Re: [PATCH] ext4: turn on i_version updates by default

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, May 15, 2012 at 07:59:11PM +0200, Marco Stornelli wrote:
> Only a really really stupid question (I don't know NFS protocol well
> enough). In 3.3 kernel, I see that only ext4 uses MS_I_VERSION, so I
> wonder: if i_version change it's needed for exportable fs and so for
> nfs, other exportable fs?

Yes, it's needed for others as well.  I believe btrfs and xfs are both
adding it.

We're currently using ctime for the nfs change attribute.  That's
effectively jiffy granularity.  So to see the problem at a minimum you'd
need two writes to be processed within one jiffy, and a stat to come
between them.  But that's a correctness problem, and we'd like to see it
fixed before it becomes more common.

More generally, it's useful to be able to ask whether a file changed
without rereading all its data, and a clock that registers every change
and is consistent across a filesystem sounds difficult to scale.  We may
eventually find we need something like this outside nfs.

--b.