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From"Petrucci, Joseph" <>
SubjectRE: lack of raw disk devices
DateTue, 6 Jan 1998 08:16:46 -0600
Wouldn't this solution be just as difficult to implement as Raw
devices???

> -----Original Message-----
	>From:	linux kernel account [SMTP:linker@nightshade.z.ml.org]
	>Sent:	Monday, January 05, 1998 8:51 PM
	>To:	pmonta@imedia.com
	>Cc:	jhohertz@golden.net; h.milz@seneca.muc.de;
linux-kernel@vger.rutgers.edu
	>Subject:	Re: lack of raw disk devices>
	>.
	>On Mon, 5 Jan 1998, Peter Monta wrote:>
	>
	>> A raw device doesn't go through the buffer cache.  For
example, if you
	>> issue two reads for the same block, a raw device will
physically go
	>> out to disk each time, whereas a buffered device will satisfy
the
	>> second request from the cache.
	>> 
	>> I've read all of the discussion I can find about this, in
linux-kernel
	>> archives and elsewhere, and while I don't want to beat a dead
horse, I
	>> disagree with those who say raw devices are useless or
intrinsically
	>> bad.  Granted, the vast majority of the time a buffered
device is the
	>> right thing.  But I have one application in mind that cries
out for a
	>> raw device---it involves moving digital video as fast as
possible from
	>> disk to DRAM (and subsequently out to a PCI bus master).  The
system
	>> will never again need to refer to this data, so a buffered
device both
	>> incurs a copying expense (from buffer cache to DRAM buffer)
and
	>> trashes the buffer cache to no purpose.
	>
	>They are not bad, however, the implimentation would be
sufficently
	>difficult and cumbersom, to mostly out weigh the benifits.. In
the future,
	>it would be nice if you could adjust the buffercache
characteristics on a
	>device by device basis and potentiall on each group of block
reads
	>(wouldn't it be neat if ext2fs could be told by the app 'this
is a mpg
	>movie dont cache it' while it's accessed, while the rest of the
device
	>would be cached normally)..
	>
	>> 
	>> Cheers,
	>> Peter Monta   pmonta@imedia.com
	>> Imedia Corp.
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