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Date:      Wed, 14 Mar 2001 08:36:47 -0800 (PST)
From:      "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net>
To:        gibbs@scsiguy.com (Justin T. Gibbs)
Cc:        bright@wintelcom.net (Alfred Perlstein), Helge.Oldach@de.origin-it.com (Helge Oldach), oberman@es.net, sos@freebsd.dk, mobile@FreeBSD.ORG, stable@FreeBSD.ORG
Subject:   Re: Disk I/O problem in 4.3-BETA
Message-ID:  <200103141636.IAA47422@gndrsh.dnsmgr.net>
In-Reply-To: <200103141405.f2EE5Ks71827@aslan.scsiguy.com> from "Justin T. Gibbs" at "Mar 14, 2001 07:05:20 am"

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> >First off, some disk caches are getting > 10megs, that's a lot
> >of potnetial seeking after loosing power depending on the cache
> >contents...
> 
> I've heard that most modern drives reserve a contiguous area of the
> disk the size of the cache near where the heads park to dump any
> cache contents on power outage.  This avoids most if not all seeking.
> When the disk powers up again, the reserve track is read and the
> transactions are written to the correct locations.  Any disk that does
> this should be safe to use with write caching enabled.

As some one who has spent some time working with disk drive manufactures
in the far past all sorts of work is done to make sure that the write
cache is not a failure mechanism for lost data.

Some of the designs include things like using the spindle motor as
a generator and the inertia of the platters to drive it to insure the
drive has adaquate power for a long enough period to flush any cached
data.

Other things done are in non-segmented write caches is to only cache
write data for the current track (this is actually where the write
cache does most of its good) and always flush it before doing any
seek.

Only broken WC designs loose data in a power out sitation.  There are
many broken designs out there.  There are also many that work perfectly.
The general tell tell on this is what state the WCE bit is in when the
drive comes from the factory.  Those manufactures who have good designs
tend to ship with WCE on, those that tend to loose data always ship with
WCE off.

With the wonderful breakthroughs in supercapacitors it should now be
possible to have some very large caches.

-- 
Rod Grimes - KD7CAX @ CN85sl - (RWG25)               rgrimes@gndrsh.dnsmgr.net

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