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https://issues.apache.org/jira/browse/MAPREDUCE-902?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12746200#action_12746200
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Arun C Murthy commented on MAPREDUCE-902:
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Christian - this is very interesting. How much data is the map spewing out?
> Map output merge still uses unnecessary seeks
> ---------------------------------------------
>
> Key: MAPREDUCE-902
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-902
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: task
> Affects Versions: 0.20.1
> Reporter: Christian Kunz
>
> HADOOP-3638 improved the merge of the map output by caching the index files.
> But why not also caching the data files?
> In our use-case scenario, still using hadoop-0.18.3, but HADOOP-3638 would
> only help partially, an individual map tasks finishes in less than 30
> minutes, but needs 4 hours to merge 70 spills for 20,000 partitions (with lzo
> compression), reading about 10kB from each spill file (which is re-opened for
> every partition). As this is just a merge sort, there is no reason to not
> keep the input files open and eliminate seek altogether with sequential
> access.
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