Union Filesystem for FUSE
Funionfs is a filesystem which concatenate two or more directories. These directories are hierarchised by Funionfs. Typically, you could use a mounted filesystem wich is in read-only where you only read files and an upper filesystem (empty at the start of the system) where you write modifications. Funionfs is very useful for embedded linux (the system must resist powerfail) and for live-cd Linux.