HDFS-5620 indicated a GlobalAclSet containing unique AclFeature can be de-duplicated to save the memory in NameNode. However it was not implemented at that time.
This Jira re-proposes same implementation, along with de-duplication of unique AclEntry across all ACLs.
One simple usecase is:
A mapreduce user's home directory with the set of default ACLs, under which lot of other files/directories could be created when jobs is run. Here all the default ACLs of parent directory will be duplicated till the explicit delete of those ACLs. With de-duplication,only one object will be in memory for the same Entry across all ACLs of all files/directories.