UnionFS / OverlayFS
Storage driver combining distinct read-only image layers and a read-write layer into a single unified directory tree.
Mechanism & Definition
A stackable unification filesystem driver that merges multiple directories into one virtual mount point. OverlayFS presents lower read-only image layers (`lowerdir`) and a top read-write container layer (`upperdir`) as a unified rootfs (`merged`), using Copy-on-Write (CoW) when modified files are written.