[jira] [Resolved] (HDFS-4093) In branch-1-win, AzureBlockPlacementPolicy#chooseTarget only returns one DN when replication factor is greater than 3.

["Tsz Wo (Nicholas), SZE (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HDFS-4093?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Tsz Wo (Nicholas), SZE resolved HDFS-4093.
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       Resolution: Fixed
    Fix Version/s: 1-win

I have committed this.  Thanks, Jing!
                
> 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
>             Fix For: 1-win
>
>         Attachments: HDFS-b1-win-4093.001.patch, HDFS-b1-win-4093.002.patch
>
>
> 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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