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https://issues.apache.org/jira/browse/MAPREDUCE-7500?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17939348#comment-17939348
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ASF GitHub Bot commented on MAPREDUCE-7500:
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robreeves closed pull request #7425: MAPREDUCE-7500. Support optimistic file
renames in the commit protocol
URL: https://github.com/apache/hadoop/pull/7425
> Support optimistic file renames in the commit protocol
> ------------------------------------------------------
>
> Key: MAPREDUCE-7500
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-7500
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: client
> Environment: The commit protocol in FileOutputCommitter now supports
> optimistic commits for files. This saves a FileSystem.getFileStatus call for
> cases where it is unexpected to have conflict in the destination location at
> commit time (e.g. Spark). This feature is disabled by default. To enable it
> set mapreduce.fileoutputcommitter.optimistic.file.commit.enabled=true.
> Reporter: Rob Reeves
> Priority: Minor
> Labels: pull-request-available
> Attachments: flamegraph_commit.png
>
>
> During a file commit in FileOutputCommitter, it assumes a file may be in the
> destination location and if so will delete it first. This means for every
> file commit is calls FileSystem.getFileStatus for the destination. For the
> Spark use case, there will be nothing existing in the destination location
> for the expected case so the getFileStatus call is wasted in all, but
> exceptional and unexpected cases.
> The getFileStatus call can take significant time. When I profiled a commit in
> our environment (HDFS, intermittent latency issues) the
> FileSystem.getFileStatus call takes 50% of the commit time. We have an
> aggressive auto-msync setting, but even when I disabled msync I saw the same
> behavior. I attached an example flame graph for the commit time
> (getFileStatus time is highlighted in pink).
> To avoid the time spent on getFileStatus, there should be an option to
> optimistically commit the file assuming there will be no conflict in the
> destination.
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