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https://issues.apache.org/jira/browse/HDFS-3990?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13476139#comment-13476139
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Daryn Sharp commented on HDFS-3990:
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The caching is to prevent the unnecessary dns lookups that are a multiple of
the number of datanodes - typically just to view a jsp or query json, or for
other internal operations as well. Every time a node is checked against the
include/exclude lists, it generates dns queries of 2X the datanodes. Counting
the number of nodes causes a dns query for every datanode.
Reassigning an ip should require no restart of the NN. The DN's are tracked by
their ip and storage id. If a DN registers with a previously known ip or
storage id, the existing node is updated with the fields in the new node id
which contain a refreshed lookup.
> NN's health report has severe performance problems
> --------------------------------------------------
>
> Key: HDFS-3990
> URL: https://issues.apache.org/jira/browse/HDFS-3990
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: name-node
> Affects Versions: 0.23.0, 2.0.0-alpha, 3.0.0
> Reporter: Daryn Sharp
> Assignee: Daryn Sharp
> Priority: Critical
> Attachments: HDFS-3990.patch
>
>
> The dfshealth page will place a read lock on the namespace while it does a
> dns lookup for every DN. On a multi-thousand node cluster, this often
> results in 10s+ load time for the health page. 10 concurrent requests were
> found to cause 7m+ load times during which time write operations blocked.
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