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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
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Allen Wittenauer commented on MAPREDUCE-2149:
---------------------------------------------

+1 

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
longer.
> 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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