Redundancy is an attribute that can be set upon EXAStorage Volume creation. It specifies the number of copies of the data that is hosted on Active Cluster nodes.

In practice this is either Redundancy 1 or Redundancy 2. Redundancy 1 means there is no redundancy, so if one node fails, the volume with that redundancy is no longer available. Typically that is seen with one-node systems like the Exasol Community Edition. Redundancy 2 means that each node holds a copy of data that is operated on by a neighbor node, so the volume remains available if one node fails.

If volumes are configured with redundancy 2 – which is a best practice – then each node holds a mirror of data that is operated on by a neighbor node. If e.g. n11 modifies A the mirror A‘ on n12 is synchronized over the private network. Should an active node fail, the reserve node will step in starting an instance.