From the Arch Linux wiki:
Anything-sync-daemon(asd) is a tiny shell script designed to manage target directories in tmpfs and to periodically sync them back to your physical disc (HDD/SSD). This is accomplished via a symlinking step and an innovative use of rsync to maintain back-up and synchronization between the two. One of the major design goals of asd is a completely transparent user experience.
What would it take to use this in Ubuntu?