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https://issues.apache.org/jira/browse/HDFS-7967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14372305#comment-14372305
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Daryn Sharp commented on HDFS-7967:
-----------------------------------
The current implementation is so bad that on large clusters we have to restrict
the balancer to using only one thread for block queries. Multiple threads will
destroy the performance of busy namenodes by causing call queue overflows.
> Reduce the performance impact of the balancer
> ---------------------------------------------
>
> Key: HDFS-7967
> URL: https://issues.apache.org/jira/browse/HDFS-7967
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: namenode
> Affects Versions: 2.0.0-alpha
> Reporter: Daryn Sharp
> Assignee: Daryn Sharp
>
> The balancer needs to query for blocks to move from overly full DNs. The
> block lookup is extremely inefficient. An iterator of the node's blocks is
> created from the iterators of its storages' blocks. A random number is
> chosen corresponding to how many blocks will be skipped via the iterator.
> Each skip requires costly scanning of triplets.
> The current design also only considers node imbalances while ignoring
> imbalances within the nodes's storages. A more efficient and intelligent
> design may eliminate the costly skipping of blocks via round-robin selection
> of blocks from the storages based on remaining capacity.
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