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From "Arun C Murthy (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1127) Speculative Execution and output of Reduce tasks
Date Wed, 28 Mar 2007 12:47:25 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-1127?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Arun C Murthy updated HADOOP-1127:

    Attachment: HADOOP-1127_20070328_1.patch

Early patch for review while I continue testing...

> 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
> 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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