Here is a great post from Bobby Johnson (ex-Facebooker and CTO of Interana) with his opinion on "in-memory" data stores: https://community.interania.com/t5/Blogs/The-Myth-of-In-Memo....

This was the reasoning behind a very key architectural decision at Interana that makes it different than Scuba - instead of developing an in memory system, Interana created a custom data store that is heavily optimized around using spinning disk and CPU cache. This makes it incredibly fast and less expensive to operative massive clusters at scale.