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From "Sven Groot" <sgr...@gmail.com>
Subject RE: Limitation of key-value pairs for a particular key.
Date Fri, 18 Jan 2013 09:50:19 GMT


I think I know what's going on here. It has to do with how many spills the
map task performs.


You are emitting the numbers in order, so if there is only one spill, they
stay in order. For larger number of records, the map task will create more
than one spill, which must be merged. During the merge, the original order
is not preserved.


If you want the original order to be preserved, you must set io.sort.mb
and/or io.sort.record.percent such that the map task requires only a single





From: Utkarsh Gupta [mailto:Utkarsh_Gupta@infosys.com] 
Sent: 18 January 2013 18:25
To: mapreduce-user@hadoop.apache.org
Subject: RE: Limitation of key-value pairs for a particular key.


You are right 

Actually we were expecting the values to be sorted.

We tried to reproduce the problem by this simple code

private final IntWritable one=new IntWritable(1);

        private Text word=new Text();


        public void map(LongWritable key,Text value, Context context) throws
IOException, InterruptedException {

            int N=30000;

            for(int i=0;i<N;i++)







For smaller N numbers were in order but for N 3000000 order was not


From: Harsh J [mailto:harsh@cloudera.com] 
Sent: Thursday, January 17, 2013 1:57 AM
To: mapreduce-user
Subject: RE: Limitation of key-value pairs for a particular key.


We don't sort values (only keys) nor apply any manual limits in MR. Can your
post a reproduceable test case to support your suspicion?

On Jan 16, 2013 4:34 PM, "Utkarsh Gupta" <Utkarsh_Gupta@infosys.com
<mailto:Utkarsh_Gupta@infosys.com> > wrote:


Thanks for the response. There was some issues with my code. I have checked
that in detail. 

All the values of map are present in reducer but not in sorted order. This
case happens if the number of values are too large for a key. 





From: Vinod Kumar Vavilapalli [mailto:vinodkv@hortonworks.com
<mailto:vinodkv@hortonworks.com> ] 
Sent: Thursday, January 10, 2013 11:00 PM
To: mapreduce-user@hadoop.apache.org
Subject: Re: Limitation of key-value pairs for a particular key.


There isn't any limit like that. Can you reproduce this consistently? If so,
please file a ticket.

It will definitely help if you can provide a test case which can reproduce
this issue.



On Thu, Jan 10, 2013 at 12:41 AM, Utkarsh Gupta <Utkarsh_Gupta@infosys.com
<mailto:Utkarsh_Gupta@infosys.com> > wrote:



I am using Apache Hadoop 1.0.4 on a 10 node cluster of commodity machines
with Ubuntu 12.04 Server edition. I am having a issue with my map reduce
code. While debugging I found that the reducer can take 262145 values for a
particular key. If more values are there, they seem to be corrupted. I
checked the values while emitting from map and again checked in reducer.

I am wondering is there any such kind of limitation in the Hadoop or is it a
configuration problem.



Thanks and Regards

Utkarsh Gupta



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