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Date: Thu, 20 Jul 2006 03:02:15 -0700 (PDT)
From: "Arun C Murthy (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Updated: (HADOOP-54) SequenceFile should compress blocks,
 not individual entries
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     [ http://issues.apache.org/jira/browse/HADOOP-54?page=all ]

Arun C Murthy updated HADOOP-54:
--------------------------------

    Attachment: VIntCompressionResults.txt

Here's a test run analysing compression of VInts (exported from recordio).

Essentially we get almost 50% savings (either with zlib/gzip) of compressed-VInts v/s uncompressed raw integers on disk (4bytes per int) ... 

Sounds like a good reason to go ahead with Eric's proposal to compress not only keys/values but also keyLengthsBlock/valueLengthsBlock? I'll go ahead with this for now unless anyone objects...

thanks,
Arun


> 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
>
>         Attachments: VIntCompressionResults.txt
>
>
> 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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