Comment by hifier

It seems to me that one of the core differences between MySQL/Postgres and distributed stores like Cassandra / Hbase is that with the former your data and your write workload have to fit onto a single host. If either one cannot fit then you have to partition at the application level or use a real distributed data store. Partitioning at the app level is an operational burden and complexity that would be best avoided, but there are always exceptions.