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https://issues.apache.org/jira/browse/HDFS-599?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12866751#action_12866751
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Eli Collins commented on HDFS-599:
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@Dmytro That makes sense, it takes over ten minutes for the NN to think a DN is
dead so it seems like you'd need more than a GC pause to cause this. Does this
change address the other causes of this issue (NN thinking DNs have died)
you've seen in your cluster? Besides gc Dhruba also mentioned in the opening
comment "inconsistent performance of NFS filer".
> Improve Namenode robustness by prioritizing datanode heartbeats over client
> requests
> ------------------------------------------------------------------------------------
>
> Key: HDFS-599
> URL: https://issues.apache.org/jira/browse/HDFS-599
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: dhruba borthakur
> Assignee: Dmytro Molkov
> Attachments: HDFS-599.patch
>
>
> The namenode processes RPC requests from clients that are reading/writing to
> files as well as heartbeats/block reports from datanodes.
> Sometime, because of various reasons (Java GC runs, inconsistent performance
> of NFS filer that stores HDFS transacttion logs, etc), the namenode
> encounters transient slowness. For example, if the device that stores the
> HDFS transaction logs becomes sluggish, the Namenode's ability to process
> RPCs slows down to a certain extent. During this time, the RPCs from clients
> as well as the RPCs from datanodes suffer in similar fashion. If the
> underlying problem becomes worse, the NN's ability to process a heartbeat
> from a DN is severly impacted, thus causing the NN to declare that the DN is
> dead. Then the NN starts replicating blocks that used to reside on the
> now-declared-dead datanode. This adds extra load to the NN. Then the
> now-declared-datanode finally re-establishes contact with the NN, and sends a
> block report. The block report processing on the NN is another heavyweight
> activity, thus casing more load to the already overloaded namenode.
> My proposal is tha the NN should try its best to continue processing RPCs
> from datanodes and give lesser priority to serving client requests. The
> Datanode RPCs are integral to the consistency and performance of the Hadoop
> file system, and it is better to protect it at all costs. This will ensure
> that NN recovers from the hiccup much faster than what it does now.
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