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From "Yann Collet (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-7657) Add support for LZ4 compression
Date Fri, 30 Sep 2011 12:35:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7657?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13118022#comment-13118022
] 

Yann Collet commented on HADOOP-7657:
-------------------------------------

Hi

I'm the author of LZ4.

Since it is mentionned in this issue, i feel compelled to provide some more information.
There are in fact two container modes created for LZ4, but i only cared publishing the first
one (stream mode), since there was very little interest expressed in the second one (seekable
mode).

The seekable mode has these properties :
- Selectable block size (as a power of 2, from 512 bytes to 16 MB; extension reserved up to
1GB)
- Handle uncompressed blocks
- Blocks can be forced to start at aligned positions (2 bytes, 4 bytes, 8 bytes, etc.)
- Compressed file starts with an array which provides the start position of each block
- Array value can be stored as direct position or block size (the second solution can reduce
the size of array)
- Automatic sizing of array values (16, 32 or 64 bits).
- File size limited to 256 TB

Since the array must be written at the beginning of the compressed file, it is not compatible
with "pipe mode".

The only use-case for seekable mode so far has been compressed ISO. So i've not spent much
time on it.

I'm a bit surprised it would be usefull for Hadoop since i expect Handle to handle its own
container format, and a probably much adapted one. 

Anyway, since r35 is out, i would advise to use something like 64KB blocks. This way, each
block is independant, and can be fetched/stored/duplicated/decoded independantly, in its own
thread, CPU, machine or cluster.

Hope this helps
Regards
                
> Add support for LZ4 compression
> -------------------------------
>
>                 Key: HADOOP-7657
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7657
>             Project: Hadoop Common
>          Issue Type: Improvement
>            Reporter: Mr Bsd
>              Labels: compression
>
> According to several benchmark sites, LZ4 seems to overtake other fast compression algorithms,
especially in the decompression speed area. The interface is also trivial to integrate (http://code.google.com/p/lz4/source/browse/trunk/lz4.h)
and there is no license issue.

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