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https://issues.apache.org/jira/browse/HADOOP-4012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12750137#action_12750137
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Chris Douglas commented on HADOOP-4012:
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True, there isn't threading issue with LineRecordReader. However, this is the
only codec with state, SplittableCompressionCodec won't just be used in
LineRecordReader and BZip2Codec may not be the last splittable codec. Though I
don't think most codecs are accessed by multiple threads, synchronization is
cheap, and the first user to implement a splittable reader with a shared codec
will almost certainly cargo-cult this implementation and miss the race
condition. If you'd rather leave it this way, then OK.
There was another part of the patch, in BZip2Codec::createInputStream:
{noformat}
+ if(in.getPos() <= start){
+ ((Seekable)seekableIn).seek(start);
+ in = this.createInputStream(seekableIn, readMode);
+
+
+ }
{noformat}
This drops the stream it created above, which wrapped the stream passed in with
a CBZip2InputStream and BufferedInputStream. It's not clear why the stream is
being re-created, either... particularly since the start stored in the codec is
left alone. What case is being handled here?
> 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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