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From "Allen Wittenauer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-2149) Distcp : setup with update is too slow when latency is high
Date Mon, 25 Oct 2010 17:44:22 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-2149?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12924645#action_12924645

Allen Wittenauer commented on MAPREDUCE-2149:


We were just talking about this last week, as we move from one data center to another.

> Distcp : setup with update is too slow when latency is high
> -----------------------------------------------------------
>                 Key: MAPREDUCE-2149
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2149
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: distcp
>    Affects Versions: 0.20.2, 0.21.0
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
> If you run distcp with '-update' option, for _each of the files_ present on source cluster
setup invokes a separate RPC to destination cluster to fetch file info. 
> Usually this overhead is not very noticeable when both cluster are geographically close
to each other. But if the latency is large, setup could take couple of orders of magnitude
> E.g. : source has 10k directories, each with about 10 files, round trip latency between
source and destination is 75 ms (typical for coast-to-coast clusters). 
> If we run distcp on source cluster, set up would take about _2.5 hours_ irrespective
of whether destination has these files or not. '-lsr' on the same dest dir from source cluster
would take up to 12 min (depending on how many directories already exist on dest). 
>   * A fairly simple fix to how setup() iterates should bring the set up time to same
as '-lsr'. I will have a patch for this.. (though 12 min is too large).
>   * A more scalable option is to differ update check to mappers.

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