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Vertica has a unique approach to failure recovery that is based on the distributed nature of a database. If the physical schema design of a database is 
K-Safe, the database can lose a node and keep on running as if nothing happened. When the lost node comes back online and rejoins the database, it recovers its lost objects by querying the other nodes. K-Safety is determined by whether or not the projections in the physical schema design meet certain redundancy criteria.
In 2.1.GA, a database can continue to run if one node goes down. However, another node failure causes the database to automatically shut down and can result in loss of data.
The minimum number of nodes required for K-safety is three. Vertica does not support production databases having fewer than three nodes. Vertica Systems, Inc. recommends that you use a minimum of four nodes.