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From Harsh J <ha...@cloudera.com>
Subject Re: Queries about HDFS block migration
Date Sun, 24 Apr 2011 14:42:19 GMT
Hello Amit,

On Sun, Apr 24, 2011 at 5:00 AM, Amit Gupta <gupta.v.amit@gmail.com> wrote:
> 1. I noticed there is a Source and Target, but there is also a ProxySource.
> I havent been able to grasp the conceptual difference between a source and a
> proxy source

The 'proxy' source is a DN that has a replica of a given block. This
is used in order to even out the load across the DNs in the network.
It would not be a good idea to put too much tx load onto the breaching
DN, when you can copy off a replica.

> 2. I'm also having trouble understanding the actual mechanics of a block
> being relocated from
> one datanode to the other. I understand that the communication with the
> namenode occurs to change
> metadata of the block being relocated accordingly, but have not understood
> what does the actual copy
> of the block itself.

The block movement is done from a DN to another DN directly when it
receives a command from the NameNode to do so (part of the heartbeat
mechanism).

-- 
Harsh J

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