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From "Stefan Will (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5539) o.a.h.mapred.Merger not maintaining map out compression on intermediate files
Date Fri, 20 Mar 2009 18:16:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5539?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12683973#action_12683973
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Stefan Will commented on HADOOP-5539:
-------------------------------------

I'd like to see this fixed as well since one reason I've enabled map output compression is
to reduce disk space usage by the mapreduce framework. It appears that currently the map outputs
are simply decompressed as soon as they have been downloaded by the reducer.

> o.a.h.mapred.Merger not maintaining map out compression on intermediate files
> -----------------------------------------------------------------------------
>
>                 Key: HADOOP-5539
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5539
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.19.1
>         Environment: 0.19.2-dev, r753365 
>            Reporter: Billy Pearson
>             Fix For: 0.19.2, 0.20.0
>
>
> hadoop-site.xml :
> mapred.compress.map.output = true
> map output files are compressed but when the in memory merger closes 
> on the reduce the on disk merger runs to reduce input files to <= io.sort.factor if
needed. 
> when this happens it outputs files called intermediate.x files these 
> do not maintain compression setting the writer (o.a.h.mapred.Merger.class line 432)
> passes the codec but I added some logging and its always null map output compression
set true or false.
> This causes task to fail if they can not hold the uncompressed size of the data of the
reduce its holding
> I thank this is just and oversight of the codec not getting set correctly for the on
disk merges.
> {code}
> 2009-03-20 01:30:30,005 INFO org.apache.hadoop.mapred.Merger: Merging 30 intermediate
segments out of a total of 3000
> 2009-03-20 01:30:30,005 INFO org.apache.hadoop.mapred.Merger: intermediate.1 used codec:
null
> {code}
> I added 
> {code}
>           // added my me
> 	   if (codec != null){
> 	     LOG.info("intermediate." + passNo + " used codec: " + codec.toString());
> 	   } else {
> 	     LOG.info("intermediate." + passNo + " used codec: Null");
> 	   }
> 	   // end added by me
> {code}
> Just before the creation of the writer o.a.h.mapred.Merger.class line 432
> and it outputs the second line above.
> I have confirmed this with the logging and I have looked at the files on the disk of
the tasktracker. I can read the data in 
> the intermediate files clearly telling me that there not compressed but I can not read
the map.out files direct from the map output
> telling me the compression is working on the map end but not on the on disk merge that
produces the intermediate.
> I can see no benefit for these not maintaining the compression setting and as it looks
they where intended to maintain it.

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