Speculative Execution and output of Reduce tasks
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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
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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