OverlayFS

Overlayfs (Overlay Filesystem) differs from other union filesystem implementations in that after a file is opened all operations go directly to the underlying, lower or upper, filesystems. This simplifies the implementation and allows native performance in these cases.

The option to enable OverlayFS exists in Linux kernels 3.18 and higher.

Usage
Once enabled in the kernel OverlayFS can be controlled using the mount command.

External resources

 * http://lwn.net/Articles/447650/ - A LWN article written by Jonathan Corbet in June 2011 covering vises and virtues of OverlayFS.
 * An informative AskUbuntu.com thread.
 * Overlay fs in the Linux git repository.