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https://issues.apache.org/jira/browse/HADOOP-3799?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12708122#action_12708122
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Hairong Kuang commented on HADOOP-3799:
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> Balancer does not use the configured policy but ensures that the number of
> unique racks for the block is not reduced. Thus, both these components should
> work ok with a externally configured replication policy, isn't it?
Replication policies may have other requirements other than number of racks,
for example co-location. I don't think balancer handles this right now.
> Design a pluggable interface to place replicas of blocks in HDFS
> ----------------------------------------------------------------
>
> Key: HADOOP-3799
> URL: https://issues.apache.org/jira/browse/HADOOP-3799
> Project: Hadoop Core
> Issue Type: Improvement
> Components: dfs
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
> Attachments: BlockPlacementPluggable.txt
>
>
> The current HDFS code typically places one replica on local rack, the second
> replica on remote random rack and the third replica on a random node of that
> remote rack. This algorithm is baked in the NameNode's code. It would be nice
> to make the block placement algorithm a pluggable interface. This will allow
> experimentation of different placement algorithms based on workloads,
> availability guarantees and failure models.
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