Date: Sun, 24 Nov 1996 14:54:53 +1030 (CST) From: Michael Smith <msmith@atrad.adelaide.edu.au> To: terry@lambert.org (Terry Lambert) Cc: joelh@gnu.ai.mit.edu, grog@lemis.de, terry@lambert.org, chat@FreeBSD.org Subject: Re: ATAPI (was: Who needs Perl? We do!) Message-ID: <199611240424.OAA24031@genesis.atrad.adelaide.edu.au> In-Reply-To: <199611232121.OAA19464@phaeton.artisoft.com> from Terry Lambert at "Nov 23, 96 02:21:52 pm"
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Terry Lambert stands accused of saying: > > What are these A/V drives I see nowdays? Are these just > > standard-issue SCSI drives trying to get on the 'multimedia' > > bandwagon, or is there really something else to them? > > They are standard drives which do not have an off cycl;e for thermal > recalibration. This is not strictly true. > This makes them faster to dump an incoming stream of "A/V" data > and turn around for more data, but it makes them much more sensitive > to thermal variance. An incorrect conclusion, a conjunction and a falshehood. > If you have a machine you leave on all the time, and you scsiformat > after it has reached thermal equilibrium, and never remount after a > crash until it is, again, at thermal equilibrium, and you maintain > a standard thermal profile with consistent ventilation to a controlled > environment in which the machine is placed, you can use them all day > with no difference, except they are slightly faster over a bursty short > haul. This is derived from dream-delirium facts, and should be tossed out before either baby or bathwater. > If you don't do any of these things, they are slightly faster over a > bursty short haul, but they have a *significantly* decreased MTBF. This was probably derived from the above observations, but may be true regardless. It is certainly speculation. Some theory : - harddisk platters are made of aluminium. - aluminium expands and contracts corresponding to temperature. - harddisks generate quite a lot of heat. - disk control logic uses a variety of techniques for getting the head to the right place on the disk as quickly as possible. One of these techniques calls for guesstimating "about where" to sling the head before actually looking at the disk to see where it is. - in order to compensate for the expansion/contraction of the platters, the drive logic performs a periodic operation known as "thermal recalibration", where it hops the head across the disk comparing where it thinks the head should have landed with where it actually _did_ land, and updating it's idea of what is where accordingly. - in 'conventional' disk drives, this process is uninterruptible, and can take hundreds of milliseconds. So, to deal with the "AV" crowd, whose hardware often can't handle being starved of data for several hundred ms, drive manufacturers made the recalibration process interruptible, so that data operations continue and recalibration occurs in the "background". Even with the use of a servo surface, it is not practical to abandon thermal recalibration at all. > Terry Lambert -- ]] Mike Smith, Software Engineer msmith@gsoft.com.au [[ ]] Genesis Software genesis@gsoft.com.au [[ ]] High-speed data acquisition and (GSM mobile) 0411-222-496 [[ ]] realtime instrument control. (ph) +61-8-8267-3493 [[ ]] Unix hardware collector. "Where are your PEZ?" The Tick [[
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