This is a known bug => https://issues.apache.org/jira/browse/AMBARI-10930 But what to do if you cannot upgrade Ambari but needs to enable Kerberos again with a different Realm ? You have to remove the remaining Kerberos artifacts by using the following REST call, then you can enable Kerberos with the new Realm applied: curl -H "X-Requested-By:ambari" -u <adminuser>:<adminpw> -X DELETE http://<ambari-server>:8080/api/v1/clusters/<clustername>/artifacts/kerberos_descriptor ... View more

I'm most familiar with GC tuning for HDFS, so I'll answer from that perspective. As you expected, our recommendation for the HDFS daemons is CMS. In practice, we have found that some of the default settings for CMS are sub-optimal for the NameNode's heap usage pattern. In addition to enabling CMS, we recommend tuning a few of those settings. I agree that G1 would be good to evaluate as the future direction. As of right now, we have not tested and certified with G1, so I can't recommend using it. For more details, please refer to the NameNode garbage collection deep dive article that I just posted. https://community.hortonworks.com/articles/14170/namenode-garbage-collection-configuration-best-pra.html ... View more