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From "Abdul Qadeer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4012) Providing splitting support for bzip2 compressed files
Date Thu, 04 Jun 2009 10:29:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12716223#action_12716223
] 

Abdul Qadeer commented on HADOOP-4012:
--------------------------------------

(1) Running  test-patch target on my local box does not produce release audit warnings:

[exec] 
     [exec] There appear to be 503 release audit warnings before the patch and 501 release
audit warnings after applying the patch.
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] +1 overall.  
     [exec] 
     [exec]     +1 @author.  The patch does not contain any @author tags.
     [exec] 
     [exec]     +1 tests included.  The patch appears to include 3 new or modified tests.
     [exec] 
     [exec]     +1 javadoc.  The javadoc tool did not generate any warning messages.
     [exec] 
     [exec]     +1 javac.  The applied patch does not increase the total number of javac compiler
warnings.
     [exec] 
     [exec]     +1 findbugs.  The patch does not introduce any new Findbugs warnings.
     [exec] 
     [exec]     +1 Eclipse classpath. The patch retains Eclipse classpath integrity.
     [exec] 
     [exec]     +1 release audit.  The applied patch does not increase the total number of
release audit warnings.
     [exec] 

(2) Test org.apache.hadoop.hdfs.server.datanode.TestBlockReplacement.testBlockReplacement
does not fail on my local box and
org.apache.hadoop.mapred.TestQueueCapacities.testMultipleQueues produces an error (But this
is the same even for the committed SVN Code).

These 1 or 2 errors are occurring for other patches as well on Hudson.  Additionally they
are un-related as far as bzip2 is concerned.

Can the Hadoop committers please review the patch so that I can complete this work.  Thank
you.




> Providing splitting support for bzip2 compressed files
> ------------------------------------------------------
>
>                 Key: HADOOP-4012
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4012
>             Project: Hadoop Core
>          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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