Troubleshooting NameNode: Common Errors and How to Fix Them?

 

NameNode Common Errors and Solutions

1. NameNode Out of Memory (OOM)

Error Message:

java.lang.OutOfMemoryError: Java heap space

Cause:

• Heap size allocated to NameNode is too small.
• Large number of small files consuming excessive memory.

Solution:

1. Increase heap memory in hadoop-env.sh:

   export HADOOP_NAMENODE_OPTS="-Xms4G -Xmx8G"
   

2. Enable Federation for large datasets (dfs.federation.enabled=true).
3. Use HDFS Erasure Coding instead of replication.

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2. NameNode Safe Mode Stuck

Error Message:

org.apache.hadoop.hdfs.server.namenode.SafeModeException: Cannot leave safe mode.

Cause:

• DataNodes not reporting correctly.
• Corrupt blocks preventing NameNode from exiting safe mode.

Solution:

1. Check DataNode health:

   hdfs dfsadmin -report
   

2. Force NameNode out of safe mode (if healthy):

   hdfs dfsadmin -safemode leave
   

3. Run block check and delete corrupt blocks:

   hdfs fsck / -delete
   

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3. NameNode Fails to Start Due to Corrupt Edit Logs

Error Message:

org.apache.hadoop.hdfs.server.namenode.EditLogInputStream

Cause:

• Corrupt edit logs due to improper shutdown.

Solution:

1. Try recovering logs:

   hdfs namenode -recover
   

2. If recovery fails, format NameNode metadata (last resort):

   hdfs namenode -format
   

   (⚠️ This will erase all metadata! Use only if absolutely necessary.)

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4. NameNode Connection Refused

Error Message:

java.net.ConnectException: Connection refused

Cause:

• NameNode service is not running.
• Firewall or incorrect network configuration.

Solution:

1. Restart NameNode:

   hdfs --daemon start namenode
   

2. Check firewall settings:

   iptables -L
   

3. Verify correct hostnames in core-site.xml.

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5. NameNode High CPU Usage

Cause:

• Too many open file handles.
• Insufficient NameNode memory.

Solution:

1. Increase file descriptor limit:

   ulimit -n 100000
   

2. Optimize hdfs-site.xml for large deployments:

   <property>
       <name>dfs.namenode.handler.count</name>
       <value>100</value>
   </property>