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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-1448) Setting the replication factor of a file too high causes namenode cpu overload
Date Thu, 31 May 2007 18:31:15 GMT
Setting the replication factor of a file too high causes namenode cpu overload
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                 Key: HADOOP-1448
                 URL: https://issues.apache.org/jira/browse/HADOOP-1448
             Project: Hadoop
          Issue Type: Bug
          Components: dfs
            Reporter: dhruba borthakur


The replication factor of a file in set to 300 (on a 800 node cluster). Then all mappers try
to open this file. For every open call that the namenode receives from each of these 800 clients,
it sorts all the replicas of the block(s) based on the distance from the client. This causes
CPU usage overload on the namenode.

One proposal is to make the namenode return a non-sorted list of datanodes to the client.
Information about each replica also contains the rack on which that replica resides. The client
can look at the replicas to determine if there is a copy on the local node. If not, then it
can find out if there is a replica on the local rack. If not then it can choose a replica
at random.

This proposal is scalable because the sorting and selection of replicas is done by the client
rather than the Namenode.

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