Changing hostname in fs default name property puts Hadoop in safemode

Question

I have a CDH pseudo-distributed mode Hadoop cluster. It was working fine. Then as I was studying about the configuration files, I came across fs.default.name property present in core-site.xml file. Earlier my host was localhost, but I replaced it with hadoop.

After restarting the Hadoop daemons, Hadoop show that it is in safemode.

I got the following output:

$ hadoop dfsadmin -report
...
Safe mode is ON
Configured Capacity: 0 (0 B)
Present Capacity: 0 (0 B)
DFS Remaining: 0 (0 B)
DFS Used: 0 (0 B)
DFS Used%: NaN%
Under replicated blocks: 0
Blocks with corrupt replicas: 0
Missing blocks: 0

I am not able to execute -cat or -put command. It is showing that NameNode is in safe mode. Can anyone help in understanding how I can keep the hostname as Hadoop so that external systems can connect to it & my NameNode does not enter safe mode.

Answer 1

First of all, in Safe mode the HDFS filesystem is in read-only mode. So, all the read only applications can be executed but operations like write, create, delete cannot be performed.

Next, your hostname issue is because you machine cannot resolve your domain name. To do so, you need to append this in your file /etc/hosts:

192.168.1.1 (your own ip address)     hadoop.domain.com localhost