Special-purpose file systems supported by the Linux kernel

tmpfs (short for temporary file system) is a temporary file storage paradigm implemented in many Unix-like operating systems. It is intended to appear as a mounted file system, but data is stored in volatile memory rather than on a persistent storage device.

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.