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

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Message-ID: <20120515191810.GA11028@fieldses.org>
Date:	Tue, 15 May 2012 15:18:10 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Marco Stornelli <marco.stornelli@...il.com>
Cc:	Josef Bacik <josef@...hat.com>, Andreas Dilger <adilger@...ger.ca>,
	Ted Ts'o <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] ext4: turn on i_version updates by default

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.
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