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Date:   Sun, 21 Jun 2020 09:54:08 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Masayoshi Mizuma <msys.mizuma@gmail.com>,
        Eric Sandeen <sandeen@sandeen.net>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Christoph Hellwig <hch@infradead.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs <linux-xfs@vger.kernel.org>, jlayton@redhat.com
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts
Message-ID: <20200620235408.GS2005@dread.disaster.area>
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On Fri, Jun 19, 2020 at 09:56:33PM -0400, J. Bruce Fields wrote:
> 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?

e.g.  RH BZ #1355813 when v5 format was enabled by default in RHEL7.
Numbers were 40-47% performance degradation for in-cache writes
caused by the original IVERSION implementation using iozone.  There
were others I recall, all realted to similar high-IOP small random
writes workloads typical of databases....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com