Currently the StatefulSets support only two kinds of update strategies:

• RollingUpdate: The RollingUpdate update strategy implements automated, rolling update for the Pods in a StatefulSet. It is the default strategy when .spec.updateStrategy is left unspecified. When a StatefulSet's .spec.updateStrategy.type is set to RollingUpdate, the StatefulSet controller will delete and recreate each Pod in the StatefulSet. It will proceed in the same order as Pod termination (from the largest ordinal to the smallest), updating each Pod one at a time. It will wait until an updated Pod is Running and Ready prior to updating its predecessor.

• OnDelete: The OnDelete update strategy implements the legacy (1.6 and prior) behavior. When a StatefulSet's .spec.updateStrategy.type is set to OnDelete, the StatefulSet controller will not automatically update the Pods in a StatefulSet. Users must manually delete Pods to cause the controller to create new Pods that reflect modifications made to a StatefulSet's .spec.template.

However, there is a plan to implement a MaxUnavailable Rolling Update to StatefulSet. It would allow you to update X number of replicas together based on a maxUnavailble strategy. It led to this update proposal but it is not done yet and judging from the latest comments it should be set as a milestone for Kubernetes 1.20.