Walter Su created HADOOP-11676:
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             Summary: Add API to NetworkTopology for getting all racks
                 Key: HADOOP-11676
                 URL: https://issues.apache.org/jira/browse/HADOOP-11676
             Project: Hadoop Common
          Issue Type: New Feature
            Reporter: Walter Su


The existing two NetworkTopology.chooseRandom(..) API support choosing node 
from scope and choosing outside scope. BlockPlacementPolicyDefault class use 
these two API to choose node from one rack or choose outside one rack.
We want to implement a new placement policy called 
BlockPlacementPolicyFaultTolerant which tries its best to place replicas to 
most racks. To achieve this, We need to know how many replicas each rack has. 
And first, we need to get all racks.



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