This enhancement adds improved replica placement, and protection from duplicated assignments.
This was added because, in the traditional Swift layout, the act of accidentally assigning two replicas of a partition to the same device resulted in a silent reduction of durability.
As a result, duplicate assignments are prevented, thereby adhering to calculated guarantees. However, since this requires the number of devices to be no less than the number of replicas, it is possible for certain incorrect old rings to be considered invalid. Consequently, it is still possible to have the number of zones be smaller than the number of replicas.