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https://issues.apache.org/jira/browse/HDFS-13183?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16374932#comment-16374932
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Ajay Kumar commented on HDFS-13183:
-----------------------------------

[~hexiaoqiao], this is good start but it will not redirect {{getBlocks}} 
requests to ANN. It would be good if in HA mode, ANN redirect all calls to SNN 
or signals client to direct these calls to SNN through appropriate IOE.

> Standby NameNode process getBlocks request to reduce Active load
> ----------------------------------------------------------------
>
>                 Key: HDFS-13183
>                 URL: https://issues.apache.org/jira/browse/HDFS-13183
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: balancer & mover, namenode
>    Affects Versions: 2.7.5, 3.1.0, 2.9.1, 2.8.4, 3.0.2
>            Reporter: He Xiaoqiao
>            Assignee: He Xiaoqiao
>            Priority: Major
>         Attachments: HDFS-13183-trunk.001.patch
>
>
> The performance of Active NameNode could be impact when {{Balancer}} requests 
> #getBlocks, since query blocks of overly full DNs performance is extremely 
> inefficient currently. The main reason is {{NameNodeRpcServer#getBlocks}} 
> hold read lock for long time. In extreme case, all handlers of Active 
> NameNode RPC server are occupied by one reader 
> {{NameNodeRpcServer#getBlocks}} and other write operation calls, thus Active 
> NameNode enter a state of false death for number of seconds even for minutes.
> The similar performance concerns of Balancer have reported by HDFS-9412, 
> HDFS-7967, etc.
> If Standby NameNode can shoulder #getBlocks heavy burden, it could speed up 
> the progress of balancing and reduce performance impact to Active NameNode.



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