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From "Chris Douglas (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-4012) Providing splitting support for bzip2 compressed files
Date Sun, 30 Aug 2009 21:49:32 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Chris Douglas updated HADOOP-4012:

    Status: Open  (was: Patch Available)

This is what we were looking for. Thank you.

* {{CBZip2InputStream::skipToNextMarker}} should add IOException (for bsR) to its throws list,
not Exception, and throw IllegalArgumentException if markerBitLength is larger than 63. This
permits the removal of the empty catch clauses in calls to this method. The javadoc should
include a description of this behavior, and note that marker is the EOB delimiter (w/ {{@param}}
javadoc directives). The javadoc may be self-evident, but it's a public API.
* {{updateProcessedByteCount}} and {{updateReportedByteCount}} are public methods, but they
seem very, very specialized. The comment explains that the client may manipulate the compressed
stream, but any client sophisticated enough to do that is likely bound to this class. Would
making these methods protected make sense?
* This method:
+  private static void reportCRCError() throws IOException {
+    throw new IOException("crc error");
+  }
Seems unnecessary. If the intent is to add a hook, then the method should be protected and
non-static. Otherwise, there's no reason not to simply throw the IOE at its callers.
* The API issue with InputStreamCreationResultant is clearer to me, now (I hadn't seen why
a reader might need a modified start). Other than synchronizing on the codec before creating
new streams (to avoid the race condition), I don't see a better way to do this without pushing
other API changes. Unless someone has a better idea, I think documenting this requirement
on {{SplittableCompressionCodec}} is sufficient for now (and making these methods synchronized
in {{BZip2Codec}}).

> 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-version10.patch, Hadoop-4012-version11.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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