I don't understand what you're getting at, but you can implement CoW at the page level with MMU hardware. That's how Redis dumping works, for example (the process forks into two, with one dumping to disk and the other continuing to take updates, CoW'ing pages that get modified and sharing the rest). What would CoW even mean inside a single address space? The idea of persistent data structures (as used in functional programming and other places) is that you never visibly modify anything, but instead make new objects that share structure with the old objects, relying on the GC to clean up data that stops being reachable.