It all depends
So if you have 1 master and 4 data ( 5 node cluster) and you enable HA you will have to pick a data node to become the replica so you will still have a 5 node cluster.
Enabling HA will do two things.
- As the name states it will create a replica of the master (Alarms, policies, custom groups etc) that normal only run on the Master so if you lose the master the replica will take over and all is well
- The data nodes will start to duplicate data across the nodes (this includes the Master and replica as they are also data nodes) so you will have an increase in storage requirements.
This next point is maybe where you are seeing the need to increase the node count. Once you enable HA the number of objects and metrics that each data node can handle will drop. You need to make sure that you cluster is sized correctly and may require the additional nodes to take the load of enabling HA
Take a look at VMware Knowledge Base
From the notes section
When High Availability (HA) is enabled, each object is replicated in one of the nodes of a cluster. The limit of objects for HA based environment is half compared to a non-HA configuration. vRealize Operations HA supports only one node failure and you can avoid Single-point-of-failure by adding vRealize Operations cluster nodes into different hosts in the cluster.
To enable high availability, you must have another node deployed in addition to the master node.
When you enable HA in your scenario, you've 1 Master, 1 Replica and 3 data nodes. One of the 4 data role nodes is changed to a Replica role node.
In HA, each object is replicated in some nodes of a cluster. HA will use double resources and reduce maximums in half.
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Thanks sxnxr, I only caught the creation of the replica node. Having "each object is replicated in one of the nodes of a cluster" pointed out makes it crystal clear.