unionfs-fuse is a unionfs filesystem implementation using fuse.
It is meant to be way more flexible than the current in-kernel unionfs
solution.

Why choose this stuff?
 * The filesystem has to be mounted after the roots are mounted when
   using the standard module. With unionfs-fuse, you can mount the
   roots later and their contents will appear seamlesly.
 * You get caching (provided by the underlying FUSE page cache) which
   speeds things up a lot for free.
 * Advanced features like copy-on-write and more.

Why NOT choose it?
 * Compared to kernel-space solution we need lots of useless context
   switches which makes a kernel-only solution clear speed-winner.