[1] Union mounting is supported in Linux, BSD and several of its successors, and Plan 9, with similar but subtly different behavior.
[5] Problems that union mounting on Unix-like operating systems encounters include: Early attempts to add unioning to Unix filesystems included the 3-d filesystem (Bell Labs) and the Translucent File Service in SunOS (Sun Microsystems, 1988[2]).
[2] In 2014 OverlayFS union mount implementation was added to the standard Linux kernel source code.
[7] Similarly, GlusterFS offers the ability to mount different filesystems distributed across a network, rather than being located on the same machine.
[8] MergerFS, originally released in 2014, is an actively developed open-source FUSE plugin, allowing pooling of arbitrary directories.