One of the most attractive features of Hadoop framework is its utilization of commodity hardware. However, this leads to frequent DataNode crashes in a Hadoop cluster. Another striking feature of Hadoop Framework is the ease of scale in accordance to the rapid growth in data volume. Because of these two reasons, one of the most common task of a Hadoop administrator is to commission (Add) and decommission (Remove) Data Nodes in a Hadoop Cluster.
Commissioning and Decommissioning Nodes in a Hadoop Cluster:
Above diagram shows a step by step process to decommission a DataNode in the cluster.
The first task is to update the ‘exclude‘ files for both HDFS (hdfs-site.xml) and MapReduce (mapred-site.xml).
The ‘exclude’ file:
- for jobtracker contains the list of hosts that should be excluded by the jobtracker. If the value is empty, no hosts are excluded.
- for Namenode contains a list of hosts that are not permitted to connect to the Namenode.
Here is the sample configuration for the exclude file in hdfs-site.xml and mapred-site.xml:
hdfs-site.xml
<property>
<name>dfs.hosts.exclude</name>
<value>/home/hadoop/excludes</value>
<final>true</final>
</property>
mapred-site.xml
<property>
<name>mapred.hosts.exclude</name>
<value>/home/hadoop/excludes</value>
<final>true</final>
</property>
Note: The full pathname of the files must be specified.
Similarly, we have the ‘include’ files:
- for jobtracker containing the list of nodes that may connect to the JobTracker. If the value is empty, all hosts are permitted.
- for Namenode containing a list of hosts that are permitted to connect to the Namenode. If the value is empty, all hosts are permitted.
The ‘dfsadmin’ and ‘mradmin’ commands refresh the configuration with the changes to make them aware of the new node.
The ‘slaves’ file on master server contains the list of all data nodes. This must also be updated to ensure any issues in future hadoop daemon start/stop.
The important step in data node commission process is to run the Cluster Balancer.
>hadoop balancer -threshold 40
Balancer attempts to provide a balance to a certain threshold among data nodes by copying block data from older nodes to newly commissioned nodes.
So, this is how you can do – Commissioning and Decommissioning Nodes in a Hadoop Cluster.
Get a better understanding of the Hadoop Cluster from this Big Data Course.
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