Lukas Majercak created HDFS-12288:

             Summary: Change DN xceiver count calculation
                 Key: HDFS-12288
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: datanode, hdfs
            Reporter: Lukas Majercak
            Assignee: Lukas Majercak

The problem with the ThreadGroup.activeCount() method is that the method is 
only a very rough estimate, and in reality returns the total number of threads 
in the thread group as opposed to the threads actually running.

In some DNs, we saw this to return 50~ for a long time, even though the actual 
number of DataXceiver threads was next to none.
This is a big issue as we use the xceiverCount to make decisions on the NN for 
choosing replication source DN or returning DNs to clients for R/W.

The plan is to reuse the DataNodeMetrics.dataNodeActiveXceiversCount value 
which only accounts for actual number of DataXcevier threads currently running 
and thus represents the load on the DN much better.

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