Ceph is a highly resilient, scalable, and configurable distributed storage system, which often functions through a cluster of multiple nodes. One critical component of a Ceph cluster's operation involves the use of keyring files for authentication and authorization purposes. These keyring files contain secret keys for clients and daemons to authenticate against each other. One common issue that may arise during the management or setup of a Ceph cluster is the KeyNotFoundError, particularly when the ceph.client.admin.keyring is missing.

Understanding the KeyNotFoundError

When you encounter a KeyNotFoundError, it typically means that the Ceph cluster is unable to locate the specified keyring file in its designated directory. The error message Could not find keyring file: /etc/ceph/ceph.client.admin.keyring on host node1 specifically indicates that the admin keyring file is missing on 'node1', which is one of the nodes in the Ceph cluster.

Why is ceph.client.admin.keyring Important?

The ceph.client.admin.keyring file contains the keys for the admin user of the Ceph cluster. This user has unrestricted access to the cluster, which is necessary for performing various administrative operations such as:

• Creating and deleting storage pools.
• Adjusting configuration settings.
• Accessing cluster status and reports.

Common Causes of the KeyNotFoundError

Some typical reasons why this error might occur include:

• Improper Installation: If Ceph was not installed correctly or the installation script failed to complete, some key files might not be placed in their expected locations.
• Manual Deletions or Modifications: Unintentional deletion or moving of the keyring file without updating respective configurations could lead to this error.
• Incorrect Permissions: Even if the file exists, incorrect file permissions can prevent Ceph from accessing it.

Resolving the KeyNotFoundError

Here are some steps to troubleshoot and resolve the KeyNotFoundError:

1. Check the file existence: First, ensure the file really is missing by logging into 'node1' and checking the /etc/ceph directory.

   ls -l /etc/ceph

1. Restore or recreate the keyring:
   • If you have a backup, restore the keyring file from it.
   • Alternatively, you can regenerate the admin keyring using another admin node or any node with admin privileges:

     ceph auth get-or-create client.admin osd 'allow *' mon 'allow *' -o /etc/ceph/ceph.client.admin.keyring

1. Verify correct permissions: The keyring file should be readable by the user under which the Ceph services are running.

   chmod 644 /etc/ceph/ceph.client.admin.keyring
   chown ceph:ceph /etc/ceph/ceph.client.admin.keyring

1. Copy the keyring to node1: If you regenerated or restored the keyring on a different node, ensure to copy it to node1.

   scp /etc/ceph/ceph.client.admin.keyring node1:/etc/ceph/

1. Restart Ceph services: After ensuring the keyring is in place and correctly configured, restart the Ceph services on node1.

   systemctl restart ceph.target

Summary of Key Points

By following the detailed approach above, one can effectively manage and troubleshoot the KeyNotFoundError affecting the ceph.client.admin.keyring in a Ceph cluster.