In fact I thought rsync was doing by default exactly what you described as --cow. And then I was wondering how come rsync was "defeating btrfs’ copy-on-write benefits"!
Note this good laugh from the original article: "The thing is, modern networks aren’t like that at all, they’re high bandwidth and low latency."
More seriously rsync could also feature a "--datacenter-network" recursive copy option that would do a plain, complete transfer of the files which have changed.
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