Hadoop: …be replicated to 0 nodes instead of minReplication (=1). There are 1 datanode(s) running and no node(s) are excluded in this operation

Question

I\'m getting the following error when attempting to write to HDFS as part of my multi-threaded application

could only be replicated to 0 nodes instead of min         


        

Answer 1

Check if the jps command on the computers which run the datanodes show that the datanodes are running. If they are running, then it means that they could not connect with the namenode and hence the namenode thinks there are no datanodes in the hadoop system.

In such a case, after running start-dfs.sh, run netstat -ntlp in the master node. 9000 is the port number most tutorials tells you to specify in core-site.xml. So if you see a line like this in the output of netstat

tcp        0      0 120.0.1.1:9000        0.0.0.0:*               LISTEN       4209/java

then you have a problem with the host alias. I had the same problem, so I'll state how it was resolved.

This is the contents of my core-site.xml

   
       fs.default.name
       hdfs://vm-sm:9000
   

So the vm-sm alias in the master computer maps to the 127.0.1.1. This is because of the setup of my /etc/hosts file.

127.0.0.1       localhost
127.0.1.1       vm-sm
192.168.1.1     vm-sm
192.168.1.2     vm-sw1
192.168.1.3     vm-sw2

Looks like the core-site.xml of the master system seemed to have mapped on the the 120.0.1.1:9000 while that of the worker nodes are trying to connect through 192.168.1.1:9000.

So I had to change the alias of the master node for the hadoop system (just removed the hyphen) in the /etc/hosts file

127.0.0.1       localhost
127.0.1.1       vm-sm
192.168.1.1     vmsm
192.168.1.2     vm-sw1
192.168.1.3     vm-sw2

and reflected the change in the core-site.xml, mapred-site.xml, and slave files (wherever the old alias of the master occurred).

After deleting the old hdfs files from the hadoop location as well as the tmp folder and restarting all nodes, the issue was solved.

Now, netstat -ntlp after starting DFS returns

tcp        0      0 192.168.1.1:9000        0.0.0.0:*               LISTEN ...
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