From owner-freebsd-current  Sun Jun 16 21:00:20 1996
Return-Path: owner-current
Received: (from root@localhost)
          by freefall.freebsd.org (8.7.5/8.7.3) id VAA10492
          for current-outgoing; Sun, 16 Jun 1996 21:00:20 -0700 (PDT)
Received: from freefall.freebsd.org (localhost.cdrom.com [127.0.0.1])
          by freefall.freebsd.org (8.7.5/8.7.3) with ESMTP id VAA10485;
          Sun, 16 Jun 1996 21:00:18 -0700 (PDT)
Message-Id: <199606170400.VAA10485@freefall.freebsd.org>
To: Beck Peccoz Amedeo <gea@masternet.it>
cc: freebsd-current@FreeBSD.ORG
Subject: Re: ahc syncing questions 
In-reply-to: Your message of "Sun, 16 Jun 1996 16:24:21 +0200."
             <31C41915.41C67EA6@masternet.it> 
Date: Sun, 16 Jun 1996 21:00:18 -0700
From: "Justin T. Gibbs" <gibbs@freefall.freebsd.org>
Sender: owner-current@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

>Justin T. Gibbs wrote:
>
> I've got a Seagate wide HD (ST32550W) acceptinc max sync transfer
>rate of 20 Mbytes/s (i.e. 10MHz).

Don't confuse the issue.  It syncs at a clock rate measured in MHz
not MB/s.

>If I set the ultra speed on the adapter
>(20MHz or 40Mb/s) it still syncs at 10MHz. So far so good.

Right since it can't do 20MHz.  If you have an old version of the
driver, you may actually be syncing at 5MHz though.  I fixed the
Ultra bug about a month or so ago.

> A problem arises when I run a benchmark, a very silly one, that does
>only read a big unfragmented file, as I get a transfer rate of only 7.4
>Mb/s, while I read of a test on the same drive/adapter leading to a transfer
>rate of 16Mb/s, more than twice.

Ha.  That drive will not get anywhere near 16MB/s.  16Mb/s I might believe
(2MB/s).  To figure out the maximum you can expect from the drive, you
need to look at media transfer rates, not the sync rate.  A quick
calculation using the RPM of the drive and its media density should get you
what you want.  Remember that the driver transfers short bursts of data
across the SCSI bus at the negotiated sync rate, but in most cases it
cannot sustain that rate throughout the entire transfer.

>-- 
>
>Beck-Peccoz Amedeo
>GEA Software S.r.l.
>Via Deffeyes, 1
>11025 Gressoney Saint Jean (AO)
>ITALY
>
>Tel. ++39-125-366302 Fax. ++39-125-366415
>
>

--
Justin T. Gibbs
===========================================
  FreeBSD: Turning PCs into workstations
===========================================