linux-kernel - Re: O_DIRECT question

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Message-Id: <200701251838.30796.vda.linux@googlemail.com>
Date:	Thu, 25 Jan 2007 18:38:30 +0100
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Michael Tokarev <mjt@....msk.ru>,
	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 Thursday 25 January 2007 16:44, Phillip Susi wrote:
> Denis Vlasenko wrote:
> > I will still disagree on this point (on point "use O_DIRECT, it's faster").
> > There is no reason why O_DIRECT should be faster than "normal" read/write
> > to large, aligned buffer. If O_DIRECT is faster on today's kernel,
> > then Linux' read()/write() can be optimized more.
> 
> Ahh but there IS a reason for it to be faster: the application knows 
> what data it will require, so it should tell the kernel rather than ask 
> it to guess.  Even if you had the kernel playing vmsplice games to get 
> avoid the copy to user space ( which still has a fair amount of overhead 
> ), then you still have the problem of the kernel having to guess what 
> data the application will require next, and try to fetch it early.  Then 
> when the application requests the data, if it is not already in memory, 
> the application blocks until it is, and blocking stalls the pipeline.
> 
> > (I hoped that they can be made even *faster* than O_DIRECT, but as I said,
> > you convinced me with your "error reporting" argument that reads must still
> > block until entire buffer is read. Writes can avoid that - apps can do
> > fdatasync/whatever to make sync writes & error checks if they want).
> 
> 
> fdatasync() is not acceptable either because it flushes the entire file.

If you opened a file and are doing only O_DIRECT writes, you
*always* have your written data flushed, by each write().
How is it different from writes done using
"normal" write() + fdatasync() pairs?

>   This does not allow the application to control the ordering of various 
> writes unless it limits itself to a single write/fdatasync pair at a 
> time.  Further, fdatasync again blocks the application.

Ahhh shit, are you saying that fdatasync will wait until writes
*by all other processes* to thios file will hit the disk?
Is that thue?

--
vda
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