When a cluster is in quorum, what portion of participating nodes are healthy?

Two-thirds is not the threshold used by ONTAP for quorum - the requirement is a simple majority, which is a lower threshold.

ONTAP defines quorum as a simple majority, meaning more than 50% of participating nodes must be healthy and reachable. This majority-based model ensures cluster-wide consistency and prevents split-brain scenarios. If a simple majority cannot be established, the cluster halts operations to protect data integrity.

Three-quarters is not the defined quorum threshold in ONTAP cluster architecture.

Exactly half is insufficient for quorum - a simple majority requires strictly more than half of nodes to be healthy.