Union file systems

Unionfs is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.

aufs (short for advanced multi-layered unification filesystem) implements a union mount for Linux file systems. The name originally stood for AnotherUnionFS until version 2.

OverlayFS is a union mount filesystem implementation for Linux. It combines multiple different underlying mount points into one, resulting in a single directory structure that contains underlying files and sub-directories from all sources. Common applications overlay a read/write partition over a read-only partition, such as with LiveCDs and IoT devices with limited flash memory write cycles.

functionality in computer operating systems