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From "Jing Zhao (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-4093) In branch-1-win, AzureBlockPlacementPolicy#chooseTarget only returns one DN when replication factor is greater than 3.
Date Fri, 19 Oct 2012 22:24:12 GMT
Jing Zhao created HDFS-4093:
-------------------------------

             Summary: In branch-1-win, AzureBlockPlacementPolicy#chooseTarget only returns
one DN when replication factor is greater than 3. 
                 Key: HDFS-4093
                 URL: https://issues.apache.org/jira/browse/HDFS-4093
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: Jing Zhao
            Assignee: Jing Zhao


In branch-1-win, when AzureBlockPlacementPolicy (which extends the BlockPlacementPolicyDefault)
is used, if the client increases the number of replicas (e.g., from 3 to 10), AzureBlockPlacementPolicy#chooseTarget
will return only 1 Datanode each time. Thus in FSNameSystem#computeReplicationWorkForBlock,
it is possible that the replication monitor may choose a datanode that has been chosen as
target but still in the pendingReplications (because computeReplicationWorkForBlock does not
check the pending replication before doing the chooseTarget). 

To avoid this "hit-the-same-datanode" scenario, we modify the AzureBlockPlacementPolicy#chooseTarget
to make it return multiple DN. 

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