From owner-freebsd-questions  Wed Jul  4 17:19:27 2001
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Date:	Wed, 4 Jul 2001 20:18:51 -0400
From: Kevin Hui <khui@cs.toronto.edu>
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To: "Kenneth D. Merry" <ken@kdm.org>
Cc: freebsd-questions@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG
Subject: Re: Raw disk access in userland
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Then the question is whether the kernel is copying data between userspace
and kernelspace or whether it just DMAs the data straight in/out of the
user program's address space.  In Linux raw-io, given that it is a block
device and you are doing page-aligned block I/Os on it, is smart and does
zero copies.  While it may seem to be jumping through hoops, maybe it does
have a performance advantage?

Regards,
-Kevin.

On Wed, 4 Jul 2001, Kenneth D. Merry wrote:

> On Wed, Jul 04, 2001 at 17:58:58 -0400, Kevin Hui wrote:
> > Hi all,
> > 
> > I'm wondering if *BSD have any support for user programs to access raw
> > disk devices, unbuffered.  Such support is available under Linux:
> > 
> > http://www.kernel.org/LDP/HOWTO/SCSI-2.4-HOWTO/rawdev.html
> > http://www2.linuxjournal.com/cgi-bin/linux/man.cgi?mode=search&comm=raw&sect=1
> > 
> > Any information regarding this would be appreciated.
> 
> There are no block (i.e. buffered) devices in FreeBSD, only character
> devices.
> 
> So you can access raw devices without having to go through all the hoops
> described above.
> 
> Ken
> -- 
> Kenneth Merry
> ken@kdm.org
> 


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