NetApp recommends that mixing storage node types in a cluster running Element Software should result in no single node being larger than 33% of the total cluster capacity.
For instance, in a cluster with four SF-Series 4805 nodes, the largest node that can be added by itself is an SF-Series 9605. The cluster capacity threshold is calculated based on the potential loss of the largest node in this situation.
Violating the rule of thirds will result in the following cluster alert:
A single node accounts for more than one-third of the storage cluster's capacity. Contact NetApp Support for assistance.
For more information, please refer to the managing nodes section of the user documentation.