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https://issues.apache.org/jira/browse/HDFS-3495?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Junping Du reopened HDFS-3495:
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reopen this JIRA to backport to branch-2.
> Update Balancer to support new NetworkTopology with NodeGroup
> -------------------------------------------------------------
>
> Key: HDFS-3495
> URL: https://issues.apache.org/jira/browse/HDFS-3495
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: balancer
> Affects Versions: 1.1.0, 2.0.2-alpha
> Reporter: Junping Du
> Assignee: Junping Du
> Fix For: 3.0.0
>
> Attachments: HADOOP-8473-Balancer-NodeGroup-aware.patch,
> HDFS-3495-v2.patch, HDFS-3495-v3.patch, HDFS-3495-v4.patch, HDFS-3495-v5.patch
>
>
> Since the Balancer is a Hadoop Tool, it was updated to be directly aware of
> four-layer hierarchy instead of creating an alternative Balancer
> implementation. To accommodate extensibility, a new protected method,
> doChooseNodesForCustomFaultDomain is now called from the existing chooseNodes
> method so that a subclass of the Balancer could customize the balancer
> algotirhm for other failure and locality topologies. An alternative option is
> to encapsulate the algorithm used for the four-layer hierarchy into a
> collaborating strategy class.
> The key changes introduced to support a four-layer hierarchy were to override
> the algorithm of choosing <source, target> pairs for balancing. Unit tests
> were created to test the new algorithm.
> The algorithm now makes sure to choose the target and source node on the same
> node group for balancing as the first priority. Then the overall balancing
> policy is: first doing balancing between nodes within the same nodegroup then
> the same rack and off rack at last. Also, we need to check no duplicated
> replicas live in the same node group after balancing.
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