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From "Chris Douglas (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4012) Providing splitting support for bzip2 compressed files
Date Wed, 17 Jun 2009 00:24:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12720436#action_12720436

Chris Douglas commented on HADOOP-4012:

* If an IOException is ignored:
+    if (!(in instanceof Seekable) || !(in instanceof PositionedReadable)) {
+      try {
+        this.maxAvailableData = in.available();
+      } catch (IOException e) {
+      }
+    }
Please include a comment explaining why it should be ignored. Won't this fail in the first
call to getPos() if it doesn't throw in the cstr?
* This change to TestCodec looks suspect:
-    SequenceFile.Reader reader = new SequenceFile.Reader(fs, filePath, conf);
+    SequenceFile.Reader reader = null;
+    try{
+    reader = new SequenceFile.Reader(fs, filePath, conf);
+    }
+    catch(Exception exp){}
* Instead of the tenary operatior in FSInputChecker, {{Math.max(0L, count - pos)}} is more
* With changes such as the following, will the case resolved in HADOOP-3144 continue to work
in {{LineRecordReader}}?
-      int newSize = in.readLine(value, maxLineLength,
-                                Math.max((int)Math.min(Integer.MAX_VALUE, end-pos),
-                                         maxLineLength));
+      int newSize = lineReader.readLine(value, maxLineLength, Integer.MAX_VALUE);
* Specifying the constant as {{1L}} should avoid this:
-    return (bsBuffShadow >> (bsLiveShadow - n)) & ((1 << n) - 1);
+    final long one = 1;
+    return (bsBuffShadow >> (bsLiveShadow - n)) & ((one << n) - 1);
* 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()}}?
* CBZip2InputStream::read() shouldn't allocate a new {{byte[]}} for every read, but reuse
an instance var. ({{0xFF}} also works, btw)

> 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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