linux-kernel - Re: O_DIRECT question

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Message-Id: <200701252052.38119.vda.linux@googlemail.com>
Date:	Thu, 25 Jan 2007 20:52:38 +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 20:28, Phillip Susi wrote:
> > 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.

I dont understand you here. Suppose fdatasync() is "do not return until
all cached writes to this file *done by current process* hit the disk
(i.e. cached write data from other concurrent processes is not waited for),
report succes or error code". Then

write(fd_O_DIRECT, buf, sz) - will wait until buf's data hit the disk

write(fd, buf, sz) - potentially will return sooner, but
fdatasync(fd) - will wait until buf's data hit the disk

Looks same to me.

> > 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 ).

You mean "You can use aio_write" ?
--
vda
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