Re: O_DIRECT question

[Denis Vlasenko]

On Friday 26 January 2007 19:23, Bill Davidsen wrote:
> Denis Vlasenko wrote:
> > On Thursday 25 January 2007 21:45, Michael Tokarev wrote:
> >> Phillip Susi wrote:
> >>> Denis Vlasenko wrote:
> >>>> You mean "You can use aio_write" ?
> >>> Exactly.  You generally don't use O_DIRECT without aio.  Combining the
> >>> two is what gives the big win.
> >> Well, it's not only aio.  Multithreaded I/O also helps alot -- all this,
> >> say, to utilize a raid array with many spindles.
> >>
> >> But even single-threaded I/O but in large quantities benefits from O_DIRECT
> >> significantly, and I pointed this out before.
> > 
> > Which shouldn't be true. There is no fundamental reason why
> > ordinary writes should be slower than O_DIRECT.
> > 
> Other than the copy to buffer taking CPU and memory resources.

It is not required by any standard that I know. Kernel can be smarter
and avoid that if it can.
--
vda
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