Unable to start Namenode

[#1]

While trying to start Namenode from Ambari, its failing with the below error:

  raise ExecutionFailed(err_msg, code, out, err)
resource_management.core.exceptions.ExecutionFailed: Execution of 'ambari-sudo.sh su hdfs -l -s /bin/bash -c 'ulimit -c unlimited ;  /usr/hdp/3.0.0.0-1371/hadoop/bin/hdfs --config /usr/hdp/3.0.0.0-1371/hadoop/conf --daemon start namenode'' returned 1. WARNING: HADOOP_NAMENODE_OPTS has been replaced by HDFS_NAMENODE_OPTS. Using value of HADOOP_NAMENODE_OPTS. 

ERROR: Cannot set priority of namenode process 355

Does anybody know a solution

Thanks in advance

[#2]

First you need to know the error.

- Check your namenode log file, by default located in the following directory /var/log/hadoop/hdfs

run tail -f /var/log/hadoop/hdfs/hadoop-hdfs-namenode*.log . and try to start the service again.

- Check if namenode process was already running on that node with ps -ef | grep namenode