Data storage cluster with quorum service protection

In a protective quorum service, during an initial period of normal operation in which a clustered pair of data storage nodes provide host I/O access to a data storage object and replicate write-type requests to each other, the nodes are first registered to the quorum service. Subsequently, based on the registration and in response to a first auto promote request from a first-requesting node, a success response is returned and the service enters an auto promoted condition, the success response indicating that the first-requesting node is to continue providing the host I/O access to the data storage object without write replication. In response to receiving a subsequent auto promote request from the other node when in the auto promoted condition, a failure response is returned indicating that the other node is to cease providing host I/O access to the data storage object.