OverlayFS is a union mount filesystem implementation for Linux.
The need and specification of a kernel mode Linux union mount filesystem was identified in late 2009.
[1] The initial RFC patchset of OverlayFS was submitted by Miklos Szeredi in 2010.
Otherwise, OverlayFS presents the object, if any, yielded by one or the other, with the "upper" filesystem taking precedence.
[8] OverlayFS does not support renaming files without performing a full copy-up of the file; however, renaming directories in an upper filesystem has limited support.