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From Jay Vyas <jayunit...@gmail.com>
Subject Re: Retrieve and compute input splits
Date Sat, 28 Sep 2013 00:05:18 GMT
Technically, the block locations are provided by the InputSplit which in
the FileInputFormat case, is provided by the FileSystem Interface.


The thing to realize here is that the FileSystem implementation is provided
at runtime - so the InputSplit class is responsible to create a FileSystem
implementation using reflection, and then call the getBlockLocations of on
a given file or set of files which the input split is corresponding to.

I think your confusion here lies in the fact that the input splits create a
filesystem, however, they dont know what the filesystem implementation
actually is - they only rely on the abstract contract, which provides a set
of block locations.

See the FileSystem abstract class for details on that.

On Fri, Sep 27, 2013 at 7:02 PM, Peyman Mohajerian <mohajeri@gmail.com>wrote:

> For the JobClient to compute the input splits doesn't it need to contact
> Name Node. Only Name Node knows where the splits are, how can it compute it
> without that additional call?
> On Fri, Sep 27, 2013 at 1:41 AM, Sonal Goyal <sonalgoyal4@gmail.com>wrote:
>> The input splits are not copied, only the information on the location of
>> the splits is copied to the jobtracker so that it can assign tasktrackers
>> which are local to the split.
>> Check the Job Initialization section at
>> http://answers.oreilly.com/topic/459-anatomy-of-a-mapreduce-job-run-with-hadoop/
>> To create the list of tasks to run, the job scheduler first retrieves
>> the input splits computed by the JobClient from the shared filesystem
>> (step 6). It then creates one map task for each split. The number of reduce
>> tasks to create is determined by the mapred.reduce.tasks property in the
>> JobConf, which is set by the setNumReduceTasks() method, and the
>> scheduler simply creates this number of reduce tasks to be run. Tasks are
>> given IDs at this point.
>> Best Regards,
>> Sonal
>> Nube Technologies <http://www.nubetech.co>
>>  <http://in.linkedin.com/in/sonalgoyal>
>> On Fri, Sep 27, 2013 at 10:55 AM, Sai Sai <saigraph@yahoo.in> wrote:
>>> Hi
>>> I have attached the anatomy of MR from definitive guide.
>>> In step 6 it says JT/Scheduler  retrieve  input splits computed by the
>>> client from hdfs.
>>> In the above line it refers to as the client computes input splits.
>>> 1. Why does the JT/Scheduler retrieve the input splits and what does it
>>> do.
>>> If it is retrieving the input split does this mean it goes to the block
>>> and reads each record
>>> and gets the record back to JT. If so this is a lot of data movement for
>>> large files.
>>> which is not data locality. so i m getting confused.
>>> 2. How does the client know how to calculate the input splits.
>>> Any help please.
>>> Thanks
>>> Sai

Jay Vyas

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