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Date: Tue, 19 Jun 2012 00:19:30 +0530
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Subject: Re: How does scan work internally? Does it make use of
 multi-threading/replication?
From: IGZ Nick <igznick01@gmail.com>
To: user@hbase.apache.org
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Okay. Let me ask a more specific example. Say I have 3 contiguous regions,
all server by one RS. So if I do a scan which gets data from each of the
regions, then everything has to come through this RS, which would be slow.
Or is there any optimization such that continuous regions don't end up
being server by the same regionserver?

On Tue, Jun 19, 2012 at 12:11 AM, Jean-Daniel Cryans <jdcryans@apache.org>wrote:

> On Mon, Jun 18, 2012 at 11:34 AM, IGZ Nick <igznick01@gmail.com> wrote:
> > Hi Jean,
> >
> > Thank you for your reply. So RS is a completely different entity when
> > compared to the datanode?
>
> Totally.
>
> > How does RS server the data?
>
> That's HBase 101, I recommend you read the guide
> http://hbase.apache.org/book/book.html or the book
> http://ofps.oreilly.com/titles/9781449396107/ or the bigtable paper.
>
> > I can view the
> > region directories in HDFS. So the same region must be on 3 datanodes,
> > right?
>
> Yep.
>
> > Then which regionserver gets to serve that region?
>
> HBase 101, but in short the master decides that.
>
> > Is it a
> > completely random regionserver?
>
> The master uses a few heuristics.
>
> > And if I ask that region server for all
> > keys from that region, will it have to come from the same HDFS datanode?
>
> Depends if the data is there, if it is then it will be served locally
> else it will be fetched. It doesn't really matter to the region server
> since the HDFS client handles it transparently.
>
> > As
> > far as I understand, in HDFS, if I stream a file, then I get the data
> from
> > a single datanode (the one closest to the client, usually). So, in
> HBase, I
> > ask for all keys in region reg1, then I get all the keys from the
> datanode
> > that is closest to the client?
>
> Yep
>
> J-D
>

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