Tsz Wo Nicholas Sze created HDFS-8826:
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             Summary: Balancer may not move blocks efficiently in some cases
                 Key: HDFS-8826
                 URL: https://issues.apache.org/jira/browse/HDFS-8826
             Project: Hadoop HDFS
          Issue Type: Sub-task
          Components: balancer & mover
            Reporter: Tsz Wo Nicholas Sze
            Assignee: Tsz Wo Nicholas Sze


Balancer is inefficient in the following case:
|| Datanode || Utilization || Rack ||
| D1 | 95% | A |
| D2 | 30% | B |
| D3, D4, D5 | 0% | B |

The average utilization is 25% so that D2 is within 10% threshold.  However, 
Balancer currently will first move blocks from D2 to D3, D4 and D5 since they 
are under the same rack.  Then, it will move blocks from D1.



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