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From "Yiqun Lin (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-10716) The target node should be removed in balancer when the scheduled size of bytes that firstly reduced to less than 0
Date Tue, 02 Aug 2016 13:00:22 GMT
Yiqun Lin created HDFS-10716:
--------------------------------

             Summary: The target node should be removed in balancer when the scheduled size
of bytes that firstly reduced to less than 0
                 Key: HDFS-10716
                 URL: https://issues.apache.org/jira/browse/HDFS-10716
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: balancer & mover
            Reporter: Yiqun Lin
            Assignee: Yiqun Lin


In HDFS-10602, we found a failing case that the balancer moves data always between 2 DNs.
And it made the balancer can't be finished. I debug the code for this, I found there seems
a bug in choosing pending blocks in {{Dispatcher.Source.chooseNextMove}}.

The codes:
{code}
    private PendingMove chooseNextMove() {
      for (Iterator<Task> i = tasks.iterator(); i.hasNext();) {
        final Task task = i.next();
        final DDatanode target = task.target.getDDatanode();
        final PendingMove pendingBlock = new PendingMove(this, task.target);
        if (target.addPendingBlock(pendingBlock)) {
          // target is not busy, so do a tentative block allocation
          if (pendingBlock.chooseBlockAndProxy()) {
            long blockSize = pendingBlock.reportedBlock.getNumBytes(this);
            incScheduledSize(-blockSize);
            task.size -= blockSize;
            // If the size of bytes that need to be moved was first reduced to less than 0
            // it should also be removed.
            if (task.size == 0) {
              i.remove();
            }
            return pendingBlock;
            //...
{code}
The value of task.size was assigned in {{Balancer#matchSourceWithTargetToMove}}
{code}
    long size = Math.min(source.availableSizeToMove(), target.availableSizeToMove());
    final Task task = new Task(target, size);
{code}

This value was depended on the source and target node, and this value will not always can
be reduced to 0 in choosing pending blocks. And then, it will still move the data to the target
node even if the size of bytes that needed to move has been already reduced less than 0. And
finally it will make the data imbalance again in cluster.

We can opitimize for this as this title mentioned, I think this can speed the balancer.

Can see the failling logs for this.



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