Unconvincing. LVM's snapshots are CoW whether thick or thinly provisioned. And while not yet merged in mainline, XFS devs are working on CoW as well which is us

Unconvincing. LVM's snapshots are CoW whether thick or thinly provisioned. And while not yet merged in mainline, XFS devs are working on CoW as well which is used when modifying reflinked files (shared extents). Btrfs behaves basically like that with 'nodatacow' today. It will overwrite extents if there's no reflink/snapshot. If there is, CoW happens for new writes and any subsequent modifications are overwrites until there's a reflink/snapshot in which case CoW happens. The 'nodatacow' flag can be used as either a mount option, or selectively with an xattr per subvolume, directory, or file. And in all cases, metadata writes (the file system itself) are still CoW.