News Group Posts

Slow devices on SCSI bus

Newsgroups: comp.unix.sco.misc
From: bill@wjv.com.REMOVEME (Bill Vermillion)
Subject: Re: Problem running 2 Adaptec 3950U2 with SCO 5.0.5
Message-ID: <FIroFC.1xD3@wjv.com.REMOVEME> 
References: <37EFD39C.26639F0A@bytedesigns.com> 
Date: Tue, 28 Sep 1999 11:11:36 GMT

In article <37EFD39C.26639F0A@bytedesigns.com>, Heinz Wittenbecher
(at Pioneer) <heinz@bytedesigns.com> wrote:

>I'm trying to run 2 controllers so that I don't bring the first's
>Channel B down to narrow scsi speed (tape and cdrom) for the second
>HD.

You are making assumptions of how SCSI works - perpetated by
articles in the popular PC press and even in documentation in at
least on well known certification manual.

The bus slows down to the speed of the slowest device ONLY when
that device is communicating with another device on the same bus.

There can only be two devices on the bus at one time.  The devices
negotiate their highest common speed. For example, if you had
a SCSI I then the highest bus-speed would be 5MB/sec (and so many
of the earlier devices couldn't even touch that rate in data
transfer).  Connect that to one of the newest Ultra drives
with a 40MB/sec bus speed.

The data transfer will occur at the 5MB/sec speed.

When the slow device is finished, then perhaps another 40MB
device will want to exchange data with the one from above.

At this point bus is running at 40MB.

IOW - the bus does not slow down to the slowest device on the bus
EXCEPT when that device is one of two devices communicating.  The
bus speed changes depending on device connection.

If you put your slow device on another controller and want to
send data to the controller with the slower device, then you are
still limited by the slower device as to the rate of speed of the
data transfer - effectively slowing the output - not the bus speed
- of the device on the first controller.

In smaller systems you should not see any performance difference.
You increase the cost with no performance gain.  In larger systems
if you have many slower devices you could put in a controller for
the lower speed devices and reserve the ultra style controller
for several HD's.  For small systems, two or three HD's, that's
overkill IMO.


-- 
Bill Vermillion   bv @ wjv.com 

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