Unable to Attach or Mount Volumes

In Kubernetes, persistent storage is critical for stateful applications like databases and message queues requiring consistent data access across pod restarts and rescheduling. Kubernetes supports dynamic volume provisioning through StorageClasses, which automatically creates and attaches Persistent Volumes (PVs) when applications request storage via Persistent Volume Claims (PVCs).

Due to this dynamic behavior, errors can occur during the volume attachment and detachment process. ‘Unable to attach or mount volumes’ is one such error that occurs during volume operations, typically as a FailedMount or FailedAttach event in pod events, preventing pods from starting.

This error commonly occurs in stateful workloads like databases or message queues that use PVCs to request storage. This happens when the volume controller manager or the kubelet fails to complete their respective volume operations, leading to the pods being in a pending state.

Situations in which error most commonly appears

The above error generally occurs in multiple scenarios; some of these are:

• Node affinity conflicts and topology constraints: When using topology-aware volume provisioning, volume attachment fails if the pod is scheduled to a node that doesn’t satisfy the volume’s topology requirements. For instance, with the WaitForFirstConsumer volume binding mode, when the selected node doesn’t match the storage provider’s topology constraints.

• CSI driver issues: Container Storage Interface (CSI) drivers manage the lifecycle of storage volumes. Any malfunction or error caused by misconfiguration leads to provisioning failures, attachment, or mounting volume issues. This issue may occur when the CSI node plugin pod fails, network connectivity issues between the CSI controller and storage backends, and version mismatch between the CSI driver and storage provider APIs.

• Storage class misconfiguration issue: Volume operations can fail due to StorageClass misconfiguration including invalid provisioner names, unsupported filesystem type, incompatible volumeBindingMode settings and invalid storage provider-specific parameters.

Imagine you’re running a stateful application like a MySQL database on your Azure Kubernetes Service (AKS) cluster, and the pod uses a PVC to store its data on an Azure disk. Storage mount failures can occur when the underlying Azure disk becomes inaccessible due to accidental deletion, misconfiguration, or network issues.

The process starts with Kubernetes attempting to dynamically provision an Azure disk and bind it to PVC. However, since the disk is missing or inaccessible when the pod is scheduled on a node, the kubelet fails to attach and mount the volume, resulting in the pod being stuck in “pending” state with a FailedMount error.

You may observe the following events when this error occurs:

```

 Type  Reason    Age             From            Message

  ----  ------    ----            ----            -------

  Normal   Scheduled 2m              default-scheduler  Successfully assigned default/mysql-pod to aks-nodepool-12345678-vmss000000

  Warning  FailedMount  1m (x5 over 2m) kubelet         Unable to attach or mount volumes: unmounted volumes=[mysql-data], unattached volumes=[mysql-data kube-api-access-xyz]: timed out waiting for the condition

  Warning  FailedMount  30s (x10 over 2m)  kubelet         MountVolume.SetUp failed for volume "pvc-12345678-90ab-cdef-0123-4567890abcde" : rpc error: code = NotFound desc = Volume resource not found

What Causes This Error

This error could have multiple reasons, including:

• Volume management failures: Volume attachment controller failures in the Kubernetes control plane can affect the attachment process. The controller is responsible for attaching volumes to nodes. Failure due to scenarios such as loss of connectivity to the Kubernetes API server, controller pod crashes, or resource exhaustion in the control plane can prevent the controller from managing the VolumeAttachment objects necessary for storage provisioning and cause the volume to be unavailable for new pods.

• Volume plugin issues: Volume plugins can be implemented as CSI drivers or in-tree volume plugins. These plugins act as a bridge between the Kubernetes cluster and the underlying storage system, transforming the Kubernetes volume operations into storage-specific API calls. Any bugs or misconfiguration in these plugins can lead to volume mounting or attaching failures.

• Node resource contention: Nodes execute and manage pods, relying on system resources like CPU, memory, and disk space to execute the operations effectively. However, when a node does not have enough resources, some operations, like attaching or mounting volumes, as the kubelet may fail to execute volume operations when these resources are constrained.

Potential impacts of the error

Volume attachment and mount failures can impact application availability and cluster operations.

• Application Downtime: When pods fail to deploy as a result of volume mounting, it causes a delay in application lifecycle operations. Pod startup failures prevent application initialization, inaccessible filesystems, and cascading effect on dependent services.

• Deployment rollbacks: Due to this error, pods frequently fail to run or transition to the desired state. This could result from several reasons, including:
  • Misconfigured PVC/PV and incorrect storage class settings
  • StatefulSet controller unable to progress with ordered pod creation
  • Rolling updates blocked by volume attachment limits
• Increased operational overhead: This type of error can lead to increased operational overhead because it requires troubleshooting and investigating various components (such as kubelet, CSI driver, storage backend, etc.). Increased monitoring and alerting requirements lead to manual interventions and cross-team coordination, which adds to the overall overhead.

• Resource utilization impact: Unmounted volumes in Kubernetes cause severe waste of resources. Storage PVs are assigned even when not mounted, which might increase cloud storage expenses (AWS EBS, GCP Persistent Disks) for storage provisioned but unused space. Orphaned volume resources due to misconfigurations, scheduling issues, or delays in the finalizer also affect resource usage. These unused volumes utilize storage resources, limit node disk utilization, and hinder garbage collection. This inefficiency impacts cluster performance, increased costs, and application performance degradation in shared environments.

How to Troubleshoot and Resolve

Follow these common troubleshooting strategies to detect and fix any volume error conflicts in Kubernetes:

1. Check the pod for errors and scheduling details.

Use the kubectl describe command to check the pod’s status and view events for error messages related to volume attachment or mounting failures.

```
# Examine pod events and volume mount status

kubectl describe pod <pod-name> -n <namespace>



# Check pod conditions and container states

kubectl get pod <pod-name> -n <namespace> -o yaml | grep -A 10 conditions

```

Check the Events section for any errors or warnings.

2. Verify node status and confirm that the node where the pod is scheduled is in healthy condition:

```
# View the node status

kubectl get nodes

```

3. Inspect Kubelet’s log output for errors, warnings, or messages related to the scheduled pod.

```
# View kubelet volume operations

journalctl -u kubelet -f | grep "volume"



# Filter for specific volume operations

journalctl -u kubelet -f | grep "mount volume"

```

4. Verify PV and PVC status.

```
# Check PVC status and events

kubectl get pvc -n <namespace>

kubectl describe pvc <pvc-name> -n <namespace>



# Verify PV binding and reclaim status

kubectl get pv

kubectl describe pv <pv-name>



# Examine VolumeAttachment objects

kubectl get volumeattachment

kubectl describe volumeattachment <name>



```

5. If the volume remains stuck, you may need to remove finalizers from VolumeAttachment:

```

# Get the volumeattachment name

VOLUME_ATTACHMENT=$(kubectl get volumeattachment | grep <pv-name> | awk '{print $1}')



# Remove finalizers from the VolumeAttachment

kubectl patch volumeattachment $VOLUME_ATTACHMENT -p '{"metadata":{"finalizers":null}}' --type=merge



# Force delete stuck PVC finalizers (use with caution)

kubectl patch pvc <pvc-name> -n <namespace> \

  -p '{"metadata":{"finalizers":null}}' --type=merge

```

If you encounter PVC in a Lost or Pending status, you can either rebind it or delete it.

6. Verify the node and monitor if nodes have adequate CPU, memory, and disk space by using commands:

```

# Monitor the resource usage

kubectl top node <node-name>



```

Verify that the volume plugin and CSI driver are correctly installed and configured. Check the logs of the CSI driver for any errors:

```

# Logs of the CSI driver

kubectl logs -n kube-system -l app=csi-driver



```

7. Retry the operation, as they may be temporary or transient.

```

# Delete and recreate the pod

kubectl delete pod <pod-name>

Kubectl apply -f <pod-name>.yaml



```

While the troubleshooting steps listed above will help you resolve this error, more robust and comprehensive data management solutions, such as Portworx, can help you reduce volume attachment and mounting errors by ensuring high availability and consistency through dynamic provisioning, automated recovery and multi-node volume replication.

Portworx’s backup and restoration solution, PX-Backup and STORK, automates the recovery process and offers real-time visibility of storage health, facilitating the detection and resolution of issues such as unmounted or stuck volumes.

To dynamically provision storage using Portworx, install Portworx, then define a StorageClass by following the steps below:

```

apiVersion: storage.k8s.io/v1

kind: StorageClass

metadata:

  name: portworx-sc

provisioner: kubernetes.io/portworx-volume  # Portworx provisioner

parameters:

  repl: "3"                 # Number of replicas for high availability

  io_profile: "db_remote"   # I/O profile for the volume

  io_priority: "high"       # High I/O priority for performance

  secure: "true"            # Enable encryption (optional)

allowVolumeExpansion: true # Allow volume expansion

reclaimPolicy: Delete # Delete volume when PVC is deleted

volumeBindingMode: Immediate # Consider WaitForFirstConsumer for better placement

```

And, then update your PVC configuration to use this StorageClass.

```

apiVersion: v1

kind: PersistentVolumeClaim

metadata:

  name: px-pvc

spec:

  accessModes:

- ReadWriteOnce

  storageClassName: portworx-sc

  resources:

requests:

   storage: 10Gi

```

Using Portworx can assist in the prevention of this error by:

• Clearing up unused or orphaned volumes and resources
• Managing multiple volume replicas on different nodes to achieve high availability.
• Delivering real-time monitoring of volume health and status.
• Using STORK to build storage-aware scheduling.

How to avoid this error in the future

Volume mounting and attaching issues can be avoided in the future with the help of operational strategies, best practices, and advanced storage solutions such as Portworx. Follow the

```
kubectl delete pod <pod name> -–force --grace-period=30
```

This gives the kubelet 30 seconds to safely terminate the pod, unmount volumes, and release resources. Avoid using the –force flag unless necessary, as it may lead to incomplete resource cleanup.

• Implement Pod Disruption Budgets (PDBs): Implementing PDBs ensures that volume resources are not overwhelmed by concurrent operations, preventing issues that can arise during pod rescheduling, such as volume attachment or detachment failures. PDBs minimize the possibility of volume-related errors by ensuring that the fewest replicas (pods) are consistently available during disruptions to ensure application stability.
• Volume management storage solution: Use Portworx as your storage solution to benefit from advanced features such as distributed storage management capabilities, automated volume provisioning, and data protection capabilities while avoiding volume mounting or attaching errors.

• Optimize storage class configurations: Configuring storage classes to ensure that they are compatible with the storage infrastructure and offer the requisite performance and features associated with the applications.

• Update Kubernetes and CSI drivers: Maintaining the Kubernetes cluster and CSI drivers up to date delivers advantages such as improved performance and bug fixes.

Conclusion

The “Unable to Attach or Mount Volumes” is a common but complex issue in the Kubernetes ecosystem. This could happen for several reasons, including issues with the CSI driver, incorrect storage class configuration, or issues with cleaning up the volume, to name a few.

Implementing PDBs, optimizing storage class configurations, streamlining pod termination, monitoring storage infrastructure, and using advanced storage solutions like Portworx helps you to prevent this error from occurring and ensure the seamless operation of storage orchestration.

Regular monitoring and troubleshooting can help to make the Kubernetes environment more reliable and efficient.

Get started with Portworx today