Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 2 Aug 2021 17:50:59 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>, linux-fsdevel@vger.kernel.org,
        Linux NFS list <linux-nfs@vger.kernel.org>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: A Third perspective on BTRFS nfsd subvol dev/inode number issues.
Message-ID: <20210802215059.GF6890@fieldses.org>
References: <YQNG+ivSssWNmY9O@zeniv-ca.linux.org.uk>
 <162762290067.21659.4783063641244045179@noble.neil.brown.name>
 <CAJfpegsR1qvWAKNmdjLfOewUeQy-b6YBK4pcHf7JBExAqqUvvg@mail.gmail.com>
 <162762562934.21659.18227858730706293633@noble.neil.brown.name>
 <CAJfpegtu3NKW9m2jepRrXe4UTuD6_3k0Y6TcCBLSQH7SSC90BA@mail.gmail.com>
 <162763043341.21659.15645923585962859662@noble.neil.brown.name>
 <CAJfpegub4oBZCBXFQqc8J-zUiSW+KaYZLjZaeVm_cGzNVpxj+A@mail.gmail.com>
 <162787790940.32159.14588617595952736785@noble.neil.brown.name>
 <20210802123930.GA6890@fieldses.org>
 <162793864421.32159.6348977485257143426@noble.neil.brown.name>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <162793864421.32159.6348977485257143426@noble.neil.brown.name>
User-Agent: Mutt/1.5.21 (2010-09-15)
Precedence: bulk

On Tue, Aug 03, 2021 at 07:10:44AM +1000, NeilBrown wrote:
> On Mon, 02 Aug 2021, J. Bruce Fields wrote:
> > On Mon, Aug 02, 2021 at 02:18:29PM +1000, NeilBrown wrote:
> > > For btrfs, the "location" is root.objectid ++ file.objectid.  I think
> > > the inode should become (file.objectid ^ swab64(root.objectid)).  This
> > > will provide numbers that are unique until you get very large subvols,
> > > and very many subvols.
> > 
> > If you snapshot a filesystem, I'd expect, at least by default, that
> > inodes in the snapshot to stay the same as in the snapshotted
> > filesystem.
> 
> As I said: we need to challenge and revise user-space (and meat-space)
> expectations. 

The example that came to mind is people that export a snapshot, then
replace it with an updated snapshot, and expect that to be transparent
to clients.

Our client will error out with ESTALE if it notices an inode number
changed out from under it.

I don't know if there are other such cases.  It seems like surprising
behavior to me, though.

--b.

> In btrfs, you DO NOT snapshot a FILESYSTEM.  Rather, you effectively
> create a 'reflink' for a subtree (only works on subtrees that have been
> correctly created with the poorly named "btrfs subvolume" command).
> 
> As with any reflink, the original has the same inode number that it did
> before, the new version has a different inode number (though in current
> BTRFS, half of the inode number is hidden from user-space, so it looks
> like the inode number hasn't changed).