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Copy-on-write and hard links are different things. A file clone would, though (as in cp --reflink).

So if I understand correctly, copy-on-write is implemented using hard links, but the existence of hard links alone doesn't constitute CoW.

You can also have hard links on non CoW filesystems (eg ext3). I assume hard links exists on most filesystems

Where there is a filename there is a hard link, so that's true in a trivial sense.

Right, indeed. I guess I was thinking about the functionality of having multiple hard links point to the same “file” (inode)