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https://issues.apache.org/jira/browse/HDFS-13183?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16383114#comment-16383114
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He Xiaoqiao commented on HDFS-13183:
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[~xkrogen]
{quote}if the SbNN goes down, the ANN is not aware of this, but the balancer
should start to read from the ANN instead of SbNN.{quote}
v003 can not process this situation indeed, and i think it is better if client
is able to make decision to request the proper namenode which may need to
refactor {{NameNodeConnector}}, and I review the target of HDFS-12976, maybe we
need wait for finishing.
Thanks again for your detailed code reviewed. [~xkrogen]
> 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, HDFS-13183-trunk.002.patch,
> HDFS-13183-trunk.003.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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