Fengnan Li created HDFS-15634:
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             Summary: Invalidate block on decommissioning DataNode after 
replication
                 Key: HDFS-15634
                 URL: https://issues.apache.org/jira/browse/HDFS-15634
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: hdfs
            Reporter: Fengnan Li
            Assignee: Fengnan Li


Right now when a DataNode starts decommission, Namenode will mark it as 
decommissioning and its blocks will be replicated over to different DataNodes, 
then marked as decommissioned. These blocks are not touched since they are not 
counted as live replicas.

Proposal: Invalidate these blocks once they are replicated and there are enough 
live replicas in the cluster.

Reason: A recent shutdown of decommissioned datanodes to finished the flow 
caused Namenode latency spike since namenode needs to remove all of the blocks 
from its memory and this step requires holding write lock. If we have gradually 
invalidated these blocks the deletion will be much easier and faster.



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