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From Harsh J <ha...@cloudera.com>
Subject Re: structured data split
Date Fri, 11 Nov 2011 13:54:38 GMT

This is incorrect. As Denny had explained earlier, blocks are split along byte sizes alone.
The writer does not concern itself with newlines and such. When reading, the record readers
align themselves to read till the end of lines by communicating with the next block if they
have to.

This is explained neatly under http://wiki.apache.org/Hadoop/MapReduceArch, para 2 of Map.

Regarding structured data, such as XML, one can write their custom InputFormat that returns
appropriate split points after scanning through the entire file pre-submit (say, by looking
at tags). 

However, if you want XML, then there is already an XMLInputFormat available in Mahout. For
reading N lines at a time, use NLineInputFormat.

On 11-Nov-2011, at 6:55 PM, bejoy.hadoop@gmail.com wrote:

> Donal
> In hadoop that hardly happens so. When you are storing data in hdfs it would be split
line to blocks depending on end of lines, in case of normal files. It won't be like you'd
be having half of a line in one block and the rest in next one. You don't need to worry on
that fact.
> The case you mentioned is like dependent data splits. Hadoop's massive parallel processing
could be fully utilized only in case of independent data splits. When data splits are dependent
on a file level as I pointed out you can go for WholeFileInputFormat.
> Please revert if you are still confused. Also if you have some specific scenario, please
put that across so we may be able to help you understand better on the map reduce processing
of the same.
> Hope it clarifies...
> Regards
> Bejoy K S
> From: 臧冬松 <donal0412@gmail.com>
> Date: Fri, 11 Nov 2011 20:46:54 +0800
> To: <hdfs-user@hadoop.apache.org>
> ReplyTo: hdfs-user@hadoop.apache.org
> Subject: Re: structured data split
> Thanks Bejoy!
> It's better to process the data blocks locally and separately.
> I just want to know how to deal with a structure (i.e. a word,a line) that is split into
two blocks.
> Cheers,
> Donal
> 在 2011年11月11日 下午7:01,Bejoy KS <bejoy.hadoop@gmail.com>写道:
> Hi Donal
>       You can configure your map tasks the way you like to process your input. If you
have file of size 100 mb, it would be divided into two input blocks and stored in hdfs ( if
your dfs.block.size is default 64 Mb). It is your choice on how you  process the same using
map reduce
> - With the default TextInputFormat the two blocks would be processed by two different
mappers. (under default split settings) If the blocks are in two different data nodes then
two different mappers mappers would be spanned in each data node in beat case. ie They are
data local map tasks
>  - If you want one mapper to process the whole file,change your input format to WholeFileInputFormat.
There a mapper task would be triggred on any one of the node where the blocks are located.
(best case) If both the blocks are not on the same node then one of the blocks would be transferred
to the map task location for processing.
> Hope it helps!...
> Thank You
> Bejoy.K.S
> 2011/11/11 臧冬松 <donal0412@gmail.com>
> Thanks Denny!
> So that means each map task will have to read from another DataNode inorder to read the
end line of the previous block?
> Cheers,
> Donal
> 2011/11/11 Denny Ye <dennyy99@gmail.com>
> hi
>    Structured data is always being split into different blocks, likes a word or line.

>    MapReduce task read HDFS data with the unit - line - it will read the whole line from
the end of previous block to start of subsequent to obtains that part of line record. So you
does not worry about the Incomplete structured data. HDFS do nothing for this mechanism.
> -Regards
> Denny Ye
> On Fri, Nov 11, 2011 at 3:43 PM, 臧冬松 <donal0412@gmail.com> wrote:
> Usually large file in HDFS is split into bulks and store in different DataNodes.
> A map task is assigned to deal with that bulk, I wonder what if the Structured data(i.e
a word) was split into two bulks?
> How MapReduce and HDFS deal with this?
> Thanks!
> Donal

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