With the development of hardware technology, the cost of main memory is decreasing, which makes it possible to let DBMS (Database Management System) put the whole data into main memory. Compared to traditional DRDB (DiskResident Database), MMDB (MainMemory Database) provides much faster of data storage, higher throughput of applications, stronger ability on concurrent access, and meets the demand of timely response. However, due to its volatility, MMDB has differences on system availability with DRDB. The survey focuses on main strategies of improving the availability of MMDBs, including fast recovery, redundant backup and fault tolerance mechanism.