linux-kernel - Re: O_DIRECT question

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Date:	Sun, 28 Jan 2007 18:03:08 +0100
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	Bill Davidsen <davidsen@....com>
Cc:	Michael Tokarev <mjt@....msk.ru>, Phillip Susi <psusi@....rr.com>,
	Linus Torvalds <torvalds@...l.org>, Viktor <vvp01@...ox.ru>,
	Aubrey <aubreylee@...il.com>, Hua Zhong <hzhong@...il.com>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	hch@...radead.org, kenneth.w.chen@in
Subject: Re: O_DIRECT question

On Sunday 28 January 2007 16:18, Bill Davidsen wrote:
> Denis Vlasenko wrote:
> > 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:
> 
> 	[...]
> 
> >>>> 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.
> 
> Actually, no, the whole idea of page cache is that overall system i/o 
> can be faster if data sit in the page cache for a while. But the real 
> problem is that the application write is now disconnected from the 
> physical write, both in time and order.

Not in O_SYNC case.

> No standard says the kernel couldn't do direct DMA, but since having 
> that required is needed to guarantee write order and error status linked 
> to the actual application i/o, what a kernel "might do" is irrelevant.
> 
> It's much easier to do O_DIRECT by actually doing the direct i/o than to 
> try to catch all the corner cases which arise in faking it.

I still don't see much difference between O_SYNC and O_DIRECT write
semantic.
--
vda
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