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From Julian Bui <julian...@gmail.com>
Subject Re: basic question about rack awareness and computation migration
Date Tue, 05 Mar 2013 23:50:37 GMT
Hi Rohit,

Thanks for responding.

> a task can be scheduled by hadoop to be executed on the same node which
is having data.

In my case, the mapper won't actually know where the data resides at the
time of being scheduled.  It only knows what data it will be accessing when
it reads in the keys.  In other words, the task will be already be running
by the time the mapper figures out what data must be accessed - so how can
hadoop know where to execute the code?

I'm still lost.  Please help if you can.


On Tue, Mar 5, 2013 at 11:15 AM, Rohit Kochar <mnit.rohit@gmail.com> wrote:

> Hello ,
> To be precise this is hidden from the developer and you need not write any
> code for this.
> Whenever any file is stored in HDFS than it is splitted into block size of
> configured size and each block could potentially be stored on different
> datanode.All this information of which file contains which blocks resides
> with the namenode.
> So essentially whenever a file is accessed via DFS Client it requests the
>  NameNode for metadata,
> which DFS client uses to provide the file in streaming fashion to enduser.
> Since namenode knows the location of all the blocks/files ,a task can be
> scheduled by hadoop to be executed on the same node which is having data.
> Thanks
> Rohit Kochar
> On 05-Mar-2013, at 5:19 PM, Julian Bui wrote:
> > Hi hadoop users,
> >
> > I'm trying to find out if computation migration is something the
> developer needs to worry about or if it's supposed to be hidden.
> >
> > I would like to use hadoop to take in a list of image paths in the hdfs
> and then have each task compress these large, raw images into something
> much smaller - say jpeg  files.
> >
> > Input: list of paths
> > Output: compressed jpeg
> >
> > Since I don't really need a reduce task (I'm more using hadoop for its
> reliability and orchestration aspects), my mapper ought to just take the
> list of image paths and then work on them.  As I understand it, each image
> will likely be on multiple data nodes.
> >
> > My question is how will each mapper task "migrate the computation" to
> the data nodes?  I recall reading that the namenode is supposed to deal
> with this.  Is it hidden from the developer?  Or as the developer, do I
> need to discover where the data lies and then migrate the task to that
> node?  Since my input is just a list of paths, it seems like the namenode
> couldn't really do this for me.
> >
> > Another question: Where can I find out more about this?  I've looked up
> "rack awareness" and "computation migration" but haven't really found much
> code relating to either one - leading me to believe I'm not supposed to
> have to write code to deal with this.
> >
> > Anyway, could someone please help me out or set me straight on this?
> >
> > Thanks,
> > -Julian

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