linux-ext4 - Re: should we make "-o iversion" the default on ext4 ?

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Date:   Fri, 22 Jul 2022 08:32:44 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lczerner@...hat.com, Benjamin Coddington <bcodding@...hat.com>
Subject: Re: should we make "-o iversion" the default on ext4 ?

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.

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.

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....

> 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 of
> 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]).
> 
> 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.

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.

> Thoughts?

My 2c is to behave like XFS: ignore the mount option and always turn
it on.

Cheers,

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