We use something like this at scale, similar to Riak's design. We use it to store things like the active state of millions of hospital patients in RAM for high availability (zero downtime when DCs or nodes fail). One copy per data center with a writer at one of the DCs. Our current cluster has 32 nodes (8 per DC -- we should have 5 DCs across the US but only have 4 at the moment). You can learn more about it here:
https://www.youtube.com/watch?v=pQ0CvjAJXz4.