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From "Chris Douglas (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MAPREDUCE-2094) LineRecordReader should not seek into non-splittable, compressed streams.
Date Fri, 08 May 2015 20:59:01 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Chris Douglas updated MAPREDUCE-2094:
    Summary: LineRecordReader should not seek into non-splittable, compressed streams.  (was:
org.apache.hadoop.mapreduce.lib.input.FileInputFormat: isSplitable implements unsafe default
behaviour that is different from the documented behaviour.)

> LineRecordReader should not seek into non-splittable, compressed streams.
> -------------------------------------------------------------------------
>                 Key: MAPREDUCE-2094
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2094
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>            Reporter: Niels Basjes
>            Assignee: Niels Basjes
>         Attachments: M2094.patch, MAPREDUCE-2094-2011-05-19.patch, MAPREDUCE-2094-20140727-svn-fixed-spaces.patch,
MAPREDUCE-2094-20140727-svn.patch, MAPREDUCE-2094-20140727.patch, MAPREDUCE-2094-2015-05-05-2328.patch,
> When implementing a custom derivative of FileInputFormat we ran into the effect that
a large Gzipped input file would be processed several times. 
> A near 1GiB file would be processed around 36 times in its entirety. Thus producing garbage
results and taking up a lot more CPU time than needed.
> It took a while to figure out and what we found is that the default implementation of
the isSplittable method in [org.apache.hadoop.mapreduce.lib.input.FileInputFormat | http://svn.apache.org/viewvc/hadoop/mapreduce/trunk/src/java/org/apache/hadoop/mapreduce/lib/input/FileInputFormat.java?view=markup
] is simply "return true;". 
> This is a very unsafe default and is in contradiction with the JavaDoc of the method
which states: "Is the given filename splitable? Usually, true, but if the file is stream compressed,
it will not be. " . The actual implementation effectively does "Is the given filename splitable?
Always true, even if the file is stream compressed using an unsplittable compression codec.
> For our situation (where we always have Gzipped input) we took the easy way out and simply
implemented an isSplittable in our class that does "return false; "
> Now there are essentially 3 ways I can think of for fixing this (in order of what I would
find preferable):
> # Implement something that looks at the used compression of the file (i.e. do migrate
the implementation from TextInputFormat to FileInputFormat). This would make the method do
what the JavaDoc describes.
> # "Force" developers to think about it and make this method abstract.
> # Use a "safe" default (i.e. return false)

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