[jira] [Commented] (HADOOP-13975) Allow DistCp to use MultiThreadedMapper

["Zheng Shao (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-13975?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15819514#comment-15819514 ] 

Zheng Shao commented on HADOOP-13975:
-------------------------------------

Example usage:

bin/hadoop distcp -Dmapreduce.job.user.classpath.first=true -prbugp -m 2 -numThreadsPerMap 16 hdfs://sourcehdfs/srcdir hdfs://targethdfs/

Note that "Dmapreduce.job.user.classpath.first=true" is needed if you only update the client-side hadoop-tools jar but not the server (YARN nodemanager) side yet.


> Allow DistCp to use MultiThreadedMapper
> ---------------------------------------
>
>                 Key: HADOOP-13975
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13975
>             Project: Hadoop Common
>          Issue Type: New Feature
>          Components: tools/distcp
>    Affects Versions: 3.0.0-alpha1
>            Reporter: Zheng Shao
>            Assignee: Zheng Shao
>            Priority: Minor
>         Attachments: HADOOP-distcp-multithreaded-mapper-branch26.1.patch, HADOOP-distcp-multithreaded-mapper-trunk.1.patch
>
>
> Although distcp allow users to control the parallelism via number of mappers, sometimes it's desirable to run fewer mappers but more threads per mapper.  Since distcp is network bound (either by throughput or more frequently by latency of creating connections, opening files, reading/writing files, and closing files), this can make each mapper much more efficient.
> In that way, a lot of resources can be shared so we can save memory and connections to NameNode.



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