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Date: Wed, 19 Jul 2006 03:04:16 -0700 (PDT)
From: "Doug Cutting (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Commented: (HADOOP-54) SequenceFile should compress blocks,
 not individual entries
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    [ http://issues.apache.org/jira/browse/HADOOP-54?page=comments#action_12422077 ] 
            
Doug Cutting commented on HADOOP-54:
------------------------------------

> Any way to do this without an extra copy?

There's no way to length-prefix things without knowing the length, and the length is only known after the value is serialized.  I doubt the extra copy will significantly affect overall write performance.

Note that folks who use the raw API, directly passing bytes rather than Writables, should not suffer the extra-copy penalty when adding items to a file.

> 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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