"many systems have no mechanism of determining that n bytes have changed without scanning the entire data set." This is so insanely inefficient it can't scale

"many systems have no mechanism of determining that n bytes have changed without scanning the entire data set." This is so insanely inefficient it can't scale to very large amounts of data. If you can't do data syncing at the application layer you can do it at the storage layer with high end storage arrays that duplicate all writes to a second storage array, either synchronously or asynchronously. Or duplicate snapshots to a remote array. They work really well.