About the whole no downtime thing If you have enabled Automatic Failover using ZooKeeper Failover Controllers, an additional step is required to reset the ZKFC state in ZooKeeper. Stop both the ZKFC processes and run the following command as the hdfs super user. I think some clarification would be nice Stop the ZKFC and then format zkfc znode and after that restart NameNodes? Or restart the NameNodes, then stop the ZKFC and format after that? ... View more