linux-kernel - Re: O_DIRECT question

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Date:	Thu, 25 Jan 2007 14:28:15 -0500
From:	Phillip Susi <psusi@....rr.com>
To:	Denis Vlasenko <vda.linux@...glemail.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

Denis Vlasenko wrote:
> 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?

Because you can do writes async, but not fdatasync ( unless there is an 
async version I don't know about ).

> 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?


I think all processes yes, but certainly all writes to this file by this 
process.  That means you have to sync for every write, which means you 
block.  Blocking stalls the pipeline.

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