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https://issues.apache.org/jira/browse/MAPREDUCE-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12969142#action_12969142
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Todd Lipcon commented on MAPREDUCE-2212:
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We've found that even on single rack clusters (where bandwidth is usually not
the bottleneck) LZO intermediate compression almost always helps. That
indicates that at least in many workloads we're intermediate-IO bound more than
CPU. This is consistent with what we see on most clusters with 4-6 disks.
Clusters with 12 local disks more often are bound on network or CPU.
> MapTask and ReduceTask should only compress/decompress the final map output
> file
> --------------------------------------------------------------------------------
>
> Key: MAPREDUCE-2212
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-2212
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: task
> Affects Versions: 0.23.0
> Reporter: Scott Chen
> Assignee: Scott Chen
> Fix For: 0.23.0
>
>
> Currently if we set mapred.map.output.compression.codec
> 1. MapTask will compress every spill, decompress every spill, merge and
> compress the final map output file
> 2. ReduceTask will decompress, merge and compress every map output file. And
> repeat the compression/decompression every pass.
> This causes all the data being compressed/decompressed many times.
> The reason we need mapred.map.output.compression.codec is for network traffic.
> We should not compress/decompress the data again and again during merge sort.
> We should only compress the final map output file that will be transmitted
> over the network.
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