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From "Abdul Qadeer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4012) Providing splitting support for bzip2 compressed files
Date Tue, 23 Jun 2009 09:42:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12723016#action_12723016

Abdul Qadeer commented on HADOOP-4012:

The InputStreamCreationResultant struct seems unnecessary, but perhaps I'm not following the
logic there. Of the three auxiliary params returned, end is unmodified from the actual and
areLimitsChanged only avoids a redundant assignment to start. Since CompressionInputStream
now implements Seekable, can this be replaced with start = in.getPos()?

(1) I can not use start = getPos() in the LineRecordReader's constructor.  The reason is that,
for the BZip2 compressed data getPos() does not return the actual stream value.  It is manipulated
such a way in BZip2Codecs that, this hack works with LineRecordReader's start / end / pos
stuff.  On the other hand I need to do stuff (e.g. throwing away a line if it is not the first
split) which required accurate value of start.  So two pieces of information from the method
createInputStream(...) were required:

a) the new value of start
b) the input stream

The parameter "end" was also passed just in case some other future codec had the flexibility
to change start or end or both.

So I made that InputStreamCreationResultant because I wanted to return more than one things.
 To avoid making a new class, I can use some array which has inputstream on its 0th location
and then the parameter start.  But that will not be type safe.

Another option might be to add another method in the SplitEnabledCompressionCodec to ask for
the changed start value.  So I will need to call createInputStream(...) and then call the
new method e.g. getStart().  But the semantics of such a call were not neat and clear.

Do we have any other option to avoid this "InputStreamCreationResultant"? 

(2)  I have fixed the rest of the comments.  Once this point is clear, I will prepare the
final patch.


> 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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