Nowadays instead of having the sub-systems you are referring to for storing data, many scaled databases use NoSQL and Redis for databasing and caching. The cach

Nowadays instead of having the sub-systems you are referring to for storing data, many scaled databases use NoSQL and Redis for databasing and caching. The caching is probably the more scalable of the two because caching at any level has to do with usage and not all database data is used. Caching has allowed the most used data to be readily available. The limits of scalability may only require the system you described, but bigger and more complex systems like Twitter and Pinterest have been documented using database scaling via caching with things similar to Redis and Memcache. Keywords for finding more information are "cache/caching". Problems with the sub-system of databasing are too many transactions (I/O) and the speed of getting the data and acting on it is very inefficient. If you want to talk to me about it in more detail, message me on Twitter: @compooter or get my email at mayorpear.com