Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!convex!mic!letni!rwsys!merch!cpe!adaptex!adaptx1!neese
From: neese@adaptx1.UUCP
Newsgroups: comp.periphs.scsi
Subject: Re: < Connecting Fujitsu M2266SA to SGI
Message-ID: <283400113@adaptx1>
Date: 16 May 91 03:46:46 GMT
References: <41530@unlisys.in-berlin.de>
Lines: 52
Nf-ID: #R:unlisys.in-berlin.de:41530:adaptx1:283400113:000:2636
Nf-From: adaptx1.UUCP!neese    May 15 11:40:00 1991


>/* Written  9:15 am  May 14, 1991 by utstat.UUCP!tg in adaptx1:comp.periphs.s */
>I too bought a 2266 in the hopes of connecting a BIG, FAST, and CHEAP
>drive to what is a relatively fast machine (4D220).
>
>The drive came configured from our vendor and it almost worked.
>A lot of hours later I had a drive that ran. But it ran SLOWWW.
>Top speed of about 900KB/sec. Way short of the rated 5MB/sec.
>
>>STUFF DELETED<<
>
>Still not satisfied I'm trying to get the very last drop of performance
>out of the drive. Why won't it run any faster than 900KB? The drive is
>rate for 2MB/sec in asynch mode. 
>
>A call to Fujitsu indicated that 
>	"in my configuration 1MB/sec was a good number." 
>
>The nuance of the phone call was that unless you had some VERY good
>SCSI equipement your aren't going to get anything near 2MB/sec.
>
>>STUFF DELETED<<

Bitten by the marketing bug again.  What is said or not said in the SCSI
brochures about devices is;  the continous data rate.  Although some of the
higher quality manufacturers are starting to put this into thier literature,
most do not.  While they will all virtually claim 2MB/sec async and 5MB sync,
if you look a little closer, you may find this data rate is *burst mode*
transfers.  Some won't even document this, but most do.
Another thing to realize is;  The effective data rate.  Specifying the burst or
continous data rate for the device may be measured only during the data
phase and will probably not include the other phases the SCSI bus goes
through which will lower the overall data rate.
The manufacturer of the peripheral cannot know what the effective throughput
is, this depends on all of the external variables they have no control over
(CPU speed, adapter type, driver, OS, SCSI bus implementation,...).
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
be about 900KB/sec.  Best case would be 1MB/sec, but there is some overhead
for setup on each byte, so it drops a bit.
If this drive is clocking data at 10Mhz, you are getting a decent throughput,
if not the best the drive can muster.
Oh and one other thing.  In SCSI, the terms BIG, FAST, and CHEAP are usually
contridictions in terms.  BIG and FAST are okay, but the third qualification
usually waters down the other two.

			Roy Neese
			Adaptec Senior SCSI Applications Engineer
			UUCP @  neese@adaptex
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