Is it an in-memory database system, or not?
When you hatch a database management system, it will, by design and implementation, be either an in-memory database system or an on-disk database management system. The choice affects the fundamental optimization strategies that will be baked into the database system code. To optimize an on-disk database is to minimize disk I/O, so its design will use extra CPU cycles and extra memory if doing so will reduce or eliminate I/O. Conversely, IMDSs by definition eliminate all disk I/O; their optimization is all about delivering the highest performance at a given level of processing power, thus reducing demand for CPU cycles is a key objective. And since memory is storage for an IMDS, reducing memory overhead (i.e. RAM consumed for anything other than storing data) is also a key objective.
A hybrid database
eXtremeDB enables the developer to combine in-memory and on-disk paradigms in a single database system. Specifying that data will be stored in memory (transient), or on disk (persistent), requires a simple database schema declaration. The resulting system retains in-memory strengths (speed, footprint, etc.), yet leverages the potential cost savings and durability of an on-disk database.