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From:   "NeilBrown" <neilb@suse.de>
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     "Chuck Lever" <chuck.lever@oracle.com>, "Chris Mason" <clm@fb.com>,
        "Josef Bacik" <josef@toxicpanda.com>,
        "David Sterba" <dsterba@suse.com>, linux-nfs@vger.kernel.org,
        "Wang Yugui" <wangyugui@e16-tech.com>,
        "Ulli Horlacher" <framstag@rus.uni-stuttgart.de>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH/RFC] NFSD: handle BTRFS subvolumes better.
In-reply-to: <20210715154534.GA24492@fieldses.org>
References: <20210613115313.BC59.409509F4@e16-tech.com>,
 <20210310074620.GA2158@tik.uni-stuttgart.de>,
 <162632387205.13764.6196748476850020429@noble.neil.brown.name>,
 <20210715154534.GA24492@fieldses.org>
Date:   Fri, 16 Jul 2021 09:08:27 +1000
Message-id: <162639050750.13764.15083673238872288080@noble.neil.brown.name>
Precedence: bulk

On Fri, 16 Jul 2021, J. Bruce Fields wrote:
> On Thu, Jul 15, 2021 at 02:37:52PM +1000, NeilBrown wrote:
> > To fix this, we need to report a different fsid for each subvolume, but
> > need to use the same fsid that we currently use for the top-level
> > volume.  Changing this (by rebooting a server to new code), might
> > confuse the client.  I don't think it would be a major problem (stale
> > filehandles shouldn't happen), but it is best avoided.
> ...
> > Again, we really want an API to get this from the filesystem.  Changing
> > it later has no cost, so we don't need any commitment from the btrfs team
> > that this is what they will provide if/when we do get such an API.
> 
> "No cost" makes me a little nervous, are we sure nobody will notice the
> mountd-on-fileid changing?

One cannot be 100% sure, but I cannot see how anything would depend on
it being stable.  Certainly the kernel doesn't.
'ls -i' doesn't report it - even as "ls -if".  "find -inum xx" cannot see
it.
Obviously readdir() will see it but if any application put much weight
on the number, it could already get confused when btrfs returns
non-unique numbers as I mentioned.  
I certainly wouldn't lose sleep over changing it.

NeilBrown

> 
> Fileid and fsid changes I'd worry about more, though I wouldn't rule it
> out if that'd stand in the way of a bug fix.
> 
> Thanks for looking into this.
> 
> --b.
> 
>