Setting fs.default.name in core-site.xml Sets HDFS to Safemode - hadoop

Question

I installed the Cloudera CDH4 distribution on a single machine in pseudo-distributed mode and successfully tested that it was working correctly (e.g. can run MapReduce programs, insert data on the Hive server, etc.) However, if I chance the core-site.xml file to have fs.default.name set to machine name rather than localhost and restart the NameNode service, the HDFS enters safe-mode.
Before the change of fs.default.name, I ran the following to check the state of the HDFS:
$ hadoop dfsadmin -report
...
Configured Capacity: 18503614464 (17.23 GB)
Present Capacity: 13794557952 (12.85 GB)
DFS Remaining: 13790785536 (12.84 GB)
DFS Used: 3772416 (3.60 MB)
DFS Used%: 0.03%
Under replicated blocks: 2
Blocks with corrupt replicas: 0
Missing blocks: 0
Then I made the modification to core-site.xml (with the machine name being hadoop):
<property>
<name>fs.default.name</name>
<value>hdfs://hadoop:8020</value>
</property>
I restarted the service and reran the report.
$ sudo service hadoop-hdfs-namenode restart
$ 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
An interesting note is that I can still perform some HDFS commands. For example, I can run
$ hadoop fs -ls /tmp
However, if I try to read a file using hadoop fs -cat or try to place a file in the HDFS, I am told the NameNode is in safemode.
$ hadoop fs -put somefile .
put: Cannot create file/user/hadinstall/somefile._COPYING_. Name node is in safe mode.
The reason I need the fs.default.name to be set to the machine name is because I need to communicate with this machine on port 8020 (the default NameNode port). If fs.default.name is left to localhost, then the NameNode service will not listen to external connection requests.
I am at a loss as to why this is happening and would appreciate any help.

Answer 1

The issue stemmed from domain name resolution. The /etc/hosts file needed to be modified to point the IP address of the machine of the hadoop machine for both localhost and the fully qualified domain name.
192.168.0.201 hadoop.fully.qualified.domain.com localhost

Answer 2

Safemode is an HDFS state in which the file system is mounted read-only; no replication is performed, nor can files be created or deleted. Filesystem operations that access the filesystem metadata like 'ls' in you case will work.
The Namenode can be manually forced to leave safemode with this command( $ hadoop dfsadmin -safemode leave).Verify status of safemode with ( $ hadoop dfsadmin -safemode get)and then run dfsadmin report to see if it shows data.If after getting out of safe mode the report still dose not show any data then i'm suspecting communication between namenode and datanode is not hapenning. Check namenode and datanode logs after this step.
The next steps could be to try restarting datanode process and last resort will be to format namenode which will result in loss of data.