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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-5308) Shuffling to memory can get out-of-sync when fetching multiple compressed map outputs
Date Thu, 06 Jun 2013 22:26:21 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-5308?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13677601#comment-13677601

Hadoop QA commented on MAPREDUCE-5308:

{color:green}+1 overall{color}.  Here are the results of testing the latest attachment 
  against trunk revision .

    {color:green}+1 @author{color}.  The patch does not contain any @author tags.

    {color:green}+1 tests included{color}.  The patch appears to include 1 new or modified
test files.

    {color:green}+1 javac{color}.  The applied patch does not increase the total number of
javac compiler warnings.

    {color:green}+1 javadoc{color}.  The javadoc tool did not generate any warning messages.

    {color:green}+1 eclipse:eclipse{color}.  The patch built with eclipse:eclipse.

    {color:green}+1 findbugs{color}.  The patch does not introduce any new Findbugs (version
1.3.9) warnings.

    {color:green}+1 release audit{color}.  The applied patch does not increase the total number
of release audit warnings.

    {color:green}+1 core tests{color}.  The patch passed unit tests in hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core.

    {color:green}+1 contrib tests{color}.  The patch passed contrib unit tests.

Test results: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/3746//testReport/
Console output: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/3746//console

This message is automatically generated.
> Shuffling to memory can get out-of-sync when fetching multiple compressed map outputs
> -------------------------------------------------------------------------------------
>                 Key: MAPREDUCE-5308
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5308
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: trunk, 2.0.3-alpha, 0.23.8
>            Reporter: Nathan Roberts
>            Assignee: Nathan Roberts
>         Attachments: MAPREDUCE-5308.patch
> When a reducer is fetching multiple compressed map outputs from a host, the fetcher can
get out-of-sync with the IFileInputStream, causing several of the maps to fail to fetch.
> This occurs because decompressors can return all the decompressed bytes before actually
processing all the bytes in the compressed stream (due to checksums or other trailing data
that we ignore). In the unfortunate case where these extra bytes cross an io.file.buffer.size
boundary, some extra bytes will be left over and the next map_output will not fetch correctly
(usually due to an invalid map_id).
> This scenario is not typically fatal to a job because the failure is charged to the map_output
immediately following the "bad" one and the subsequent retry will normally work. 

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