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

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Date:	Sun, 30 Aug 2009 17:58:34 +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:
> On 08/28/2009 09:44 AM, Jamie Lokier wrote:
> >(Oh, and Ulrich: Why is there a "#define O_RSYNC O_SYNC" in the Glibc
> >headers?  That doesn't make sense: O_RSYNC has nothing to do with
> >writing.)
> 
> O_SYNC is a superset of O_RSYNC.  In the absence of a true O_RSYNC 
> that's the next best thing.

That's an error - O_SYNC is not a superset of O_RSYNC.

O_SYNC (by itself) only affects writes.

O_RSYNC only affect reads.

In the absence of O_RSYNC support in the kernel, it's better to not
define O_RSYNC at all in userspace.  That tells applications they can
call fsync/fdatasync themselves before reading to get an equivalent
effect.

In fact O_RSYNC, when implemented correctly, can be used by
applications to get the effect of range-fsync/fdatasync when such
system calls aren't available (by reading a range), but not as
efficiently of course.  Defining O_RSYNC as O_SYNC fails to do that.

-- Jamie
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