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From Jean-Daniel Cryans <jdcry...@apache.org>
Subject Re: Region assignment in Hbase
Date Mon, 29 Mar 2010 20:53:05 GMT


On Mon, Mar 29, 2010 at 11:45 AM, john smith <js1987.smith@gmail.com> wrote:
> Hi all,
> I read the issue HBase-57 ( https://issues.apache.org/jira/browse/HBASE-57 )
> . I don't really understand the use of assigning regions keeping DFS in
> mind. Can anyone give an example usecase showing its advantages

A region is composed of files, files are composed of blocks. To read
data, you need to fetch those blocks. In HDFS you normally have access
to 3 replicas and you fetch one of them over the network. If one of
the replica is on the local datanode, you don't need to go through the
network. This means less network traffic and better response time.

> Can
> map-reduce exploit it's advantage in any way (if data is distributed in the
> above manner)  or is it just the read-write performance that gets improved .

MapReduce works in the exact same way, it always tries to put the
computation next to where the data is. I recommend reading the
MapReduce tutorial

> Can some one please help me in understanding this.
> Regards
> JS

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