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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-15281) Distcp to add no-rename copy option
Date Thu, 24 Jan 2019 18:20:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-15281?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16751403#comment-16751403
] 

Steve Loughran commented on HADOOP-15281:
-----------------------------------------

I see it. Hit the "submit patch" button and jenkins will build it

> Distcp to add no-rename copy option
> -----------------------------------
>
>                 Key: HADOOP-15281
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15281
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: tools/distcp
>    Affects Versions: 3.0.0
>            Reporter: Steve Loughran
>            Assignee: Andrew Olson
>            Priority: Major
>         Attachments: HADOOP-15281-001.patch
>
>
> Currently Distcp uploads a file by two strategies
> # append parts
> # copy to temp then rename
> option 2 executes the following sequence in {{promoteTmpToTarget}}
> {code}
>     if ((fs.exists(target) && !fs.delete(target, false))
>         || (!fs.exists(target.getParent()) && !fs.mkdirs(target.getParent()))
>         || !fs.rename(tmpTarget, target)) {
>       throw new IOException("Failed to promote tmp-file:" + tmpTarget
>                               + " to: " + target);
>     }
> {code}
> For any object store, that's a lot of HTTP requests; for S3A you are looking at 12+ requests
and an O(data) copy call. 
> This is not a good upload strategy for any store which manifests its output atomically
at the end of the write().
> Proposed: add a switch to write direct to the dest path. either a conf option or a CLI
option



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