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https://issues.apache.org/jira/browse/HDFS-8875?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14662250#comment-14662250
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Chris Trezzo commented on HDFS-8875:
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bq. I thought the balancer is going to wait until one namespace finish moving
before going to the next namespace/iteration, no?
Good point. I was thinking in terms of a setup where the balancer is killed
periodically and the first namespace never finishes... If the balancer is never
restarted then you are right, I don't see a point to the shuffle.
> Optimize the wait time in Balancer for federation scenario
> ----------------------------------------------------------
>
> Key: HDFS-8875
> URL: https://issues.apache.org/jira/browse/HDFS-8875
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Ming Ma
> Assignee: Chris Trezzo
>
> Balancer has wait time between two consecutive iterations. That is to give
> some time for block movement to be fully committed ( return from replaceBlock
> doesn't mean the NN's blockmap has been updated and the block has been
> invalidated on the source node.).
> This wait time could be 23 seconds if {{dfs.heartbeat.interval}} is set to 10
> and {{dfs.namenode.replication.interval}} is to 3. In the case of federation,
> given we iterate through all namespaces in each iteration, this wait time
> becomes unnecessary as while balancer is processing the next namespace, it
> gives the previous namespace it just finished time to commit.
> In addition, Balancer calls {{Collections.shuffle(connectors);}} It doesn't
> seem necessary.
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