This is designed to allow running a read-only root
filesystem while keeping some r/w locations (/tmp,
var/log, ...). It might be useful on embedded systems.
Compared to tmpfs, ram data are compressed, files on
real file-system and tmp filesystem coexist thanks to
unionfs, and an effort is done to allow synchronisation
of the ram data onto the real filesystem. This is not
automatic though, processes keeping opened files needs
to be cleanly stopped for syncronisation and restarted
after.
This is combining unionfs-fuse, btrfs and tmpfs.
This is so far designed on and for debian derivatives.