Kubernetes 1.23: Prevent PersistentVolume leaks when deleting out of order
Author: Deepak Kinni (VMware)
PersistentVolume (or PVs for short) are
associated with Reclaim Policy.
The Reclaim Policy is used to determine the actions that need to be taken by the storage
backend on deletion of the PV.
Where the reclaim policy is
Delete, the expectation is that the storage backend
releases the storage resource that was allocated for the PV. In essence, the reclaim
policy needs to honored on PV deletion.
With the recent Kubernetes v1.23 release, an alpha feature lets you configure your cluster to behave that way and honor the configured reclaim policy.
How did reclaim work in previous Kubernetes releases?
PersistentVolumeClaim (or PVC for short) is a request for storage by a user. A PV and PVC are considered Bound if there is a newly created PV or a matching PV is found. The PVs themselves are backed by a volume allocated by the storage backend.
Normally, if the volume is to be deleted, then the expectation is to delete the PVC for a bound PV-PVC pair. However, there are no restrictions to delete a PV prior to deleting a PVC.
First, I'll demonstrate the behavior for clusters that are running an older version of Kubernetes.
Retrieve an PVC that is bound to a PV
Retrieve an existing PVC
kubectl get pvc example-vanilla-block-pvc
The following output shows the PVC and it's
Bound PV, the PV is shown under the
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE example-vanilla-block-pvc Bound pvc-6791fdd4-5fad-438e-a7fb-16410363e3da 5Gi RWO example-vanilla-block-sc 19s
When I try to delete a bound PV, the cluster blocks and the
kubectl tool does
not return back control to the shell; for example:
kubectl delete pv pvc-6791fdd4-5fad-438e-a7fb-16410363e3da
persistentvolume "pvc-6791fdd4-5fad-438e-a7fb-16410363e3da" deleted ^C
Retrieving the PV:
kubectl get pv pvc-6791fdd4-5fad-438e-a7fb-16410363e3da
It can be observed that the PV is in
NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE pvc-6791fdd4-5fad-438e-a7fb-16410363e3da 5Gi RWO Delete Terminating default/example-vanilla-block-pvc example-vanilla-block-sc 2m23s
kubectl delete pvc example-vanilla-block-pvc
The following output is seen if the PVC gets successfully deleted:
persistentvolumeclaim "example-vanilla-block-pvc" deleted
The PV object from the cluster also gets deleted. When attempting to retrieve the PV it will be observed that the PV is no longer found:
kubectl get pv pvc-6791fdd4-5fad-438e-a7fb-16410363e3da
Error from server (NotFound): persistentvolumes "pvc-6791fdd4-5fad-438e-a7fb-16410363e3da" not found
Although the PV is deleted the underlying storage resource is not deleted, and needs to be removed manually.
To sum it up, the reclaim policy associated with the Persistent Volume is currently
ignored under certain circumstance. For a
Bound PV-PVC pair the ordering of PV-PVC
deletion determines whether the PV reclaim policy is honored. The reclaim policy
is honored if the PVC is deleted first, however, if the PV is deleted prior to
deleting the PVC then the reclaim policy is not exercised. As a result of this behavior,
the associated storage asset in the external infrastructure is not removed.
PV reclaim policy with Kubernetes v1.23
The new behavior ensures that the underlying storage object is deleted from the backend when users attempt to delete a PV manually.
How to enable new behavior?
To make use of the new behavior, you must have upgraded your cluster to the v1.23 release of Kubernetes.
You need to make sure that you are running the CSI
4.0.0, or later.
You must also enable the
HonorPVReclaimPolicy feature gate for the
external-provisioner and for the
If you're not using a CSI driver to integrate with your storage backend, the fix isn't available. The Kubernetes project doesn't have a current plan to fix the bug for in-tree storage drivers: the future of those in-tree drivers is deprecation and migration to CSI.
How does it work?
The new behavior is achieved by adding a finalizer
external-provisioner.volume.kubernetes.io/finalizer on new and existing PVs, the finalizer is only removed after the storage from backend is deleted.
An example of a PV with the finalizer, notice the new finalizer in the finalizers list
kubectl get pv pvc-a7b7e3ba-f837-45ba-b243-dec7d8aaed53 -o yaml
apiVersion: v1 kind: PersistentVolume metadata: annotations: pv.kubernetes.io/provisioned-by: csi.vsphere.vmware.com creationTimestamp: "2021-11-17T19:28:56Z" finalizers: - kubernetes.io/pv-protection - external-provisioner.volume.kubernetes.io/finalizer name: pvc-a7b7e3ba-f837-45ba-b243-dec7d8aaed53 resourceVersion: "194711" uid: 087f14f2-4157-4e95-8a70-8294b039d30e spec: accessModes: - ReadWriteOnce capacity: storage: 1Gi claimRef: apiVersion: v1 kind: PersistentVolumeClaim name: example-vanilla-block-pvc namespace: default resourceVersion: "194677" uid: a7b7e3ba-f837-45ba-b243-dec7d8aaed53 csi: driver: csi.vsphere.vmware.com fsType: ext4 volumeAttributes: storage.kubernetes.io/csiProvisionerIdentity: 1637110610497-8081-csi.vsphere.vmware.com type: vSphere CNS Block Volume volumeHandle: 2dacf297-803f-4ccc-afc7-3d3c3f02051e persistentVolumeReclaimPolicy: Delete storageClassName: example-vanilla-block-sc volumeMode: Filesystem status: phase: Bound
The presence of the finalizer prevents the PV object from being removed from the cluster. As stated previously, the finalizer is only removed from the PV object after it is successfully deleted from the storage backend. To learn more about finalizers, please refer to Using Finalizers to Control Deletion.
What about CSI migrated volumes?
The fix is applicable to CSI migrated volumes as well. However, when the feature
HonorPVReclaimPolicy is enabled on 1.23, and CSI Migration is disabled, the finalizer
is removed from the PV object if it exists.
- The fix is applicable only to CSI volumes and migrated volumes. In-tree volumes will exhibit older behavior.
- The fix is introduced as an alpha feature in the external-provisioner under the feature gate
HonorPVReclaimPolicy. The feature is disabled by default, and needs to be enabled explicitly.
How do I get involved?
The Kubernetes Slack channel SIG Storage communication channels are great mediums to reach out to the SIG Storage and migration working group teams.
Special thanks to the following people for the insightful reviews, thorough consideration and valuable contribution:
- Jan Šafránek (jsafrane)
- Xing Yang (xing-yang)
- Matthew Wong (wongma7)
Those interested in getting involved with the design and development of CSI or any part of the Kubernetes Storage system, join the Kubernetes Storage Special Interest Group (SIG). We’re rapidly growing and always welcome new contributors.