In Balancer, the target task should be removed when its size < 0.

    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;
            //...

    long size = Math.min(source.availableSizeToMove(), target.availableSizeToMove());
    final Task task = new Task(target, size);

2016-08-01 16:51:57,492 [pool-51-thread-1] INFO  balancer.Dispatcher (Dispatcher.java:chooseNextMove(799)) - TargetNode: 58794, bytes scheduled to move, after: -67, before: 33