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
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?
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
