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

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Date:   Wed, 20 Jul 2022 17:22:57 +0200
From:   Lukas Czerner <lczerner@...hat.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>,
        Benjamin Coddington <bcodding@...hat.com>
Subject: Re: should we make "-o iversion" the default on ext4 ?

On Wed, Jul 20, 2022 at 10:38:31AM -0400, Jeff Layton wrote:
> On Wed, 2022-07-20 at 16:15 +0200, Lukas Czerner wrote:

--snip--

> > How would we approach making iversion a default? libmount is passing
> > this option to the kernel as just a MS_I_VERSION flag that is set when
> > -o iversion is used and left empty when the -o noiversion is used. This
> > means that while we could make it a default in ext4, we don't have any
> > way of knowing whether the user asked for -o noiversion. So that's not
> > really an option.
> > 
> > Updating the mke2fs/tune2fs to allow setting iversion as a default mount
> > option I think has the same problem.
> > 
> > So the only way I can see ATM would be to introduce another mountflag
> > for libmount to indicate -o noiversion. This way we can make iversion a
> > default on ext4 without loosing the information about user provided -o
> > noiversion option.
> > 
> > Is there a different way I am not seeing?
> > 
> 
> Right, implementing this is the difficult bit actually since this uses a
> MS_* flag. If we do make this the default, we'd definitely want to
> continue allowing "-o noiversion" to disable it.
> 
> Could we just reverse the default in libmount? It might cause this to
> suddenly be enabled in some deployments, but in most cases, people
> wouldn't even notice and they could still specify -o noiversion to turn
> it off.

Can be done, but that would change the default for everyone. Not sure if
that desirable. Also I can image this being a bit confusing. I still
think the best approach would be to introduce another MS_ flag for
noiversion case. I think there is precedence in the case of
MS_STRICTATIME - not exactly the same but similar enough.

> Another idea would be to introduce new mount options for this, but
> that's kind of nasty from a UI standpoint.
> 
> > 
> > If we can do reasonably extensive testing that will indeed show
> > negligible impact when nothing is querying i_version, then I would be in
> > favor of the change.
> > 
> 
> Excellent! I think that would be best if we can get away with it. A lot
> of people are currently running ext4-backed nfs servers and aren't using
> that mount option.

Could you provide some performance numbers for iversion case?

Thanks!
-Lukas

> -- 
> Jeff Layton <jlayton@...nel.org>
>