Copy on write has huge benefits in space savings, not just in disk space, but more importantly in memory, particularly for virtualized systems. For example, the vserver project already implements a solution to this which allows many virtual servers to share the same files securely. This means that if you have 1000 servers running the same copy of apache, not only can you have only one copy on disk, but the kernel will also only keep one copy in memory (of the shared stuff like program text, of course). While you could achieve a similar sharing with hard links, this would be less secure since a breach in one system would allow the file to be modified in all the other systems. With COW, this is avoided.