linux-kernel - Re: atomically swap two files

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Message-ID: <20101126141911.GJ12187@dastard>
Date:	Sat, 27 Nov 2010 01:19:11 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Pádraig Brady <P@...igBrady.com>
Cc:	"Frank A. Kingswood" <frank@...gswood-consulting.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: atomically swap two files

On Fri, Nov 26, 2010 at 11:47:52AM +0000, Pádraig Brady wrote:
> On 25/11/10 20:11, Frank A. Kingswood wrote:
> > Hi!
> > 
> > Is it possible to swap two files in an atomic way, so that after the
> > syscall either the two files have exchanged names or neither has been
> > modified?
> > 
> > I suspect that btrfs could do it, but I'm wondering about ext3/4.
> 
> I don't think there is anything equivalent to exchangedata() on Linux
> http://developer.apple.com/library/mac/#DOCUMENTATION/Darwin/Reference/ManPages/man2/exchangedata.2.html

XFS can do that, though the interface is an ioctl (XFS_IOC_SWAPEXT)
whose parameters are closely tied to the functionality of the only
user: the online defragmenter (xfs_fsr). It would be pretty easy to
wire it up to such a syscall if one existed, though....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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