- ONTAP 9
- Cluster HA (2 node cluster)
What is RDB Quorum?
RDB Quorum is a precondition for a fully functioning cluster
- Configurations that have greater than 2 nodes in the cluster, the entire cluster maintains quorum using a voting mechanism where each node has a vote that decides which node will maintain the master copy (read/write) of the replicated databases (also known as the RDB Master)
- When a cluster is in quorum, a simple majority of nodes are healthy and can communicate with each other
- When quorum is lost, the cluster loses the ability to accomplish normal cluster operations
What is epsilon?
Epsilon is a tie breaker
- Because there is the possibility of a tie in a cluster that has an even number of nodes, one node has an extra fractional voting weight called epsilon
- If the connectivity between two equal portions of a large cluster fails, the group of nodes containing epsilon maintains quorum, assuming that all of the nodes are healthy
What is Cluster HA?
Cluster HA allows continuous monitoring of a 2 node Cluster's state
- A 2 node cluster presents some unique challenges for maintaining quorum
- 2 node clusters use cluster HA, in which neither node holds epsilon
- Instead, both nodes are continuously polled to ensure that if one node fails, the other has full read-write access to data, as well as access to logical interfaces and management functions
- Quorum is maintained by a successful takeover