[jira] [Commented] (HADOOP-15281) Distcp to add no-rename copy option

["Andrew Olson (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-15281?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16742311#comment-16742311 ] 

Andrew Olson commented on HADOOP-15281:
---------------------------------------

We've successfully tested a quick+dirty solution where in the RetriableFileCopyCommand class, renames are specifically avoided for any S3A target destinations in {{doCopy}} - I'm not sure there are cases where it's actually beneficial or desirable when writing to S3. The gist of the logic is simply,

{noformat}
        final boolean toAppend = action == FileAction.APPEND;
        final boolean directWriteToS3 = target.toUri().getScheme().equals("s3a");
        final boolean useTmpTarget = !toAppend && !directWriteToS3;
        Path targetPath = useTmpTarget ? getTmpFile(target, context) : target;
{noformat}

(Then replacing a couple instances of {{!toAppend}} with {{useTmpTarget}} later on in the method)

If that's a viable & reasonable solution I can work on a patch with that change and the corresponding test/docs updates. Otherwise design guidance would be appreciated for how and where to add the write-directly no-rename configuration option.

> 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
>            Priority: Major
>
> 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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