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From Philippe Kernévez <pkerne...@octo.com>
Subject Re: Anatomy of read in hdfs
Date Mon, 10 Apr 2017 08:20:32 GMT
Hi Sidharth,

As it has been explained, HDFS is not just a file system. It's a part of
the Hadoop platform. To take advantage of HDFS you have to understand how
Hadoop storage (HDFS) AND Yarn processing (say MapReduce) work all together
to implements jobs and parallel processing.
That says that you will have to rethink the design of your programs to take
advantage of HDFS.

You may start with this kind of tutorial
https://www.tutorialspoint.com/map_reduce/map_reduce_introduction.htm

Then have a deeper read of the Hadoop documentation
http://hadoop.apache.org/docs/r2.7.3/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html

Regards,
Philippe



On Sun, Apr 9, 2017 at 11:13 PM, daemeon reiydelle <daemeonr@gmail.com>
wrote:

> Readers ARE parallel processes, one per map task. There are defaults in
> map phase, about how many readers there are for the input file(s). Default
> is one mapper task block (or file, where any file is smaller than the hdfs
> block size). There is no java framework per se for splitting up an file
> (technically not so, but let's simplify, outside of your own custom code).
>
>
> *.......*
>
>
>
> *Daemeon C.M. ReiydelleUSA (+1) 415.501.0198 <(415)%20501-0198>London
> (+44) (0) 20 8144 9872 <+44%2020%208144%209872>*
>
> On Sun, Apr 9, 2017 at 2:50 AM, Sidharth Kumar <
> sidharthkumar2707@gmail.com> wrote:
>
>> Thanks Tariq, It really helped me to understand but just one another
>> doubt that if reading is not a parallel process then to ready a file of
>> 100GB and  hdfs block size is 128MB. It will take lot much to read the
>> complete file but it's not the scenerio in the real time. And second
>> question is write operations as well is sequential process ? And will every
>> datanode have their own data streamer which listen to data queue to get the
>> packets and create pipeline. So, can you kindly help me to get clear idea
>> of hdfs read and write operations.
>>
>> Regards
>> Sidharth
>>
>> On 08-Apr-2017 12:49 PM, "Mohammad Tariq" <dontariq@gmail.com> wrote:
>>
>> Hi Sidhart,
>>
>> When you read data from HDFS using a framework, like MapReduce, blocks of
>> a HDFS file are read in parallel by multiple mappers created in that
>> particular program. Input splits to be precise.
>>
>> On the other hand if you have a standalone java program then it's just a
>> single thread process and will read the data sequentially.
>>
>>
>> On Friday, April 7, 2017, Sidharth Kumar <sidharthkumar2707@gmail.com>
>> wrote:
>>
>>> Thanks for your response . But I dint understand yet,if you don't mind
>>> can you tell me what do you mean by "*With Hadoop, the idea is to
>>> parallelize the readers (one per block for the mapper) with processing
>>> framework like MapReduce.*"
>>>
>>> And also how the concept of parallelize the readers will work with hdfs
>>>
>>> Thanks a lot in advance for your help.
>>>
>>>
>>> Regards
>>> Sidharth
>>>
>>> On 07-Apr-2017 1:04 PM, "Philippe Kernévez" <pkernevez@octo.com> wrote:
>>>
>>> Hi Sidharth,
>>>
>>> The reads are sequential.
>>> With Hadoop, the idea is to parallelize the readers (one per block for
>>> the mapper) with processing framework like MapReduce.
>>>
>>> Regards,
>>> Philippe
>>>
>>>
>>> On Thu, Apr 6, 2017 at 9:55 PM, Sidharth Kumar <
>>> sidharthkumar2707@gmail.com> wrote:
>>>
>>>> Hi Genies,
>>>>
>>>> I have a small doubt that hdfs read operation is parallel or sequential
>>>> process. Because from my understanding it should be parallel but if I read
>>>> "hadoop definitive guide 4" in anatomy of read it says "*Data is
>>>> streamed from the datanode back **to the client, which calls read()
>>>> repeatedly on the stream (step 4). When the end of the **block is
>>>> reached, DFSInputStream will close the connection to the datanode, then
>>>> find **the best datanode for the next block (step 5). This happens
>>>> transparently to the client, **which from its point of view is just
>>>> reading a continuous stream*."
>>>>
>>>> So can you kindly explain me how read operation will exactly happens.
>>>>
>>>>
>>>> Thanks for your help in advance
>>>>
>>>> Sidharth
>>>>
>>>>
>>>
>>>
>>> --
>>> Philippe Kernévez
>>>
>>>
>>>
>>> Directeur technique (Suisse),
>>> pkernevez@octo.com
>>> +41 79 888 33 32 <+41%2079%20888%2033%2032>
>>>
>>> Retrouvez OCTO sur OCTO Talk : http://blog.octo.com
>>> OCTO Technology http://www.octo.ch
>>>
>>>
>>>
>>
>> --
>>
>>
>> [image: http://]
>>
>> Tariq, Mohammad
>> about.me/mti
>> [image: http://]
>> <http://about.me/mti>
>>
>>
>>
>>
>


-- 
Philippe Kernévez



Directeur technique (Suisse),
pkernevez@octo.com
+41 79 888 33 32

Retrouvez OCTO sur OCTO Talk : http://blog.octo.com
OCTO Technology http://www.octo.ch

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