Use tmpfs.

(cherry picked from commit a6e72247bf3887fe573215ca4be4dcb485e22ee4)
(cherry picked from commit c45710fad0044feb8d8c2ded4a6adef391666074)
Bump up default TMPFS size to one third of physical memory.

(cherry picked from commit eaef0173bc35406c5c936e884558f846092f31a4)