Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!rex!uflorida!travis!garyb
From: garyb@SSD.CSD.HARRIS.COM (Gary Barton)
Newsgroups: comp.periphs.scsi
Subject: Re: < Connecting Fujitsu M2266SA to SGI
Message-ID: <3415@travis.csd.harris.com>
Date: 19 May 91 00:17:15 GMT
References: <41530@unlisys.in-berlin.de> <283400113@adaptx1>
Sender: news@travis.csd.harris.com
Organization: Harris Computers Systems Division, Fort Lauderdale,FL
Lines: 41

In article <283400113@adaptx1> neese@adaptx1.UUCP writes:
>...
>You can figure out the continous data rate yourself, by looking at the clock
>rate the data is being transferred to/from the media.  The continous data
>rate cannot be greater than the clock rate of the head.  If the clock rate
>of the head is 10Mhz, the continous data rate over the SCSI bus will usually
>...

There is a much easier way to figure out what the approximate
sustainable transfer rate of a disk drive is, but you have to know
something about the geometry of the disk drive.  An upper bound on the
sustainable transfer rate of any disk drive is given by T:

	           RPM
 T (bytes/sec)  =  --- tracks/sec  *  N Sectors/track  *  M bytes/Sector
	           60

So if the drive manual does not describe the sustained transfer rate
of the drive, but does specify the rotational rate, number of tracks
per sector, and sector size, the sustained transfer rate can be
derived.  This assumes that the imbedded SCSI interface can maintain
at least this rate.  If it cannot, calculating the sustainable rate is
much more complicated because you now have have to account for an
extra rotational latency every time the disk heads get more than the
drives internal buffer size ahead of the SCSI interface.  Ggenerally
speaking, most disk drive vendors supply embedded SCSI controllers
that can keep up with the media (at least using SCSI synchronous
transfers), so the computation above is usually a good approximation.

Of course drives that have variable geometries will have variable
sustained transfer rates depending on where you happen to be
transferring to/from on the disk.

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| Gary Barton                    | System Software Development      |
| Lead Engineer			 | Harris Computer Systems Division |
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