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From sudhakara st <sudhakara...@gmail.com>
Subject Re: Decommissioning Nodes in Production Cluster.
Date Tue, 12 Feb 2013 15:30:57 GMT
The decommissioning process is controlled by an exclude file, which for 
HDFS is set by the* dfs.hosts.exclude* property, and for MapReduce by the*mapred.hosts.exclude
* property. In most cases, there is one shared file,referred to as the 
exclude file.This  exclude file name should be specified as a configuration 
parameter *dfs.hosts.exclude *in the name node start up.

To remove nodes from the cluster:
1. Add the network addresses of the nodes to be decommissioned to the 
exclude file.

2. Restart the MapReduce cluster to stop the tasktrackers on the nodes being
3. Update the namenode with the new set of permitted datanodes, with this
% hadoop dfsadmin -refreshNodes
4. Go to the web UI and check whether the admin state has changed to 
In Progress” for the datanodes being decommissioned. They will start copying
their blocks to other datanodes in the cluster.

5. When all the datanodes report their state as “Decommissioned,” then all 
the blocks
have been replicated. Shut down the decommissioned nodes.
6. Remove the nodes from the include file, and run:
% hadoop dfsadmin -refreshNodes
7. Remove the nodes from the slaves file.

 Decommission data nodes in small percentage(less than 2%) at time don't 
cause any effect on cluster. But it better to pause MR-Jobs before you 
triggering Decommission to ensure  no task running in decommissioning 
subjected nodes.
 If very small percentage of task running in the decommissioning node it 
can submit to other task tracker, but percentage queued jobs  larger then 
threshold  then there is chance of job failure. Once triggering the 'hadoop 
dfsadmin -refreshNodes' command and decommission started, you can resume 
the MR jobs.

*Source : The Definitive Guide [Tom White]*

On Tuesday, February 12, 2013 5:20:07 PM UTC+5:30, Dhanasekaran Anbalagan 
> Hi Guys,
> It's recommenced do with removing one the datanode in production cluster.
> via Decommission the particular datanode. please guide me.
> -Dhanasekaran,
> Did I learn something today? If not, I wasted it.

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