The problem is, it varies... you have what are effectively key/value stores to document databases, column stores and everything in between. You have systems built on other systems. RethinkDB and Cockroach have different approaches than Redis, Mongo, Cassandra or others.

CockroachDB gives an SQL interface over the top of a distributed data store with better consistency and relations. Cassandra has no real relations over a BigTable/Column-Store solution. It really just depends.

I think with what's coming out of SSD/NVMe and even Optain DIMMS, that there will be databases directly tuned to control/set their own data storage in these environments.