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From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Hard disk transfer rates
Message-ID: <1938@lpami.wimsey.bc.ca>
Date: 7 Sep 90 14:31:22 GMT
Lines: 57
Return-Path: <lpami!lphillips>
To: van-bc!rnews

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

The discussion was confused, to say the least. Here are some of the salient
points.

The A3000 SCSI controller is capable of handling 3.5 MegaBYTES/second to/from
memory.

The SCSI bus itself is capable of handling about 5 MegaBYTES/second.

Currently, the ultimate limitation (ie.  sustained throughput) is limited by
the rate at which data comes off the disk.

Not all SCSI disks are limited to the figures posted in the message you replied
to.

Some (a very few) expensive SCSI disks can transfer data at sustained rates of
about 3 MegaBYTES/second.

Some SCSI disks use fairly large buffers, and this gives them a short-term
transfer rate of 4.5-5 MegaBYTES/second.

The best DiskSpeed performance figures I have seen on an A2000 are about 1.5
MegaBYTES/second.

-larry

--
It is not possible to both understand and appreciate Intel CPUs.
    -D.Wolfskill
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|   //   Larry Phillips                                                 |
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