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From "Harsh J (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-2149) Distcp : setup with update is too slow when latency is high
Date Thu, 24 Aug 2017 23:56:00 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-2149?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16140953#comment-16140953

Harsh J commented on MAPREDUCE-2149:

I don't appear to have rights to change the resolution status here, but note that the true
resolution of this JIRA is not "Fixed" as indicated, but instead "Duplicate" (of MAPREDUCE-2765,
which introduces DistCp2 for YARN/MR2).

> 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
>         Attachments: MAPREDUCE-2149.patch
> 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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