Re: O_DIRECT performance impact on 2.4.18 (was: Re: [PATCH] 2.5.14 IDE 56)

[Jens Axboe]

On Sat, May 11 2002, Roy Sigurd Karlsbakk wrote:
> On Friday 10 May 2002 17:55, Linus Torvalds wrote:
> > On Fri, 10 May 2002, Lincoln Dale wrote:
> > > so O_DIRECT in 2.4.18 still shows up as a 55% performance hit versus no
> > > O_DIRECT. anyone have any clues?
> >
> > Yes.
> >
> > O_DIRECT isn't doing any read-ahead.
> >
> > For O_DIRECT to be a win, you need to make it asynchronous.
> 
> Will the use of O_DIRECT affect disk elevatoring?

No, the I/O scheduler can't even tell whether it's being handed
O_DIRECT buffers or not.

Jens