TFS introduced copy-on-write in 1986 (with SunOS 3). This is probably the first such implementation. Not sure if there was anything comparable before that, meet

TFS introduced copy-on-write in 1986 (with SunOS 3). This is probably the first such implementation. Not sure if there was anything comparable before that, meeting the same definition of layered and copy-on-write. WAFL came a few years later - in development in the early '90s, reached production in 1994.