Received-SPF: pass (google.com: domain of linux-ext4-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 20 Jul 2022 17:22:57 +0200
From:   Lukas Czerner <lczerner@redhat.com>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Benjamin Coddington <bcodding@redhat.com>
Subject: Re: should we make "-o iversion" the default on ext4 ?
Message-ID: <20220720152257.t67grnm4wdi3dpld@fedora>
References: <69ac1d3ef0f63b309204a570ef4922d2684ed7f9.camel@kernel.org>
 <20220720141546.46l2d7bxwukjhtl7@fedora>
 <ad7218a41fa8ac26911a9ccb79c87609d4279fea.camel@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <ad7218a41fa8ac26911a9ccb79c87609d4279fea.camel@kernel.org>
Precedence: bulk

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@kernel.org>
>