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From Tsz Wo Sze <szets...@yahoo.com>
Subject Re: Data Locality and WebHDFS
Date Mon, 17 Mar 2014 18:30:30 GMT
The file offset is considered in WebHDFS redirection.  It redirects to a datanode with
the first block the client going to read, not the first block of the file.

Hope it helps.
Tsz-Wo



On Monday, March 17, 2014 10:09 AM, Alejandro Abdelnur <tucu@cloudera.com> wrote:
 
actually, i am wrong, the webhdfs rest call has an offset. 
>
>Alejandro
>(phone typing)
>
>On Mar 17, 2014, at 10:07, Alejandro Abdelnur <tucu@cloudera.com> wrote:
>
>
>dont recall how skips are handled in webhdfs, but i would assume that you'll get to the
first block As usual, and the skip is handled by the DN serving the file (as webhdfs doesnot
know at open that you'll skip)
>
>Alejandro
>(phone typing)
>
>On Mar 17, 2014, at 9:47, RJ Nowling <rnowling@gmail.com> wrote:
>
>
>Hi Alejandro,
>>
>>
>>The WebHDFS API allows specifying an offset and length for the request.  If I specify
an offset that start in the second block for a file (thus skipping the first block all together),
will the namenode still direct me to a datanode with the first block or will it direct me
to a namenode with the second block?  I.e., am I assured data locality only on the first
block of the file (as you're saying) or on the first block I am accessing?
>>
>>
>>If it is as you say, then I may want to reach out the WebHDFS developers and see if
they would be interested in the additional functionality.
>>
>>
>>Thank you,
>>RJ
>>
>>
>>
>>On Mon, Mar 17, 2014 at 2:40 AM, Alejandro Abdelnur <tucu@cloudera.com> wrote:
>>
>>I may have expressed myself wrong. You don't need to do any test to see how locality
works with files of multiple blocks. If you are accessing a file of more than one block over
webhdfs, you only have assured locality for the first block of the file.
>>>
>>>
>>>Thanks.
>>>
>>>
>>>
>>>On Sun, Mar 16, 2014 at 9:18 PM, RJ Nowling <rnowling@gmail.com> wrote:
>>>
>>>Thank you, Mingjiang and Alejandro.
>>>>
>>>>
>>>>This is interesting.  Since we will use the data locality information for
scheduling, we could "hack" this to get the data locality information, at least for the first
block.  As Alejandro says, we'd have to test what happens for other data blocks -- e.g.,
what if, knowing the block sizes, we request the second or third block?
>>>>
>>>>
>>>>Interesting food for thought!  I see some experiments in my future!  
>>>>
>>>>
>>>>Thanks!
>>>>
>>>>
>>>>
>>>>On Sun, Mar 16, 2014 at 10:14 PM, Alejandro Abdelnur <tucu@cloudera.com>
wrote:
>>>>
>>>>well, this is for the first block of the file, the rest of the file (blocks
being local or not) are streamed out by the same datanode. for small files (one block) you'll
get locality, for large files only the first block, and by chance if other blocks are local
to that datanode. 
>>>>>
>>>>>
>>>>>
>>>>>Alejandro
>>>>>(phone typing)
>>>>>
>>>>>On Mar 16, 2014, at 18:53, Mingjiang Shi <mshi@gopivotal.com> wrote:
>>>>>
>>>>>
>>>>>According to this page: http://hortonworks.com/blog/webhdfs-%E2%80%93-http-rest-access-to-hdfs/
>>>>>>
>>>>>>Data Locality: The file read and file write calls 
are redirected to the corresponding datanodes. It uses the full 
bandwidth of the Hadoop cluster for streaming data.
>>>>>>>A HDFS Built-in Component: WebHDFS is a first class 
built-in component of HDFS. It runs inside Namenodes and Datanodes, 
therefore, it can use all HDFS functionalities. It is a part of HDFS – 
there are no additional servers to install
>>>>>>
>>>>>>So it looks like the data locality is built-into webhdfs, client will
be redirected to the data node automatically. 
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>On Mon, Mar 17, 2014 at 6:07 AM, RJ Nowling <rnowling@gmail.com>
wrote:
>>>>>>
>>>>>>Hi all,
>>>>>>>
>>>>>>>
>>>>>>>I'm writing up a Google Summer of Code proposal to add HDFS support
to Disco, an Erlang MapReduce framework.  
>>>>>>>
>>>>>>>
>>>>>>>We're interested in using WebHDFS.  I have two questions:
>>>>>>>
>>>>>>>
>>>>>>>1) Does WebHDFS allow querying data locality information?
>>>>>>>
>>>>>>>
>>>>>>>2) If the data locality information is known, can data on specific
data nodes be accessed via Web HDFS?  Or do all Web HDFS requests have to go through a single
server?
>>>>>>>
>>>>>>>Thanks,
>>>>>>>RJ
>>>>>>>
>>>>>>>
>>>>>>>-- 
>>>>>>>em rnowling@gmail.com
>>>>>>>c 954.496.2314 
>>>>>>
>>>>>>
>>>>>>-- 
>>>>>>
>>>>>>Cheers
>>>>>>-MJ
>>>>>>
>>>>
>>>>
>>>>
>>>>-- 
>>>>em rnowling@gmail.com
>>>>c 954.496.2314 
>>>
>>>
>>>
>>>-- 
>>>Alejandro 
>>
>>
>>
>>-- 
>>em rnowling@gmail.com
>>c 954.496.2314 
>
>
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