These are the assumptions under which VoltDB was architected:
First, it should be noted that main memory is getting very cheap. It is straightforward to put 50 Gbytes of memory on a $5,000 server. Beefy servers these days have 10 times that amount. Moreover, many (but not all) transactional databases don’t require massive storage volumes. An OLTP application with more than a few Tbytes of data is quite rare. The same can be said for new OLTP “fire hose” applications that require ultra-high write throughput and ACID transactions (e.g., digital advertising, wireless, real-time monitoring) — these systems rarely need to manage more than a few Tbytes of hot state. Hence, it is plausible to buy enough main memory to store the data for the vast majority of OLTP applications.
Original title and link: VoltDB Assumptions: Memory vs Disk ( ©myNoSQL)