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https://issues.apache.org/jira/browse/HADOOP-1127?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Arun C Murthy updated HADOOP-1127:
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Status: Patch Available (was: Open)
> Speculative Execution and output of Reduce tasks
> ------------------------------------------------
>
> Key: HADOOP-1127
> URL: https://issues.apache.org/jira/browse/HADOOP-1127
> Project: Hadoop
> Issue Type: Improvement
> Components: mapred
> Affects Versions: 0.12.0
> Reporter: Arun C Murthy
> Assigned To: Arun C Murthy
> Fix For: 0.13.0
>
> Attachments: HADOOP-1127_20070328_1.patch,
> HADOOP-1127_20070331_2.patch, HADOOP-1127_20070402_3.patch,
> HADOOP-1127_20070403_4.patch
>
>
> We've recently seen instances where jobs run with 'speculative execution'
> tend to be quite unstable and fail with *AlreadyBeingCreatedException*
> noticed at the NameNode. Also potentially we could have hairy situations
> where a failed Reduce tasks's output could clash with a successful task's
> (same tip) output.
> As it exists, speculative execution relies on the PhasedFileSystem which
> creates a temp output file and then on task-completion that file is 'moved'
> to its final position via a call to PhasedFileSystem.commit from
> ReduceTask.run(). This has lead to issues such as the above.
> Proposal:
> Basically the idea is to due this uniformly for all Reduce tasks i.e. all
> reducers create temp files and then have a serialized 'commit' done by the
> JobTracker which moves the temp file to it's final position.
> We create the temp file in the job's output directory itself:
> <output_dir>/_<taskid> (emphasis on the leading '_')
> On task completion we'll add that temp file's path to the TaskStatus and then
> the JobTracker moves that file to it's final position.
> Thoughts?
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