linux-kernel - Re: adding proper O_SYNC/O_DSYNC, was Re: O

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Message-ID: <20090828164657.GB8036@shareable.org>
Date:	Fri, 28 Aug 2009 17:46:58 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: adding proper O_SYNC/O_DSYNC, was Re: O_DIRECT and barriers

Ulrich Drepper wrote:
> >  - O_RSYNC basically means we need to commit atime updates before a
> >    read returns, right?
> 
> No, that's not it.
> 
> O_RSYNC on its own just means the data is successfully transferred to 
> the calling process (always the case).
> 
> O_RSYNC|O_DSYNC means that if a read request hits data that is currently 
> in a cache and not yet on the medium, then the write to medium is 
> successful before the read succeeds.
> 
> O_RSYNC|O_SYNC means the same plus the integrity of file meta 
> information (access time etc).

On several unixes, O_RSYNC means it will send the read to the
hardware, not relying on the cache.  This can be used to verify the
data which was written earlier, whether by O_DSYNC or fdatasync.

-- Jamie
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