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Date:   Tue, 13 Sep 2022 15:02:26 -0400
To:     NeilBrown <neilb@suse.de>
Cc:     Dave Chinner <david@fromorbit.com>,
        Jeff Layton <jlayton@kernel.org>,
        Theodore Ts'o <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        adilger.kernel@dilger.ca, djwong@kernel.org,
        trondmy@hammerspace.com, viro@zeniv.linux.org.uk,
        zohar@linux.ibm.com, xiubli@redhat.com, chuck.lever@oracle.com,
        lczerner@redhat.com, brauner@kernel.org, fweimer@redhat.com,
        linux-man@vger.kernel.org, linux-api@vger.kernel.org,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, ceph-devel@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-xfs@vger.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field
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From:   bfields@fieldses.org (J. Bruce Fields)
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On Tue, Sep 13, 2022 at 11:49:03AM +1000, NeilBrown wrote:
> Invalidating the client cache on EVERY unmount/mount could impose
> unnecessary cost.  Imagine a client that caches a lot of data (several
> large files) from a server which is expected to fail-over from one
> cluster node to another from time to time.  Adding extra delays to a
> fail-over is not likely to be well received.
> 
> I don't *know* this cost would be unacceptable, and I *would* like to
> leave it to the filesystem to decide how to manage its own i_version
> values.  So maybe XFS can use the LSN for a salt.  If people notice the
> extra cost, they can complain.

I'd expect complaints.

NFS is actually even worse than this: it allows clients to reacquire
file locks across server restart and unmount/remount, even though
obviously the kernel will do nothing to prevent someone else from
locking (or modifying) the file in between.

Administrators are just supposed to know not to allow other applications
access to the filesystem until nfsd's started.  It's always been this
way.

You can imagine all sorts of measures to prevent that, and if anyone
wants to work on ways to prevent people from shooting themselves in the
foot here, great.

Just taking away the ability to cache or lock across reboots wouldn't
make people happy, though....

--b.