Re: [PATCH] btrfs: derive f_fsid with dev_t only when temp_fsid is active

[David Sterba <dsterba@suse.cz>]

On Mon, Sep 14, 2026 at 12:47:53PM +0200, David Woodhouse wrote:
> On Mon, 2026-09-14 at 16:28 +0800, Anand Suveer Jain wrote:
> > 
> > 
> > > > . For stable kernels, this flag will keep the new f_fsid derivation
> > > > disabled by default.
> > 
> > 
> > > No. That still gives you a broken fsid on upgrading to the next major kernel, doesn't it?
> > 
> > Upgrading to a _major_ kernel would require an extra step to re-encrypt
> > with the new FSID _for some configs_.
> > A one-time operational overhead.
> 
> I'm not keen on that. The documentation is fairly poor, but historical
> behaviour and expectation is that the FSID is supposed to be *stable*.
> You're not supposed to randomly change it from one kernel to the next.

That's what I consider sane and would like to preserve. We allow users
to change the UUID by tools, but this means the user is aware of that
and initiating such action.

> Won't this also trigger some incremental backup schemes to re-archive
> every file as it seems the {fsid, ino} has changed?

This is a good point, I don't know if backup tools do that but it's
quite natual thing to do.