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From: telam@pyrps5.pyramid.com (Thomas Elam)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Hard disk transfer rates
Message-ID: <126212@pyramid.pyramid.com>
Date: 8 Sep 90 02:29:01 GMT
Sender: daemon@pyramid.pyramid.com
Reply-To: telam@pyrps5.pyramid.com (Thomas Elam)
Organization: Pyramid Technology Corp., Mountain View, CA
Lines: 52

In article <126198@pyramid.pyramid.com> telam@pyrps5.pyramid.com (Thomas Elam) writes:

[I'm adding to my own follow-up.]

>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.

I'm sorry.  I was wrong.  I asked a guru here.  The "fast synchronous
data transfers" of SCSI use a clock that sets the transfer rate
somewhere between 4 and 5 Megabytes per second, not faster or slower.
For example, an HP SCSI drive we have here is specified to have fast
synchronous data transfer rate of 4 MB.  I think it would require an
incredibly fast microcode-software-programmed controller to sustain a 4
MB transfer rate:  not just the software would have to be efficient,
but the controller's CPU and other hardware would have to be fast.

>>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

Tom