From owner-freebsd-stable  Sun Sep 23  8:47:11 2001
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To: "Chad R. Larson" <chad@DCFinc.com>
Cc: "Daniel O'Connor" <doconnor@gsoft.com.au>,
	j mckitrick <jcm@FreeBSD-uk.eu.org>, freebsd-stable@FreeBSD.ORG,
	Nuno Teixeira <nuno.mailinglists@pt-quorum.com>,
	"David W. Chapman Jr." <dwcjr@inethouston.net>
Subject: Re: hw.ata.wc && hw.ata.tags && softupdates short question 
In-reply-to: Your message of "Sat, 22 Sep 2001 18:20:52 PDT."
             <20010922182052.B16388@freeway.dcfinc.com> 
Date: Sun, 23 Sep 2001 08:44:11 -0700
From: "Kevin Oberman" <oberman@es.net>
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Chad,

Many older removable SMD drives had enough momentum to keep spinning
for minutes and used a dummy load across the motor to stop in a
reasonable time.

The big issue with these drives was parking the massive voice coil
mounted heads stack. This took a LOT of energy and CDC drives used the
capacitor bank to do the job, but some smaller drives (Diablo,
Century) did use the motor as a generator.

Of course, this was not an attempt to flush 2 MB of cache. They only
had 512 bytes of cache to contain one sector of data and this was
written out before the heads retracted on most disks. These drives
lacked a smart controller and any drive optimization was entirely done
in the driver.

Modern ATA disks are tiny. The momentum in a spinning disk is also
tiny and I doubt you could get enough energy to flush the entire cache
with multiple seeks and many rotations at near full speed required to
do the job. I strongly suspect that capacitance is the only game in
town.

R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net			Phone: +1 510 486-8634


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