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From "Jeremy Karn (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-9184) Some reducers failing to write final output file to s3.
Date Mon, 07 Jan 2013 19:52:12 GMT
Jeremy Karn created HADOOP-9184:

             Summary: Some reducers failing to write final output file to s3.
                 Key: HADOOP-9184
                 URL: https://issues.apache.org/jira/browse/HADOOP-9184
             Project: Hadoop Common
          Issue Type: Bug
    Affects Versions: 0.20.2
            Reporter: Jeremy Karn
         Attachments: task_log.txt

We had a Hadoop job that was running 100 reducers with most of the reducers expected to write
out an empty file. When the final output was to an S3 bucket we were finding that sometimes
we were missing a final part file.  This was happening approximately 1 job in 3 (so approximately
1 reducer out of 300 was failing to output the data properly). I've attached the pig script
we were using to reproduce the bug.

After an in depth look and instrumenting the code we traced the problem to moveTaskOutputs
in FileOutputCommitter.  

The code there looked like:

    if (fs.isFile(taskOutput)) {
	… do stuff …       
    } else if(fs.getFileStatus(taskOutput).isDir()) {
	… do stuff … 

And what we saw happening is that for the problem jobs neither path was being exercised. 
I've attached the task log of our instrumented code.  In this version we added an else statement
and printed out the line "THIS SEEMS LIKE WE SHOULD NEVER GET HERE …".

The root cause of this seems to be an eventual consistency issue with S3.  You can see in
the log that the first time moveTaskOutputs is called it finds that the taskOutput is a directory.
 It goes into the isDir() branch and successfully retrieves the list of files in that directory
from S3 (in this case just one file).  This triggers a recursive call to moveTaskOutputs for
the file found in the directory.  But in this pass through moveTaskOutput the temporary output
file can't be found resulting in both branches of the above if statement being skipped and
the temporary file never being moved to the final output location.

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