[jira] [Created] (MAPREDUCE-5308) Shuffling to memory can get out-of-sync when fetching multiple compressed map outputs

["Nathan Roberts (JIRA)" <ji...@apache.org>]

Nathan Roberts created MAPREDUCE-5308:
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             Summary: 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: 0.23.8, 2.0.3-alpha, trunk
            Reporter: Nathan Roberts
            Assignee: Nathan Roberts


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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