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Message-ID: <7ae8a678f59a49ffeaaa39265d5108135911eeb3.camel@kernel.org>
Subject: Re: should we make "-o iversion" the default on ext4 ?
From:   Jeff Layton <jlayton@kernel.org>
To:     Dave Chinner <david@fromorbit.com>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lczerner@redhat.com, Benjamin Coddington <bcodding@redhat.com>
Date:   Mon, 25 Jul 2022 12:22:21 -0400
In-Reply-To: <20220721223244.GP3600936@dread.disaster.area>
References: <69ac1d3ef0f63b309204a570ef4922d2684ed7f9.camel@kernel.org>
         <20220721223244.GP3600936@dread.disaster.area>
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On Fri, 2022-07-22 at 08:32 +1000, Dave Chinner wrote:
> On Tue, Jul 19, 2022 at 09:51:33AM -0400, Jeff Layton wrote:
> > Back in 2018, I did a patchset [1] to rework the inode->i_version
> > counter handling to be much less expensive, particularly when no-one is
> > querying for it.
>=20
> Yup, there's zero additional overhead for maintaining i_version in
> XFS when nothing is monitoring it. Updating it comes for free in any
> transaction that modifies the inode, so when writes
> occur i_version gets bumped if timestamps change or allocation is
> required.
>=20
> And when something is monitoring it, the overhead is effectively a
> single "timestamp" update for each peek at i_version the monitoring
> agent makes. This is also largely noise....
>=20
> > Testing at the time showed that the cost of enabling i_version on ext4
> > was close to 0 when nothing is querying it, but I stopped short of
> > trying to make it the default at the time (mostly out of an abundance o=
f
> > caution). Since then, we still see a steady stream of cache-coherency
> > problems with NFSv4 on ext4 when this option is disabled (e.g. [2]).
> >=20
> > Is it time to go ahead and make this option the default on ext4? I don'=
t
> > see a real downside to doing so, though I'm unclear on how we should
> > approach this. Currently the option is twiddled using MS_I_VERSION flag=
,
> > and it's unclear to me how we can reverse the sense of such a flag.
>=20
> XFS only enables SB_I_VERSION based on an on disk format flag - you
> can't turn it on or off by mount options, so it completely ignores
> MS_I_VERSION.
>=20
> > Thoughts?
>=20
> My 2c is to behave like XFS: ignore the mount option and always turn
> it on.

I'd be fine with that, personally.

They could also couple that with a tune2fs flag or something, so you
could still disable it if it were a problem for some reason.

It's unlikely that anyone will really notice however, so turning it on
unconditionally may be the best place to start.
--=20
Jeff Layton <jlayton@kernel.org>