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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-599) Improve Namenode robustness by prioritizing datanode heartbeats over client requests
Date Sat, 05 Sep 2009 07:47:57 GMT
Improve Namenode robustness by prioritizing datanode heartbeats over client requests
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                 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: dhruba borthakur


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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