Mailing-List: contact hadoop-dev-help@lucene.apache.org; run by ezmlm
Precedence: bulk
Reply-To: hadoop-dev@lucene.apache.org
Message-ID: <31235710.1174000329228.JavaMail.jira@brutus>
Date: Thu, 15 Mar 2007 16:12:09 -0700 (PDT)
From: "Arun C Murthy (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Created: (HADOOP-1127) Speculative Execution and output of
 Reduce tasks
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit

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.