[jira] Commented: (HADOOP-1448) Setting the replication factor of a file too high causes namenode cpu overload

["dhruba borthakur (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-1448?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12501313 ] 

dhruba borthakur commented on HADOOP-1448:
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In the short term, I like Doug's proposal #1. The current code already supports a configuration setting named dfs.replication.max. It is set to 512.

For installations that are seeing this problem, maybe they can change dfs.replication.max to a smaller number, say 10 - 20 or so. An application that was successfully able to set a replication factor of 512 with the current configuration will now experience an exception when dfs.replication.max is set to 20.

> Setting the replication factor of a file too high causes namenode cpu overload
> ------------------------------------------------------------------------------
>
>                 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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