Anything-sync-daemon (asd) is a tiny pseudo-daemon designed to manage user defined dirs in tmpfs and to periodically sync back to the 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.
Consult the man page or the wiki page: https://wiki.archlinux.org/index.php/Anything-sync-daemon
To build from source, see the included INSTALL text document.