The Under replicated partitions metric displays the number of partitions that do not have enough replicas to meet the desired replication factor. A partition will also be considered under-replicated if the correct number of replicas exist, but one or more of the replicas have fallen significantly behind the partition leader. The two most common causes of under-replicated partitions are that one or more brokers is unresponsive, or the cluster is experiencing performance issues and one or more brokers have fallen behind.