> I know that supporting all sorts of corner cases and features has caused > major pains for union mount, but this restriction sounds a bit too
> restrictive to me :)
This also will cause problems if you want to use union mount in server consolidation environments where you have multiple servers working out of a common base distribution as their lower layer. With your scheme, I will not be able to do updates (like security updates) to the base distribution and see it getting effected in all the servers.