How to resolve a failed startup of a Hadoop datanode?

If a Hadoop datanode fails to start, you can try the following steps to resolve the issue:

1. Check the network connection of the slave node to ensure it can communicate with the master node.
2. Check if the configuration files of the secondary nodes are correct. For example, check the core-site.xml, hdfs-site.xml, and yarn-site.xml configuration files of the secondary nodes to ensure that their settings are consistent with the primary node.
3. Check the file system permissions on the slave nodes. Make sure that the Hadoop directories on the slave nodes, such as the HDFS data directory and log directory, have the correct permissions so that the Hadoop processes can read and write to these directories.
4. Check if the resources allocation for the slave node is reasonable. For example, check if there is enough memory and CPU resources on the slave node, and make sure that these resources are not being used by other processes.
5. Check the log files of the slave nodes. The log files of Hadoop are usually located in the log directory of the slave nodes, and reviewing these log files can help identify the reasons for startup failures.
6. Try to restart the secondary node. Sometimes, restarting the secondary node can resolve the issue of startup failure.

If the above methods do not resolve the issue, you can conduct a more in-depth investigation based on the specific error information, or seek help from the Hadoop community.