Reflinks and copy_file_range() are just landing in OpenZFS now I think? (Block cloning)
Block cloning support has indeed recently landed in git and already allows for reflinks under FreeBSD. Still has to be wired up for Linux though.
Really excited about this.
Once support hits in Linux, a little app of mine[0] will support block cloning for its "roll forward" operation, where all previous snapshots are preserved, but a particular snapshot is rolled forward to the live dataset. Right now, data is simply diff copied in chunks. When this support hits, there will be no need to copy any data. Blocks written to the live dataset can just be references to the underlying snapshot blocks, and no extra space will need to be used.