Implement feature of Rolling Decommission for HDFS Datanodes.
Use "Under-Replicated Blocks" as parameter for Rolling Decommission, Instead of Wait Time.
E.g. - To decommission 100 HDFS Datanodes - with rolling decommission of 3 nodes at a time, a batch of decommission should trigger when a threshold of - "Minimum number of Under-Replicated Blocks" has reached.
*Note - Minimum Threshold of - "Under-Replicated Blocks" has to be taken care so that we don't end up in a situation with huge load on HDFS Namenode and high number of Under Replicated Blocks - Specially with Datanodes of huge Data size ( ~ TB )