Determining the optimal number of Oracle RAC nodes
 

Question: We are using RAC for High Availability.  What is better two nodes or three nodes?  It's been suggested that "the more nodes, the better".  What is the optimal number of RAC nodes? 

Answer:  First, remember that RAC only protects against server failure, and you must still mirror the disk and have network redundancy for guaranteed 100% availability.  In my experience, RAC is not as good as some other options for scalability (i.e. the "scale up" approach), and the most common use of RAC is for HA, and covering server failover.

In deciding about the number of nodes for HA, just multiply the Mean Time Between Failure's (MTBF) for each node together and get an overall probability of failure.  It's all about covering the probability of server failure.  When used exclusively for HA, two-node RAC is ideal, especially when the nodes are geographically distributed and connected via super-fast dark fiber networks.

Oracle ACE Andy Kerber notes that two node clusters are ideal under some circumstances:

"Well, 3 nodes can be better than two if your workload is fully scalable, and there is no resource contention among the three nodes. I would venture to say that the majority of RAC systems are 2 node clusters, and are set up that way as part of a HA system instead of a workload scaling system."

 

Read more about choosing the optimal number of RAC nodes here:

http://www.dba-oracle.com/t_optimal_number_rac_nodes.htm

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