HDFS Operations

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HDFS Operations

Starting HDFS

Format the configured HDFS file system and then open the namenode (HDFS server) and execute the following command.

$ hadoop namenode -format

Start the distributed file system and follow the command listed below to start the namenode as well as the data nodes in cluster.

$ start-dfs.sh

Listing Files in HDFS

Finding the list of files in a directory and the status of a file using ‘ls’ command in the terminal. Syntax of ls can be passed to a directory or a filename as an argument which are displayed as follows:

$ $HADOOP_HOME/bin/hadoop fs -ls 

Inserting Data into HDFS

Below mentioned steps are followed to insert the required file in the Hadoop file system.

Step1: Create an input directory

$ $HADOOP_HOME/bin/hadoop fs -mkdir /user/input

Step2: Use put command transfer and store the data file from the local systems to the HDFS using the following commands in the terminal.

$ $HADOOP_HOME/bin/hadoop fs -put /home/intellipaat.txt /user/input

Step3: Verify the file using ls command.

$ $HADOOP_HOME/bin/hadoop fs -ls /user/input

Retrieving Data from HDFS

For an instance if you have a file in HDFS called Intellipaat. Then retrieve the required file from the Hadoop file system by carrying out:

Step1: View the data from HDFS using cat command.

$ $HADOOP_HOME/bin/hadoop fs -cat /user/output/intellipaat

Step2: Gets the file from HDFS to the local file system using get command as shown below

$ $HADOOP_HOME/bin/hadoop fs -get /user/output/ /home/hadoop_tp/

Shutting Down the HDFS

Shut down the HDFS files by following the below command

$ stop-dfs.sh

Multi-Node Cluster

Installing Java

Syntax of java version command

$ java -version

Following output is presented.

java version “1.7.0_71”

Java(TM) SE Runtime Environment (build 1.7.0_71-b13)

Java HotSpot(TM) Client VM (build 25.0-b02, mixed mode)

Creating User Account

System user account is used on both master and slave systems for the Hadoop installation.

# useradd hadoop
# passwd hadoop

Mapping the nodes

Hosts files should be edited in /etc/ folder on each and every nodes and IP address of each system followed by their host names must be specified mandatorily.

# vi /etc/hosts

Enter the following lines in the /etc/hosts file. hadoop-master hadoop-slave-1 hadoop-slave-2

Configuring Key Based Login

Ssh should be setup in each node so they can easily converse with one another without any prompt for password.

# su hadoop
$ ssh-keygen -t rsa
$ ssh-copy-id -i ~/.ssh/id_rsa.pub tutorialspoint@hadoop-master
$ ssh-copy-id -i ~/.ssh/id_rsa.pub hadoop_tp1@hadoop-slave-1
$ ssh-copy-id -i ~/.ssh/id_rsa.pub hadoop_tp2@hadoop-slave-2
$ chmod 0600 ~/.ssh/authorized_keys
$ exit

Installation of Hadoop

Hadoop should be downloaded in the master server using the following procedure.

# mkdir /opt/hadoop
# cd /opt/hadoop/
# wget http://apache.mesi.com.ar/hadoop/common/hadoop-1.2.1/hadoop-1.2.0.tar.gz
# tar -xzf hadoop-1.2.0.tar.gz
# mv hadoop-1.2.0 hadoop
# chown -R hadoop /opt/hadoop
# cd /opt/hadoop/hadoop/

Configuring Hadoop

Hadoop server must be configured in core-site.xml and should be edited where ever required.


hdfs-site.xml file should be editted.

















mapred-site.xml file should be edited as per the requirement example is being shown.








JAVA_HOME, HADOOP_CONF_DIR, and HADOOP_OPTS should be edited as follows:


export JAVA_HOME=/opt/jdk1.7.0_17

export HADOOP_OPTS=-Djava.net.preferIPv4Stack=true

export HADOOP_CONF_DIR=/opt/hadoop/hadoop/conf

Installing Hadoop on Slave Servers

Hadoop should be installed on all the slave servers

# su hadoop

$ cd /opt/hadoop

$ scp -r hadoop hadoop-slave-1:/opt/hadoop

$ scp -r hadoop hadoop-slave-2:/opt/hadoop

Configuring Hadoop on Master Server

Master server configuration

# su hadoop

$ cd /opt/hadoop/hadoop

Master Node Configuration

$ vi etc/hadoop/masters


Slave Node Configuration

$ vi etc/hadoop/slaves



Name Node format on Hadoop Master


# su hadoop

$ cd /opt/hadoop/hadoop

$ bin/hadoop namenode –format

11/10/14 10:58:07 INFO namenode.NameNode: STARTUP_MSG:



STARTUP_MSG: Starting NameNode

STARTUP_MSG: host = hadoop-master/

STARTUP_MSG: args = [-format]

STARTUP_MSG: version = 1.2.0

STARTUP_MSG: build =

https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1.2 -r 1479473; compiled by 'hortonfo' on Monday May 6 06:59:37 UTC 2013

STARTUP_MSG: java = 1.7.0_71



11/10/14 10:58:08 INFO util.GSet: Computing capacity for map BlocksMap editlog=/opt/hadoop/hadoop/dfs/name/current/edits



11/10/14 10:58:08 INFO common.Storage: Storage directory /opt/hadoop/hadoop/dfs/name has been successfully formatted.

11/10/14 10:58:08 INFO namenode.NameNode: SHUTDOWN_MSG:


SHUTDOWN_MSG: Shutting down NameNode at hadoop-master/


Hadoop Services

Starting Hadoop services on the Hadoop-Master procedure explains its setup.


$ cd $HADOOP_HOME/sbin

$ start-all.sh

Addition of a New DataNode in the Hadoop Cluster is as follows:


Add new nodes to an existing Hadoop cluster with some suitable network configuration. Consider the following network configuration for new node Configuration:

IP address :

netmask :

hostname : slave3.in

Adding a User and SSH Access

Add a user working under “hadoop” domain and the user must have the access added and password of Hadoop user can be set to anything one wants.

useradd hadoop

passwd hadoop

To be executed on master



mkdir -p $HOME/.ssh

chmod 700 $HOME/.ssh

ssh-keygen -t rsa -P '' -f $HOME/.ssh/id_rsa

cat $HOME/.ssh/id_rsa.pub >> $HOME/.ssh/authorized_keys

chmod 644 $HOME/.ssh/authorized_keys

Copy the public key to new slave node in hadoop user $HOME directory

scp $HOME/.ssh/id_rsa.pub hadoop@

Execution done on slaves

su hadoop ssh -X hadoop@

Content of public key must be copied into file “$HOME/.ssh/authorized_keys” and then the permission for the same must be changed as per the requirement.

cd $HOME

mkdir -p $HOME/.ssh

chmod 700 $HOME/.ssh

cat id_rsa.pub >>$HOME/.ssh/authorized_keys

chmod 644 $HOME/.ssh/authorized_keys

ssh login must be changed from the master machine. It is possible that the ssh to the new node without a password from the master must be verified.

ssh hadoop@ or hadoop@slave3

Setting  Hostname for New Node

Hostname is setup in the file directory  /etc/sysconfig/network

On new slave3 machine



Machine must be restarted again or hostname command should be run under new machine with the corresponding hostname to make changes effectively.

On slave3 node machine:

hostname slave3.in

/etc/hosts must be updated on all machines of the cluster slave3.in slave3

ping the machine with hostnames to check whether it is resolving to IP address.

ping master.in

Start the DataNode on New Node

Datanode daemon should be started manually using $HADOOP_HOME/bin/hadoop-daemon.sh script. Master (NameNode) should correspondingly join the cluster after automatically contacted. New node should be added to the configuration/slaves file in the master server. New node will be identified by script-based commands.

Login to new node

su hadoop or ssh -X hadoop@

HDFS is started on a newly added slave node

./bin/hadoop-daemon.sh start datanode

 jps command output must be checked on a new node.

$ jps

7141 DataNode

10312 Jps

Removing a DataNode

Node can be removed from a cluster while it is running, without any worries of data loss. A decommissioning feature is made available by HDFS which ensures that removing a node is performed securely.

Step 1

Login to master machine so that the user can check Hadoop is being installed.

$ su hadoop

Step 2

Before starting the cluster an exclude file must be configured where a key named dfs.hosts.exclude should be added to our$HADOOP_HOME/etc/hadoop/hdfs-site.xmlfile.

NameNode’s local file system contains a list of machines which are not permitted to connect to HDFS receives full path by this key and the value associated with it as follows.


<name>dfs.hosts.exclude</name><value>/home/hadoop/hadoop-1.2.1/hdfs_exclude.txt</value><description>>DFS exclude</description>


Step 3

Hosts with respect to decommission are determined.

File reorganization by the hdfs_exclude.txt for each and every machine to be decommissioned which will results in preventing them from connecting to the NameNode.


Step 4

Force configuration reloads.

“$HADOOP_HOME/bin/hadoop dfsadmin -refreshNodes” should be run

$ $HADOOP_HOME/bin/hadoop dfsadmin -refreshNodes

NameNode will be forced made to re-read its configuration, as this is inclusive for the newly updated ‘excludes’ file. Nodes will be decommissioned over a period of time intervals, and allowing time for each node’s blocks to be replicated onto machines which are scheduled to be active.jps command output should be checked on slave2.in. Once the work is done DataNode process will shutdown automatically.

Step 5

Shutdown nodes.

The decommissioned hardware can be carefully shut down for maintenance purpose after the decommission process has been finished.


$ $HADOOP_HOME/bin/hadoop dfsadmin -report

Step 6

Excludes are edited again and once the machines have been decommissioned, they are removed from the ‘excludes’ file. “$HADOOP_HOME/bin/hadoop dfsadmin -refreshNodes” will read the excludes file back into the NameNode.

Data Nodes will rejoin the cluster after the maintenance has been completed, or if additional capacity is needed in the cluster again is being informed.

To run/shutdown tasktracker


$ $HADOOP_HOME/bin/hadoop-daemon.sh stop tasktracker

$ $HADOOP_HOME/bin/hadoop-daemon.sh start tasktracker

Add a new node with the following steps

1) Take a new system which gives access to create a new username and password

2) Install the SSH and with master node setup ssh connections

3) Add sshpublic_rsa id key having an authorized keys file

4) Add the new data node hostname, IP address and other informative details in /etc/hosts slaves file192.168.1.102 slave3.in slave3

5) Start the DataNode on the New Node

6) Login to the new node command like suhadoop or Ssh -X hadoop@

7) Start HDFS of newly added in the slave node by using the following command ./bin/hadoop-daemon.sh start data node

8) Check the output of jps command on a new node.