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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-54) SequenceFile should compress blocks, not individual entries
Date Thu, 20 Jul 2006 09:34:18 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-54?page=comments#action_12422374 ] 
            
Doug Cutting commented on HADOOP-54:
------------------------------------

Re documentation: perhaps we should add file-format documentation to the javadoc as a part
of this change.  This could look something like Lucene's file-formats documentation (except
simpler).  That mixes a pseudo-formal BNF-like syntax with commentary:

http://lucene.apache.org/java/docs/fileformats.html

This should link to other javadoc to describe things like the zero-compressed int format.

Eric's proposal looks good to me.  I assume zero-compressed ints are meant most places he
says 'delta compressed'.  Is that right?  It may make sense to delta compress the lists of
key and value lengths, but it probably does not make sense to delta compress numPairs and
the block lengths.

> SequenceFile should compress blocks, not individual entries
> -----------------------------------------------------------
>
>                 Key: HADOOP-54
>                 URL: http://issues.apache.org/jira/browse/HADOOP-54
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: io
>    Affects Versions: 0.2.0
>            Reporter: Doug Cutting
>         Assigned To: Arun C Murthy
>             Fix For: 0.5.0
>
>
> SequenceFile will optionally compress individual values.  But both compression and performance
would be much better if sequences of keys and values are compressed together.  Sync marks
should only be placed between blocks.  This will require some changes to MapFile too, so that
all file positions stored there are the positions of blocks, not entries within blocks.  Probably
this can be accomplished by adding a getBlockStartPosition() method to SequenceFile.Writer.

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