Hadoop HDFS Namenode is unresponsive or fails to start.

What is Hadoop HDFS Namenode is unresponsive or fails to start.

Understanding Hadoop HDFS

Hadoop Distributed File System (HDFS) is a distributed file system designed to run on commodity hardware. It is highly fault-tolerant and designed to be deployed on low-cost hardware. HDFS is the primary storage system used by Hadoop applications and provides high throughput access to application data.

Identifying the Symptom

One common issue that can occur in HDFS is when the Namenode becomes unresponsive or fails to start. This is a critical problem as the Namenode is responsible for managing the metadata and directory structure of all files and directories in the HDFS.

Observed Error

When attempting to start or interact with the Namenode, you may encounter errors indicating that the Namenode cannot be reached or is not functioning properly. This can manifest as an inability to access HDFS data or perform file operations.

Details About the Issue

The error code HDFS-011: Namenode Disk Failure indicates a failure in the disk where the Namenode's metadata is stored. This disk failure can lead to the Namenode being unable to read or write the necessary metadata, resulting in the observed symptoms.

Root Cause Analysis

The root cause of this issue is typically a hardware failure in the disk used by the Namenode. This can be due to physical damage, wear and tear, or other hardware-related issues that prevent the disk from functioning correctly.

Steps to Fix the Issue

To resolve the HDFS-011: Namenode Disk Failure issue, follow these steps:

Step 1: Identify the Failed Disk

Check the Namenode logs for any disk-related errors. The logs are usually located in the /var/log/hadoop-hdfs/ directory. Use disk diagnostic tools such as smartctl to check the health of the disks. For example, run smartctl -a /dev/sdX where /dev/sdX is the disk identifier.

Step 2: Replace the Failed Disk

Physically replace the failed disk with a new one. Ensure that the new disk is properly mounted and recognized by the operating system.

Step 3: Restore Metadata from Backup

Restore the Namenode metadata from a recent backup. This can be done using the hdfs namenode -restore command. Ensure that the backup is up-to-date and consistent with the current state of the HDFS.

Step 4: Restart the Namenode

Once the disk is replaced and metadata is restored, restart the Namenode using the hadoop-daemon.sh start namenode command. Verify that the Namenode starts successfully and that HDFS operations can be performed without errors.

Additional Resources

For more information on managing HDFS and troubleshooting common issues, refer to the following resources:

HDFS User Guide HDFS Architecture and Design Cloudera Community: Recovering from Namenode Failure