Unionfs

Well, aufs is a great replacement for unionfs. That particular code should be in kernel IMHO,
as it's working much better. It is documented in aufs.5 man page: http://aufs.sf.net/aufs.html

Knoppix switched to AUFS already, all linux-live based distributions including Slax are
switching to AUFS as well. Junjiro Okajima (AUFS author) sadly doesn't wish to submit the code
to kernel yet, while unionfs developers try very hard to get it inthere, have no idea why is
that so important for them. The inclusion won't make unionfs code better.

If you review the unionfs versus aufs development process, AUFS adds cool features (like
balancing of writable branches), while unionfs fixes locking, mmap code, NULL pointer
dereferences, and so on. ..