Tsz Wo Nicholas Sze created HDFS-8361:
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             Summary: Choose SSD over DISK in block placement
                 Key: HDFS-8361
                 URL: https://issues.apache.org/jira/browse/HDFS-8361
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: namenode
            Reporter: Tsz Wo Nicholas Sze
            Assignee: Tsz Wo Nicholas Sze


BlockPlacementPolicyDefault chooses the StorageType by iterating the given 
StorageType EnumMap in its natural order (the order in which the enum constants 
are declared).  So DISK will be chosen over SSD in One-SSD policy since DISK is 
declared before SSD as shown below.  We should choose SSD first.

{code}
public enum StorageType {
  DISK(false),
  SSD(false),
  ARCHIVE(false),
  RAM_DISK(true);

  ...
}
{code}




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