How to restart failed Namenode

Question

I have created a hadoop cluster with shared storage for having HA following edureka article in https://www.edureka.co/blog/how-to-set-up-hadoop-cluster-with-hdfs-high-availability/

I noticed that Namenode on one of the host failed and the other namenode became active. So currently it's running only one namenode. There are no bad blocks or loss of data. How do I restart the namenode failed on the host in standby mode without losing any data? Any backup steps need to be followed to be safe before proceeding further?

Answer 1

You need to solve the issue which caused your active name node to stop. Once that is done please run the command 

hdfs namenode -bootstrapStandby

 on the NameNode still down, then turn in on again. This node becomes Standby NameNode

Please follow the below steps:

Copy the HDFS Metadata from active name node to standby namenode.

Command: 

hdfs namenode -bootstrapStandby

Copy the HDFS Metadata from Active name node to Standby Namenode. Once you run this command, you will get the information from which node and location the metadata is copying and whether it is copying successfully or not.

Information on Active namenode details.

Once Metadata is copied from Active namenode to standby namenode, you will get the message shown below in the screenshot.

Information regarding HDFS in Standby Namenode.

Start the namenode daemon in Standby namenode machine.

Command: 

hadoop-daemon.sh start namenode

Now the name node which was down will become passive name node.