>> OTOH do we guarantee unique inode numbers for all filesystems?
>> IIRC union mounts done generate unique numbers - and mkisofs can get
>> confused.
> Oh dear.  If true, mkisofs won't be the only thing to be confused.
> rsync and tar spring immediately to mind.

getcwd() as implemented by walking up .. chains is another.  (And
that's probably done by a lot of existing statically-linked programs.)

Anything that causes distinct simultaneously-existing objects to have
identical <st_dev,st_ino> pairs breaks one of the longest-standing
undocumented Unix invariants I know of.  Lots of code will break.
What's worse, it will probably break (a) silently and (b) subtly.

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