linux-ext4 - Re: [PATCH v3 1/7] iversion: update comments with info about atime updates

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Message-ID: <20220830183244.GG26330@fieldses.org>
Date:   Tue, 30 Aug 2022 14:32:44 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "xiubli@...hat.com" <xiubli@...hat.com>,
        "brauner@...nel.org" <brauner@...nel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "neilb@...e.de" <neilb@...e.de>,
        "david@...morbit.com" <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "chuck.lever@...cle.com" <chuck.lever@...cle.com>,
        "linux-ceph@...r.kernel.org" <linux-ceph@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
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        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
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        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "walters@...bum.org" <walters@...bum.org>
Subject: Re: [PATCH v3 1/7] iversion: update comments with info about atime
 updates

On Tue, Aug 30, 2022 at 01:02:50PM -0400, Jeff Layton wrote:
> The fact that NFS kept this more loosely-defined is what allowed us to
> elide some of the i_version bumps and regain a fair bit of performance
> for local filesystems [1]. If the change attribute had been more
> strictly defined like you mention, then that particular optimization
> would not have been possible.
> 
> This sort of thing is why I'm a fan of not defining this any more
> strictly than we require. Later on, maybe we'll come up with a way for
> filesystems to advertise that they can offer stronger guarantees.

Yeah, the afs change-attribute-as-counter thing seems ambitious--I
wouldn't even know how to define what exactly you're counting.

My one question is whether it'd be worth just defining the thing as
*increasing*.  That's a lower bar.

(Though admittedly we don't quite manage it now--see again 1631087ba872
"Revert "nfsd4: support change_attr_type attribute"".)

--b.