Similarly to HDFS-5027, an improvement can be made for getVomeMap(). This is the phase in which ReplicaMap.is populated. But it will be even better if datanode scans only once and do both.
Allow DU to be initialized with an initial value
Add quick-restart option for datanode
Always read DU value from the cached "dfsUsed" file on datanode startup
On startup, DN should scan volumes in parallel