Systemd Change Allows For Stateless Systems With Tmpfs
Added to systemd last night was a change to the fstab-generator component to support the root file-system on tmpfs or other deviceless file-systems. As said by the commit message, "this allows for stateless systems."
Among the deviceless file-systems recognized are tmpfs, overlayfs, ramfs, rootfs, and others that aren't physical file-systems intended to be backed by a HDD/SSD. Up to now just an error would be generated.
For those out of the loop, systemd is working towards stateless Linux and factory reset support among other features for the growing init manager. The systemd crew is also working on new security-focused Linux distribution setups using systemd and Btrfs.