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Date:   Mon, 31 Jan 2022 15:37:42 -0500
From:   bfields <bfields@fieldses.org>
To:     Richard Weinberger <richard@nod.at>
Cc:     linux-nfs <linux-nfs@vger.kernel.org>,
        luis turcitu <luis.turcitu@appsbroker.com>,
        chris chilvers <chris.chilvers@appsbroker.com>,
        david young <david.young@appsbroker.com>,
        Daire Byrne <daire@dneg.com>,
        david oberhollenzer <david.oberhollenzer@sigma-star.at>,
        david <david@sigma-star.at>, goliath@sigma-star.at,
        trond myklebust <trond.myklebust@hammerspace.com>,
        anna schumaker <anna.schumaker@netapp.com>
Subject: Re: [RFC PATCH] mountd: export: Deal with NFS filesystems
Message-ID: <20220131203742.GE30119@fieldses.org>
References: <20220131104316.10357-1-richard@nod.at>
 <20220131170125.GB30119@fieldses.org>
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On Mon, Jan 31, 2022 at 06:15:41PM +0100, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
> > Von: "bfields" <bfields@fieldses.org>
> >> Using /proc/fs/nfsfs/volumes it is possible to find the NFS fsid
> >> from the backend and use it as seed for mountd's UUID mechanism.
> > 
> > Sorry, I haven't checked--what is that number, really?  It's probably
> > not the fsid returned from the server, as that wouldn't be guaranteed
> > unique across multiple servers.  But that means it's probably generated
> > in a way that doesn't guarantee it's stable across reboots.  And we need
> > filehandles to work across reboots.
> 
> Unless I badly misunderstood the code it comes from fs/nfs/client.c's
> nfs_create_server() where the NFS client fetches NFS_ATTR_FATTR via getattr().
> So it should be unique:
> https://datatracker.ietf.org/doc/html/rfc7530#section-5.8.1.9

Unique within one server, but not across different servers.

Depending on how your servers assign fsids, collisions might be quite
likely.  Results of a collision would be pretty bad.

--b.