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Date:      Fri, 20 Oct 1995 12:12:16 +0930 (CST)
From:      Michael Smith <msmith@atrad.adelaide.edu.au>
To:        questions@freebsd.org
Subject:   Re: SCSI HD recommendation
Message-ID:  <199510200242.MAA17063@genesis.atrad.adelaide.edu.au>
In-Reply-To: <199510191817.LAA02975@phaeton.artisoft.com> from "Terry Lambert" at Oct 19, 95 11:17:51 am

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Terry Lambert stands accused of saying:
> > ERRR??? AV means ``Audio/Video'' (or something), and they do strange
> > things (probably have a whacking great big on-board cache) so that
> > when the drive auto-recalibrates it doesn't slow down any transfers
> > too much, so that digital audio/video applications won't notice the
> > disk going off and doing housekeeping.
> > 
> > Hence, they are normally (at least) 5% >>MORE<< than the ordinary
> > drives.
> 
> Not all AV implementations use cache.
> 
> Most, in fact, skip the thermal recalibration, reducing the MTBF for
> the drive.
> 
> As a matter of fact, I don't know of an AV drive that uses a cache
> to prevent thermal recalibration delays instead of skipping the
> recalibration entirely.

Most actually just support _interrupting_ the recalibration.  Not performing
thermal recal at all would result in a totally unusable drive. (Consider
the width of a track and the coefficient of expansion of aluminium over a 
60C operating range (being conservative))

> In my experience, AV drives are less reliable, a trade-off for real-time
> streaming response -- not a typical concern for your average non-AV user.

I can't comment there; AV drives are still at a premium here, but a lot
of "mainstream" drives seem to be offering interruptible recal as well now,
so I think the difference is closing rapidly.

> 					Terry Lambert

-- 
]] Mike Smith, Software Engineer        msmith@atrad.adelaide.edu.au    [[
]] Genesis Software                     genesis@atrad.adelaide.edu.au   [[
]] High-speed data acquisition and                                      [[
]] realtime instrument control          (ph/fax) +61-8-267-3039         [[
]] My car has "demand start" -Terry Lambert  UNIX: live FreeBSD or die! [[



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