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From "Abdul Qadeer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4012) Providing splitting support for bzip2 compressed files
Date Fri, 09 Jan 2009 15:41:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12662406#action_12662406
] 

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

Chris Douglas:

I understand that any change to e.g. LineRecordReader (LRR) should be carried out with care
because it the most frequently used code.  It might not have been impossible to implement
bzip2 codecs with the older implementation of LRR but with performance penalty for bzip2 (because
it then had to decompress one more block per mapper) and the code in the LRR constructor and
the next() method would have been a mess.

HADOOP-4010 was separately created before finishing bzip2 work but unfortunately that couldn't
get in.  Owen O'Malley  mentioned some concerns , to which I provided some logical hand waiving.
 I guess Owen has been busy or is not convinced with those arguments but he didn't said anything.
 I plan to ask him again that should I provide some kind of test cases just to make sure that
his concerns are addressed.

So going systematically I first plan to get 4010 approved and then move on by doing recommended
changes.  I have successfully used the bzip2 codecs for a binary compressed bzip2 files. 
So I hope by going step by step, I will be able to get this work approved.

> 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
>            Reporter: Abdul Qadeer
>            Assignee: Abdul Qadeer
>         Attachments: Hadoop-4012-version1.patch, Hadoop-4012-version2.patch, Hadoop-4012-version3.patch,
Hadoop-4012-version4.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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