linux-ext4 - Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

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Date:   Fri, 19 Jun 2020 21:56:33 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Masayoshi Mizuma <msys.mizuma@...il.com>,
        Eric Sandeen <sandeen@...deen.net>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs <linux-xfs@...r.kernel.org>, jlayton@...hat.com
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

On Sat, Jun 20, 2020 at 11:49:57AM +1000, Dave Chinner wrote:
> On Fri, Jun 19, 2020 at 06:28:43PM -0400, J. Bruce Fields wrote:
> > On Sat, Jun 20, 2020 at 08:10:44AM +1000, Dave Chinner wrote:
> > > On Fri, Jun 19, 2020 at 04:40:33PM -0400, J. Bruce Fields wrote:
> > > > On Fri, Jun 19, 2020 at 12:44:55PM +1000, Dave Chinner wrote:
> > > > > On Thu, Jun 18, 2020 at 10:20:05PM -0400, J. Bruce Fields wrote:
> > > > > > My memory was that after Jeff Layton's i_version patches, there wasn't
> > > > > > really a significant performance hit any more, so the ability to turn it
> > > > > > off is no longer useful.
> > > > > 
> > > > > Yes, I completely agree with you here. However, with some
> > > > > filesystems allowing it to be turned off, we can't just wave our
> > > > > hands and force enable the option. Those filesystems - if the
> > > > > maintainers chose to always enable iversion - will have to go
> > > > > through a mount option deprecation period before permanently
> > > > > enabling it.
> > > > 
> > > > I don't understand why.
> > > > 
> > > > The filesystem can continue to let people set iversion or noiversion as
> > > > they like, while under the covers behaving as if iversion is always set.
> > > > I can't see how that would break any application.  (Or even how an
> > > > application would be able to detect that the filesystem was doing this.)
> > > 
> > > It doesn't break functionality, but it affects performance.
> > 
> > I thought you just agreed above that any performance hit was not
> > "significant".
> 
> Yes, but that's just /my opinion/.
> 
> However, other people have different opinions on this matter (and we
> know that from the people who considered XFS v4 -> v5 going slower
> because iversion a major regression), and so we must acknowledge
> those opinions even if we don't agree with them.

Do you have any of those reports handy?  Were there numbers?

--b.