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From "Dmytro Molkov (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-599) Improve Namenode robustness by prioritizing datanode heartbeats over client requests
Date Thu, 17 Jun 2010 20:49:28 GMT

    [ https://issues.apache.org/jira/browse/HDFS-599?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12879927#action_12879927
] 

Dmytro Molkov commented on HDFS-599:
------------------------------------

Again, running tests by hand:

     [exec] +1 overall.
     [exec]
     [exec]     +1 @author.  The patch does not contain any @author tags.
     [exec]
     [exec]     +1 tests included.  The patch appears to include 10 new or modified tests.
     [exec]
     [exec]     +1 javadoc.  The javadoc tool did not generate any warning messages.
     [exec]
     [exec]     +1 javac.  The applied patch does not increase the total number of javac compiler
warnings.
     [exec]
     [exec]     +1 findbugs.  The patch does not introduce any new Findbugs warnings.
     [exec]
     [exec]     +1 release audit.  The applied patch does not increase the total number of
release audit warnings.


Running the ant test now

> 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
>             Fix For: 0.22.0
>
>         Attachments: HDFS-599.3.patch, 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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