Kubernetes Persistent Volumes

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Kubernetes Persistent Volumes

Persistent Volumes in Kubernetes allow the operators to store data outside the Pods that will be persistent across updates or reboots. It also allows these Persistent Volumes to be mounted on multiple pods.

The lifecycle of a volume and claim and the communication between PVs and PVCs  consists of four stages:

• Provisioning: The process of creating a storage volume, provisioning can be done in two ways: statically or dynamically. Leaseweb Managed Kubernetes chose Dynamic Provisioning which you can find more detail in the Dynamic Provisioning section.
• Binding: When a PersistentVolumeClaim with a specific amount of storage is requested and with certain access modes, a control loop in the control plane finds a matching PV and binds it together.
• Using: Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a Pod
• Reclaiming: When a user is done with the volume, they can delete the PVC objects from the API that allows the reclamation of the resource. To tell what to do with the released PersistentVolumes it is important what to set for the Reclaim Policy. There are three reclaim policies:
  • Retain: Manual deletion by the cluster administrator. The volume will not be automatically deleted.
  • Delete: The volume will be automatically deleted when the corresponding PersistentVolumeClaim is deleted.
  • Recycle: Deprecated. Instead, the recommended approach is to use dynamic provisioning which is available in our clusters. (Not Supported in our environment)

See the official Kubernetes documentation on Persistent Volumes for more information. 

Dynamic Provisioning

The dynamic way of storage provisioning involves creating PVs automatically, using StorageClass instead of the manual provisioning of PersistentVolumes. Kubernetes will dynamically provision a volume specifically for this storage request, with the help of a StorageClass. In the LeaseWeb Managed Kubernetes, we chose this approach and you can find "cloudstack-custom" as the default StorageClass for now.

StorageClass

A StorageClass is a Kubernetes resource that enables dynamic storage provisioning. StorageClass provides a way for administrators to describe the "classes" of storage they offer. Each StorageClass contains the fields "provisioned", "parameters", and "reclaimPolicy", which are used when a PersistentVolume belonging to the class needs to be dynamically provisioned.

In Leaseweb Manage Kubernetes, StorageClass "cloudstack-custom" is the one you can choose which is 

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: cloudstack-custom
parameters:
  csi.cloudstack.apache.org/disk-offering-id: <your domain offering id>
provisioner: csi.cloudstack.apache.org
volumeBindingMode: WaitForFirstConsumer
reclaimPolicy: Delete

Using Kubernetes Persistent Volumes

1. In order to use Persistent Volumes, you must declare a PersistentVolumeClaim in order to request some space.

   apiVersion: v1
   kind: PersistentVolumeClaim
   metadata:
     name: example-pvc
   spec:
     storageClassName: cloudstack-custom
     accessModes:
       - ReadWriteOnce
     resources:
       requests:
         storage: 1Gi

   The only part which is special here as it was mentioned in previous section is "cloudstack-custom" which is a pre-defined storage class declared by Leaseweb when we have provisioned your cluster.
   The other interesting part is the spec.resources.requests.storage which requests exactly 1 Gi for the volume, which allows choosing the specific size you desire for that volume.
   
   

2. Then, to use this PersistentVolume in a container, you can use the PersistentVolume's name to specify in your Pod, such as : 

   apiVersion: v1
   kind: Pod
   metadata:
     name: example-pod
   spec:
     containers:
       - name: example
         image: busybox
         volumeMounts:
           - mountPath: "/data"
             name: example-volume
         stdin: true
         stdinOnce: true
         tty: true
     volumes:
       - name: example-volume
         persistentVolumeClaim:
           claimName: example-pvc

3. Once you have applied both of these configurations using kubectl apply -f, a new pod will be added, and the volume will be mounted under /data.

Persistent Volume Deletion

During Cluster Destruction

When a cluster is being destroyed, all attached PVCs (Persistent Volume Claims) are intended to be deleted along with the corresponding volumes. However, this deletion does not occur in the following two scenarios:

• When Static pods with volumes are deleted without concurrently deleting the related PVC.
• When scaling a Deployment or StatefulSet to zero.

In both cases mentioned above, it is crucial to remove all dangling PVCs or those associated with the Deployment and StatefulSet that have been scaled down before initiating cluster destruction. Neglecting to do this can lead to the persistence of leaked volumes within the infrastructure, consequently incurring additional charges.

Manual PersistentVolume Deletion

To delete a volume in a Kubernetes cluster, you typically delete the Persistent Volume Claim (PVC) that is using the volume. When you delete a PVC, the associated Persistent Volume (PV) and the actual storage resource will be deleted as our reclaimPolicy set to Delete in the StorageClass of the PV. 

# Listing PV and PVC
❯ kubectl get pvc                                                                                                                            
NAME                                                                                                   STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS        AGE
example-pvc                                                                                             Bound    pvc-fad0126a-811b-4815-823c-7758fe25fd33   1Gi        RWO            cloudstack-custom   8m
❯ kubectl get pv
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                                                                                                             STORAGECLASS        REASON   AGE
pvc-fad0126a-811b-4815-823c-7758fe25fd33   1Gi        RWO            Delete           Bound    monitoring/example-pvc                                                                                              cloudstack-custom            8m

# Deleting PVC
❯ kubectl delete pvc example-pvc                                                                                                                
persistentvolumeclaim "example-pvc" deleted
# As you can see it is completely removed from cluster
❯ kubectl get pv pvc-fad0126a-811b-4815-823c-7758fe25fd33                                                                                                 
Error from server (NotFound): persistentvolumes "pvc-fad0126a-811b-4815-823c-7758fe25fd33" not found

Relevant documentation about Kubernetes Persistent Volumes