Dynamic provisioning in Kubernetes allows volumes to be created on demand, and you expect the system to handle volume attachments automatically. However, during the process, if a pod is scheduled on a node that is unable to access the volume due to an existing attachment, the provisioning will fail with a volume attachment error.
It occurs when a node attempts to attach to a persistent volume with ReadWriteOnce (RWO) or ReadWriteOncePod (RWOP) access mode that is already attached to another node or pod. The Container Storage Interface (CSI) driver prevents this attachment, resulting in this error.
When the volume controller attempts to create a VolumeAttachment object, it first checks with the CSI driver for existing attachments since RWO and RWOP volumes can only be mounted to a single node or pod at a time. If an exclusive attachment is detected, the controller marks the new attachment as failed, thus preventing the volume from being attached to a pod on a node.
Situations In Which This Error Commonly Appears
- Node Failure and Recovery: During unexpected node failures, Kubernetes might not have completely detached the volume from the failed node. When the node recovers or when pods are scheduled to attach to other nodes, the volume logically remains attached to the failed node, which prevents any new attachments.
- Aggressive Pod Rescheduling: When pods are scheduled or evicted due to aggressive scheduling policies, there are instances when the volumes aren’t properly detached. At the same time, Kubernetes attempts to reschedule the pod to another node, which leads to a race condition between the old and new nodes.
- Storage Provider Latency: API latency at the storage provider can also affect the volume detachment process. This conflict arises in situations when the API takes too long to process a detachment request, but Kubernetes has already initiated a new attachment request.
- Cluster Scaling Operations: A lot of operations take place simultaneously during a cluster autoscaling process. If the timing of any of these operations isn’t properly coordinated, the volume conflict error can arise.
Reproducing the Error
You can reproduce this error in a controlled environment by simulating a node failure while a volume is attached. We used a GKE cluster to reproduce the error.
Below are the steps that we followed to reproduce the error:
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- Create basic storage resources:
apiVersion: storage.k8s.io/v1 kind: StorageClass metadata: name: test-sc provisioner: pd.csi.storage.gke.io # Adjust based on your cloud provider parameters: type: pd-standard volumeBindingMode: WaitForFirstConsumer --- apiVersion: v1 kind: PersistentVolumeClaim metadata: name: test-pvc spec: accessModes: - ReadWriteOnce resources: requests: storage: 10Gi storageClassName: test-sc
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- Create a deployment using a volume
apiVersion: apps/v1 kind: Deployment metadata: name: test-deployment spec: replicas: 1 selector: matchLabels: app: test-app template: metadata: labels: app: test-app spec: containers: - name: test-container image: nginx volumeMounts: - name: test-volume mountPath: /data volumes: - name: test-volume persistentVolumeClaim: claimName: test-pvc
- With the resources already deployed, we created a shell script to simulate the events that would lead to the error.
# Get node and zone information
NODE_NAME=$(kubectl get pod -l app=test-app -o jsonpath='{.spec.nodeName}')
ZONE=$(gcloud compute instances list --filter="name:'${NODE_NAME}'" --format="value(zone)")
# Simulate node failure by stopping the instance
gcloud compute instances stop "${NODE_NAME}" --zone "${ZONE}"
# Force delete the pod
kubectl delete pod -l app=test-app --force --grace-period=0
# Create new pod
kubectl scale deployment test-deployment --replicas=0
kubectl scale deployment test-deployment --replicas=1
In the above script, we do the following:
- Simulate node failure by stopping the instance by providing the node and zone.
- The node fails unexpectedly (simulated by stopping the instance).
- Force delete the pod and then try to create a new pod.
- Kubernetes tries to reschedule the pod to another node.
- The volume can’t be attached to the new node because it’s still exclusively attached to the failed node.
In another terminal window, execute the following code
`kubectl get events --sort-by='.lastTimestamp' -w`
We will see the list of events where you can find the FailedAttachVolume – Volume Is Already Exclusively Attached To One Node And Can’t Be Attached To Another
What Causes This Error
In the previous section we looked at the various situations when the error occurs, but let us now look at some of the reasons that cause this error.
- Limitations of Storage Architecture: Modern cloud storage solution providers like AWS EBS, Azure Disks, etc., implement exclusive write access at the infrastructure level allowing only one node to write to a volume at a time. Coupled with Kubernetes’ RWO policy, this can cause node failures leaving volumes in an attached state preventing other nodes from accessing it.
- Control Plane Race Conditions: The volume controller operates asynchronously with the actual storage provider operations. This architectural pattern leads to situations where the Kubernetes control plane’s volume attachment becomes inconsistent with the actual storage provider.
- Node Communication Issues: In situations where the nodes cannot communicate their volume attachment status with the Kubernetes control plane, the volume controller might take decisions based on the incorrect or stale information that can lead to this error.
Potential Impact
A volume attachment error in a Kubernetes cluster can affect multiple aspects of your application and infrastructure. Understanding these impacts is crucial for assessing the severity of the situation.
- Application Availability:
- Extended downtime of applications, as pods will be in a pending state for longer.
- Data access interruptions for stateful applications
- Data inconsistency issues of dependent services
- Operational Impact
- Increased manual interventions
- Extended debugging sessions
- Downtime can affect the agreed upon SLAs
- Cluster Resource Management
- Increased load on the control plane for repeated volume attachments
- Storage quota consumption from orphaned volumes
- Potential impact on cluster autoscaling operations
How Troubleshoot And Resolve
When you encounter this error, you need to have a systematic approach to troubleshoot and resolve the issue. Below are a few steps that you can follow to troubleshoot and resolve the issue.
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- First and foremost, find the issue, check the pod and volume status.
# Check pod status and events kubectl describe pod <pod-name> # Check PVC status kubectl get pvc kubectl describe pvc <pvc-name> # Check volume attachments kubectl get volumeattachment
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- Next, verify the nodes and volume stats.
# Check node status kubectl get nodes kubectl describe node <node-name> # Verify instance status gcloud compute instances list --filter="name=<node-name>"
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- If the volume is still stuck , you can try removing finalizers from the VolumeAttachment
# Get the volumeattachment name
VOLUME_ATTACHMENT=$(kubectl get volumeattachment | grep <pv-name> | awk '{print $1}')
# Remove finalizer
kubectl patch volumeattachment $VOLUME_ATTACHMENT -p '{"metadata":{"finalizers":null}}' --type=merge
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- You might even have to force delete the VolumeAttachment
kubectl delete volumeattachment $VOLUME_ATTACHMENT --force --grace-period=0
- If the above steps didn’t work, you might need to detach the volume at the cloud provider level
# For GCP gcloud compute instances detach-disk <node-name> --disk <disk-name> --zone <zone>
While the above manual troubleshooting steps can help resolve this volume attachment issue, a more robust solution is to use Portworx with STORK to manage storage for Kubernetes environments . Along with STORK, Portworx can help prevent this error by managing the volume attachments synchronously across multiple nodes and implementing failover mechanisms preventing attachment errors.
You can install Portworx on your cluster and create a Portworx.StorageClass in a way described below:
apiVersion: storage.k8s.io/v1 kind: StorageClass metadata: name: portworx-sc provisioner: pxd.portworx.com parameters: io_profile: db_remote repl: "3" io_priority: "high" allowVolumeExpansion: true reclaimPolicy: Delete volumeBindingMode: Immediate
And then you can update your PVC to use this StorageClass.
apiVersion: v1 kind: PersistentVolumeClaim metadata: name: px-pvc spec: accessModes: - ReadWriteOnce storageClassName: portworx-sc resources: requests: storage: 10Gi
Using Portworx helps prevent this error by
- Handling node failures gracefully
- Maintaining volume replicas across multiple nodes
- STORK helps seamless migration of pods in the event of failures and automatically handles volume attachments and detachments
How To Avoid
To avoid this error, you need a combination of good infrastructure management and operational practices, along with the right tooling:
- Use Pod Disruption Budgets (PDBs) to control rescheduling and termination of pods.
- Use pod anti-affinity rules to distribute stateful workloads across nodes.
- Configure appropriate terminationGracePeriods to allow enough time for proper cleanup.
- Implement proper node draining rules to ensure volumes are cleanly detached during maintenance and shutdowns.
- Lastly, you can implement Portworx as your storage solution to leverage its benefits, including distributed storage management capabilities.
The “volume is already exclusively attached to one node and can’t be attached to another” error can be a frustrating issue. Understanding its causes, identifying warning signs, and implementing correct measures can help reduce the likelihood of the error.
Whether you choose a solution like Portworx or plan to handle manually, you must follow the best practices and a clear troubleshooting strategy to prevent this error and ensure your applications are resilient and highly available.