Hadoop HDFS Namenode Metadata Sync Failure

What is Hadoop HDFS Namenode Metadata Sync Failure

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 provides high throughput access to application data and is suitable for applications that have large data sets.

Identifying the Symptom

In a Hadoop High Availability (HA) setup, you might encounter an error where the Namenode metadata fails to sync between the active and standby Namenodes. This issue is often indicated by error messages in the logs or a failure in the failover process.

Common Error Messages

"Namenode metadata sync failure" "Standby Namenode not updating" "Zookeeper quorum not reachable"

Exploring the Issue

The issue HDFS-043: Namenode Metadata Sync Failure arises when there is a failure in syncing metadata between Namenodes in an HA setup. This can lead to inconsistencies and potential data loss if not addressed promptly. The root cause is often related to misconfiguration or connectivity issues with Zookeeper, which is used to manage the HA state.

Root Causes

Misconfigured HA settings in the Hadoop configuration files. Network issues preventing Zookeeper from maintaining quorum. Improperly configured Zookeeper ensemble.

Steps to Resolve the Issue

To resolve the Namenode metadata sync failure, follow these steps:

Step 1: Verify HA Configuration

Ensure that the HA configuration is correctly set up in the hdfs-site.xml file. Check for the following properties:

<property> <name>dfs.nameservices</name> <value>mycluster</value></property><property> <name>dfs.ha.namenodes.mycluster</name> <value>namenode1,namenode2</value></property>

Ensure that each Namenode is correctly listed and configured.

Step 2: Check Zookeeper Status

Verify that the Zookeeper ensemble is running and reachable. Use the following command to check the status:

zkServer.sh status

Ensure that all Zookeeper nodes are in a healthy state and can communicate with each other.

Step 3: Review Network Connectivity

Ensure that there are no network issues preventing communication between the Namenodes and Zookeeper. Use tools like ping or telnet to verify connectivity.

Step 4: Restart Namenodes

If the configuration and network are correct, try restarting the Namenodes to re-establish the sync process:

hadoop-daemon.sh stop namenodehadoop-daemon.sh start namenode

Further Reading

For more detailed information on configuring HA in Hadoop, refer to the official Hadoop High Availability Guide. Additionally, check the Zookeeper Administration Guide for managing Zookeeper ensembles.