On Fri, 30 Jul 2021 at 09:34, NeilBrown [off-list ref] wrote:

But I'm curious about your reference to "some sort of subvolume
structure that the VFS knows about".  Do you have any references, or can
you suggest a search term I could try?

Found this:
https://lore.kernel.org/linux-fsdevel/20180508180436.716-1-mfasheh@suse.de/ (local)

I also remember discussing it at some LSF/MM with the btrfs devs, but
no specific conclusion.

Thanks,
Miklos