Subject: Re: RAW access to files
To: Ignatios Souvatzis <>
From: Arne H Juul <>
List: tech-kern
Date: 12/12/2001 00:54:31
On Tue, 11 Dec 2001, Ignatios Souvatzis wrote:
 > On Mon, Dec 10, 2001 at 07:42:07PM +0100, Wojciech Puchar wrote:
 > > > this is more often called "direct i/o" or "unbuffered i/o".
 > > > it's on the list of things I want to add, but I haven't had time yet.
 > > 
 > > it would be really good! with support in common mpeg/divx players etc..
 > > and with option in cp and dd
 > Why do you think that "raw" I/O would be more efficient than the filesystem
 > + buffer cache doing readahead? 

Because that's what benchmarks and real production use of some
applications with and without direct io show :-)  I don't think
"cp" is such a good example though, since you lose the readahead.
For some (typically database-like) apps it can be a pretty big boost.

Disclaimer:  We've actually *measured* the performance difference
for our app only on FreeBSD, Windows, Solaris, and Linux, AFAIK.

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