A main memory database system is a DBMS that primarily relies on main memory for computer data storage. In contrast, normal database management systems employ hard disk based persisntent storage.
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L. Liu (Lei) , M. Tamer Özsu
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Database Architectures

Boncz, P. (2008). Main Memory DBMS. In L. Liu & M. Tamer Özsu (Eds.), Encyclopedia of Database Systems. Springer-Verlag.