top button
Flag Notify
    Connect to us
      Site Registration

Site Registration

What does mean of NameNode in Hadoop?

+1 vote
348 views
What does mean of NameNode in Hadoop?
posted Dec 4, 2014 by Amit Kumar Pandey

Share this question
Facebook Share Button Twitter Share Button LinkedIn Share Button

1 Answer

0 votes

The NameNode is the centerpiece of an HDFS file system. It keeps the directory tree of all files in the file system, and tracks where across the cluster the file data is kept. It does not store the data of these files itself.
There is only One NameNode process run on any hadoop cluster. NameNode runs on its own JVM process. In a typical production cluster its run on a separate machine.

The NameNode is a Single Point of Failure for the HDFS Cluster. When the NameNode goes down, the file system goes offline.

Client applications talk to the NameNode whenever they wish to locate a file, or when they want to add/copy/move/delete a file. The NameNode responds the successful requests by returning a list of relevant DataNode servers where the data lives.

answer Dec 29, 2014 by Kali Mishra
Similar Questions
+2 votes

I am running hadoop-2.4.0 cluster. Each datanode has 10 disks, directories for 10 disks are specified in dfs.datanode.data.dir.

A few days ago, I modified dfs.datanode.data.dir of a datanode () to reduce disks. so two disks were excluded from dfs.datanode.data.dir, after the datanode was restarted, I expected that the namenode would update block locations. In other words, I thought the namenode should remove from block locations associated with blocks which were stored on excluded disks, but the namenode didnt update the block locations...

In my understanding, datanode send a block report to the namenode when datanode start so the namenode should update block locations immediately.

Is a bug? Could anyone please explain?

+1 vote

I have an old Hadoop install that Im looking to update to Hadoop 2. In the old setup, I have a /conf/masters file that specifies the secondary namenode.

Looking through the Hadoop 2 documentation I cant find any mention of a "masters" file, or how to setup a secondary namenode.

Thanks in advance?

+2 votes
public class MaxMinReducer extends Reducer {
int max_sum=0; 
int mean=0;
int count=0;
Text max_occured_key=new Text();
Text mean_key=new Text("Mean : ");
Text count_key=new Text("Count : ");
int min_sum=Integer.MAX_VALUE; 
Text min_occured_key=new Text();

 public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
       int sum = 0;           

       for (IntWritable value : values) {
             sum += value.get();
             count++;
       }

       if(sum < min_sum)
          {
              min_sum= sum;
              min_occured_key.set(key);        
          }     


       if(sum > max_sum) {
           max_sum = sum;
           max_occured_key.set(key);
       }          

       mean=max_sum+min_sum/count;
  }

 @Override
 protected void cleanup(Context context) throws IOException, InterruptedException {
       context.write(max_occured_key, new IntWritable(max_sum));   
       context.write(min_occured_key, new IntWritable(min_sum));   
       context.write(mean_key , new IntWritable(mean));   
       context.write(count_key , new IntWritable(count));   
 }
}

Here I am writing minimum,maximum and mean of wordcount.

My input file :

high low medium high low high low large small medium

Actual output is :

high - 3------maximum

low - 3--------maximum

large - 1------minimum

small - 1------minimum

but i am not getting above output ...can anyone please help me?

...