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https://issues.apache.org/jira/browse/MAPREDUCE-7500?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17929234#comment-17929234
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ASF GitHub Bot commented on MAPREDUCE-7500:
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robreeves opened a new pull request, #7425:
URL: https://github.com/apache/hadoop/pull/7425
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### Description of PR
This PR adds a new feature to commit files optimistically (assumes no
conflicting file/dir in the destination) to avoid a `FileSystem.getFileStatus`
RPC. The default behavior has not been changed. To use this feature this config
must be set `mapreduce.fileoutputcommitter.optimistic.file.commit.enabled=true`.
This is useful for cases like Spark where no destination conflict is
expected and the `FileSystem.getFileStatus` RPC is wasted time. TODO add more
details.
### How was this patch tested?
TODO add unit tests
I tested the performance of the changes using Spark writing to HDFS for
partitioned and non-partitioned datasets. TODO add more details.
### For code changes:
- [x] Does the title or this PR starts with the corresponding JIRA issue id
(e.g. 'HADOOP-17799. Your PR title ...')?
- [ ] Object storage: have the integration tests been executed and the
endpoint declared according to the connector-specific documentation?
- [ ] If adding new dependencies to the code, are these dependencies
licensed in a way that is compatible for inclusion under [ASF
2.0](http://www.apache.org/legal/resolved.html#category-a)?
- [ ] If applicable, have you updated the `LICENSE`, `LICENSE-binary`,
`NOTICE-binary` files?
> 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
> Reporter: Rob Reeves
> Priority: Minor
> 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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