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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7967) Reduce the performance impact of the balancer
Date Wed, 25 Mar 2015 22:19:55 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14380899#comment-14380899
] 

Daryn Sharp commented on HDFS-7967:
-----------------------------------

Providing a more deterministic ordering of blocks causes tests to fail because delete acks
are not promptly sent after invalidation of a balanced block.  The NN continues to hand out
the location on a datanode which already deleted the block but has not ack-ed.  The DNs return
non-existent replicas exceptions to the balancer.

Current tests "work" because the balancer keeps re-querying random ranges until it eventually
(accidentally?) works.

> 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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