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From "Yuri Pradkin (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4012) Providing splitting support for bzip2 compressed files
Date Fri, 10 Jul 2009 18:45:15 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12729764#action_12729764
] 

Yuri Pradkin commented on HADOOP-4012:
--------------------------------------

Abdul, I think at this point it's OK to assume that Chris has accepted your explanation. 
Can you please post your latest patch with all the fixes.  Beware that repository has changed:
you'll need to do switch to
http://svn.apache.org/repos/asf/hadoop/common/trunk/ or checkout a fresh copy and reapply
your patch.

Guys, can we please wrap this up?  It's taking way too long for this patch to make it.  Please.
 Thanks.

> Providing splitting support for bzip2 compressed files
> ------------------------------------------------------
>
>                 Key: HADOOP-4012
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4012
>             Project: Hadoop Common
>          Issue Type: New Feature
>          Components: io
>    Affects Versions: 0.21.0
>            Reporter: Abdul Qadeer
>            Assignee: Abdul Qadeer
>             Fix For: 0.21.0
>
>         Attachments: Hadoop-4012-version1.patch, Hadoop-4012-version2.patch, Hadoop-4012-version3.patch,
Hadoop-4012-version4.patch, Hadoop-4012-version5.patch, Hadoop-4012-version6.patch, Hadoop-4012-version7.patch,
Hadoop-4012-version8.patch, Hadoop-4012-version9.patch
>
>
> Hadoop assumes that if the input data is compressed, it can not be split (mainly due
to the limitation of many codecs that they need the whole input stream to decompress successfully).
 So in such a case, Hadoop prepares only one split per compressed file, where the lower split
limit is at 0 while the upper limit is the end of the file.  The consequence of this decision
is that, one compress file goes to a single mapper. Although it circumvents the limitation
of codecs (as mentioned above) but reduces the parallelism substantially, as it was possible
otherwise in case of splitting.
> BZip2 is a compression / De-Compression algorithm which does compression on blocks of
data and later these compressed blocks can be decompressed independent of each other.  This
is indeed an opportunity that instead of one BZip2 compressed file going to one mapper, we
can process chunks of file in parallel.  The correctness criteria of such a processing is
that for a bzip2 compressed file, each compressed block should be processed by only one mapper
and ultimately all the blocks of the file should be processed.  (By processing we mean the
actual utilization of that un-compressed data (coming out of the codecs) in a mapper).
> We are writing the code to implement this suggested functionality.  Although we have
used bzip2 as an example, but we have tried to extend Hadoop's compression interfaces so that
any other codecs with the same capability as that of bzip2, could easily use the splitting
support.  The details of these changes will be posted when we submit the code.

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