level software, so this
*is* the throughput of the hard disk system.  This works out to be 3.2
Megabits per second (400,000 x 8 = 3,200,000) or higher.  I believe
some people thought SCSI systems have throughputs of 4 Mega-*bytes*
per second.  I'm not on the SCSI committee, but I'll bet an Amiga that
the throughputs are really about 4 Mega-*bits* per second.

>Does this involve data being cached?  3.5 meg/sec is considerably faster
>than what I had thought possible for hard disk r/w.
>
>Suppose a hard disk had 34 sectors/track.  A hard disk spins 60 rev/sec.
>So the fastest disk transfer rate should be:
>    34 sectors/track * 512 bytes/sector * 60 rev/sec = approx. 1 meg/sec.
>
>Can someone explain how 3.5 meg/sec transfer rate can be possible?

I can't address myself to your other questions, but I have seen
calculations made by UNIX-guru-grade I/O software engineers that work
like your calculation.  And I think the numbers are right, too.

>Joon Song
>c60c-1gd@web.berkeley.edu

Regards,
Tom