Fallthrus as full-length symlinks?

[Valerie Aurora <vaurora@redhat.com>]

Fallthrus were invented as a placeholders for readdir() on a
union-mounted directory - basically, to use the top-level file
system's readdir() cookie mechanism.  Fallthrus are persistent
directory entries and are implemented by the underlying file system -
such as ext2 or tmpfs - in whatever way it sees fit.  We've
implemented them for ext2 in two ways: as a regular directory entry
with a magic inode number, and as a regular directory entry with a
special file type.

Recently, David Woodhouse suggested implementing fallthrus as
full-length symlinks with a special flag.  The interesting thing about
this idea is that it could theoretically let us rename a file from the
low level file system to another place in the low-level file system
without copying the contents of the file up.  Basically, we can
arbitrarily swizzle the namespace of the low-level by maintaining a
set of symlinks above.

Is this useful?  Is it implementable?

Background reading:

http://valerieaurora.org/union/

-VAL