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https://issues.apache.org/jira/browse/MAPREDUCE-5308?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13677574#comment-13677574
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Nathan Roberts commented on MAPREDUCE-5308:
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Patch fixes this issue by attempting to read() one additional byte from the
input. This should always return EOF, but the side-effect is that it forces the
decompressor to finish processing all of the compressed stream. This allows the
IFileInputStream to stay in-sync and be completely processed (e.g. verification
of IFileInputStream checksum) as part of this map_id. If we do get something
other than EOF then something is truly wrong and we should fail this particular
map.
> 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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