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https://issues.apache.org/jira/browse/HADOOP-5299?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12675440#action_12675440
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Arun C Murthy commented on HADOOP-5299:
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Caveat emptor:
# Typical mid-to-large applications have reducers which process multiple
gigabytes of data (e.g. our friend, the sort<nodes>, has each reducer
generating 5GB) which means we'll need atleast tens of intermediate merged
files per reducer (assuming a heap-size of 512M).
# io.sort.factor defaults to 100, which means we might open upto a 100 files
for the final merge which then feeds the 'reduce' call. Given that we might
have every reducer in the cluster open a 100 files at time; for a mid-size
cluster of 500 nodes with 4 reduce slots that would be 2000 * 100 ...
> Reducer inputs should be spilled to HDFS rather than local disk.
> ----------------------------------------------------------------
>
> Key: HADOOP-5299
> URL: https://issues.apache.org/jira/browse/HADOOP-5299
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Affects Versions: 0.19.0
> Environment: All
> Reporter: Milind Bhandarkar
>
> Currently, both map outputs and reduce inputs are stored on local disks of
> tasktrackers. (Un) Availability of local disk space for intermediate data is
> seen as a major factor in job failures.
> The suggested solution is to store these intermediate data on HDFS (maybe
> with replication factor of 1). However, the main blocker issue with that
> solution is that lots of temporary names (proportional to total number of
> maps), can overwhelm the namenode, especially since the map outputs are
> typically small (most produce one block output).
> Also, as we see in many applications, the map outputs can be estimated more
> accurately, and thus users can plan accordingly, based on available local
> disk space.
> However, the reduce input sizes can vary a lot, especially for skewed data
> (or because of bad partitioning.)
> So, I suggest that it makes more sense to keep map outputs on local disks,
> but the reduce inputs (when spilled from reducer memory) should go to HDFS.
> Adding a configuration variable to indicate the filesystem to be used for
> reduce-side spills would let us experiment and compare the efficiency of this
> new scheme.
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