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From: jeremy@perf2.asd.sgi.com (Jeremy Higdon)
Newsgroups: comp.periphs.scsi
Subject: Re: < Connecting Fujitsu M2266SA to SGI
Message-ID: <105234@sgi.sgi.com>
Date: 21 May 91 00:43:57 GMT
References: <41530@unlisys.in-berlin.de> <283400113@adaptx1> <3415@travis.csd.harris.com>
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Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <3415@travis.csd.harris.com>, garyb@SSD.CSD.HARRIS.COM (Gary Barton) writes:
> 
> 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.

Two other things that will affect throughput for large transfers
are head switch time, and single track seek time.  I've seen quite
a difference between two drives that have 85 sectors per track and
spin at 3600 RPM (2.2 vs 2.55 MB/s).  If the vendor gives only the
raw media transfer rate, you also have to know about format efficiency.
Again, I've seen two 24mhz drives at 3600 rpm, that differed by 5%
(81 vs. 85 sectors per track).