Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!caen!umich!sharkey!fmsrl7!art-sy!news
From: chap@art-sy.detroit.mi.us (j chapman flack)
Newsgroups: comp.periphs.scsi
Subject: Re: Separate controllers to enhance performance?
Message-ID: <9105011118.aa03402@art-sy.detroit.mi.us>
Date: 1 May 91 15:19:12 GMT
References: <1991Apr26.174456.810@homecare.uucp>
Sender: chap@art-sy.detroit.mi.us (j chapman flack)
Reply-To: chap@art-sy.detroit.mi.us (j chapman flack)
Organization: Appropriate Roles for Technology
Lines: 48

In article <1991Apr26.174456.810@homecare.uucp> jessea@homecare.UUCP (Jesse W. Asher) writes:
>I'm not real familiar with SCSI drives so I have a question about their
>performance (actually it's on the controller).  When you run MFM or RLL,
 ...
>it wouldn't make any difference unless the drives were on separate
>controllers.  Does this apply to SCSI controllers as well or does it
>access drives simultaneously (or almost so)?

I'm not certain whether you meant to say "controller" or "host adapter" there.
Please forgive me if I assume you meant "host adapter," as it's a common
confusion.  The part of a SCSI subsystem that plugs into the host computer's
bus is the host adapter, and it handles the interface between the host's bus
and the SCSI bus.  A SCSI controller is a module that interfaces one (or
more) device(s) to a SCSI bus.  Notice the difference: with ST506 (using
whatever recording format: MFM, RLL, or other), the thing you plug into your
host bus is in fact the disk controller; with SCSI, it's the host adapter.
The SCSI disk controller is either: a module built into an embedded-SCSI
disk drive, or a standalone SCSI controller module, which can control
several non-intelligent drives (ST506, say) using one ID on the SCSI bus.

Since a SCSI bus can connect 8 devices, and one (at least) will be a host
adapter, you can attach seven other devices, each of which could be
  a) one embedded-SCSI drive, or
  b) several dumb drives on a standalone SCSI controller
  c) something else (lots of things talk SCSI).

Now to your question:  an embedded-SCSI controller can only control one drive
at a time, naturally (the one it's built into).  A standalone SCSI
controller may or may not actually perform operations concurrently on N of
its attached drives; probably depends on the controller.  The host adapter,
though, which I think is what you meant to ask about, spends very little
time waiting for anyone.  If it sends a message to controller 2 saying
"read me block 3456 from your drive #0", controller 2 will say "No problem.
It'll take me a few milliseconds to find the block--I'll let you know when
I'm ready."  Controller 2 then releases the bus and works on its own, while
the host can initiate other operations with other devices.  Eventually
controller 2 sends a message to the host saying "I found the data you wanted;
here it is."

So yes, you can almost certainly boost performance by judiciously spreading
files across different devices on a SCSI.

Hope this helped...
-- 
Chap Flack                         Their tanks will rust.  Our songs will last.
chap@art-sy.detroit.mi.us                                   -Mikos Theodorakis

Nothing I say represents Appropriate Roles for Technology unless I say it does.