After you test the functional aspects of the cluster, you must finalize the cluster upgrade, which is otherwise left in upgrade start status state. Major changes to the file system HDFS are not complete until the process is finalized, and the system cannot reclaim space from any clean up efforts to follow. You cannot run the balancer, and major HBase compactions will consume massive amounts of space. Even smaller cluster upgrades should be finalized promptly.

If you had remove unhealthy hosts from the cluster before the upgrade, you can now add them back, as if they were new additions. You cannot run the balancer, and major HBase compactions will consume massive amounts of space. Even smaller cluster upgrades should be finalized promptly.