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Kubernetes Persistent Volumes and Claimes

PersistentVolume

Create a directory called /mnt/data on the k8s node. Within that directory, create index.html file with some text.

pradeep@learnk8s$ minikube ssh -p k8s
                         _             _
            _         _ ( )           ( )
  ___ ___  (_)  ___  (_)| |/')  _   _ | |_      __
/' _ ` _ `\| |/' _ `\| || , <  ( ) ( )| '_`\  /'__`\
| ( ) ( ) || || ( ) || || |\`\ | (_) || |_) )(  ___/
(_) (_) (_)(_)(_) (_)(_)(_) (_)`\___/'(_,__/'`\____)

$ sudo mkdir /mnt/data
$ sudo sh -c "echo 'Hello from Kubernetes storage' > /mnt/data/indext.html"
$ cat /mnt/data/indext.html
Hello from Kubernetes storage
$ exit
logout

Now create a PersistentVolume named my-pv-volume of 1Gi using the hostPath pointing to the newly created directory. There are three types of accessModes, for this example use, RWO.

RWO ReadWriteOnce
RWX ReadWriteMany
ROX ReadyOnlyMany 
pradeep@learnk8s$ cat persistent-volume-demo.yaml
apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv-volume
  labels:
    type: local
spec:
  storageClassName: manual
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/mnt/data"

Create the PV using this YAML file.

pradeep@learnk8s$ kubectl create -f persistent-volume-demo.yaml
persistentvolume/my-pv-volume created

Verify the available PVs.

pradeep@learnk8s$ kubectl get pv
NAME           CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS   REASON   AGE
my-pv-volume   1Gi        RWO            Retain           Available           manual                  4s

Describe the PersistentVolume for addtional details.

pradeep@learnk8s$ kubectl describe pv
Name:            my-pv-volume
Labels:          type=local
Annotations:     <none>
Finalizers:      [kubernetes.io/pv-protection]
StorageClass:    manual
Status:          Available
Claim:
Reclaim Policy:  Retain
Access Modes:    RWO
VolumeMode:      Filesystem
Capacity:        1Gi
Node Affinity:   <none>
Message:
Source:
    Type:          HostPath (bare host directory volume)
    Path:          /mnt/data
    HostPathType:
Events:            <none>

:top:

PersistentVolumeClaim

pradeep@learnk8s$ cat persistent-volume-claim-demo.yaml
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 300Mi

pradeep@learnk8s$ kubectl create -f persistent-volume-claim-demo.yaml
persistentvolumeclaim/my-pv-claim created

pradeep@learnk8s$ kubectl get pvc
NAME          STATUS   VOLUME         CAPACITY   ACCESS MODES   STORAGECLASS   AGE
my-pv-claim   Bound    my-pv-volume   1Gi        RWO            manual         40s

pradeep@learnk8s$ kubectl describe pvc
Name:          my-pv-claim
Namespace:     default
StorageClass:  manual
Status:        Bound
Volume:        my-pv-volume
Labels:        <none>
Annotations:   pv.kubernetes.io/bind-completed: yes
               pv.kubernetes.io/bound-by-controller: yes
Finalizers:    [kubernetes.io/pvc-protection]
Capacity:      1Gi
Access Modes:  RWO
VolumeMode:    Filesystem
Used By:       <none>
Events:        <none>

pradeep@learnk8s$ kubectl get pv
NAME           CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                 STORAGECLASS   REASON   AGE
my-pv-volume   1Gi        RWO            Retain           Bound    default/my-pv-claim   manual                  12m

pradeep@learnk8s$ kubectl describe pv
Name:            my-pv-volume
Labels:          type=local
Annotations:     pv.kubernetes.io/bound-by-controller: yes
Finalizers:      [kubernetes.io/pv-protection]
StorageClass:    manual
Status:          Bound
Claim:           default/my-pv-claim
Reclaim Policy:  Retain
Access Modes:    RWO
VolumeMode:      Filesystem
Capacity:        1Gi
Node Affinity:   <none>
Message:
Source:
    Type:          HostPath (bare host directory volume)
    Path:          /mnt/data
    HostPathType:
Events:            <none>

The next step is to create a Pod that uses this PersistentVolumeClaim as a volume.

Here is the configuration file for the Pod:

pradeep@learnk8s$ cat my-pv-pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: my-pv-pod
spec:
  volumes:
    - name: my-pv-storage
      persistentVolumeClaim:
        claimName: my-pv-claim
  containers:
    - name: my-pv-container
      image: nginx
      ports:
        - containerPort: 80
          name: "http-server"
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: my-pv-storage

pradeep@learnk8s$ kubectl create -f my-pv-pod.yaml
pod/my-pv-pod created

The scheduler has placed this Pod on k8s-m02 node.

pradeep@learnk8s$ kubectl get pods my-pv-pod -o wide
NAME        READY   STATUS    RESTARTS   AGE   IP           NODE      NOMINATED NODE   READINESS GATES
my-pv-pod   1/1     Running   0          10s   10.244.1.2   k8s-m02   <none>           <none>

Let us describe the pod and look at the Volumes section.

pradeep@learnk8s$ kubectl describe pods my-pv-pod
Name:         my-pv-pod
Namespace:    default
Priority:     0
Node:         k8s-m02/192.168.177.30
Start Time:   Sun, 27 Feb 2022 22:35:45 +0530
Labels:       <none>
Annotations:  <none>
Status:       Running
IP:           10.244.1.2
IPs:
  IP:  10.244.1.2
Containers:
  my-pv-container:
    Container ID:   docker://9dc92c5c1037ad4d534fe6440275f62a68f521e3bd22fe47b57c06a8b3414e24
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:2834dc507516af02784808c5f48b7cbe38b8ed5d0f4837f16e78d00deb7e7767
    Port:           80/TCP
    Host Port:      0/TCP
    State:          Running
      Started:      Sun, 27 Feb 2022 22:35:49 +0530
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /usr/share/nginx/html from my-pv-storage (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-xk6kh (ro)
Conditions:
  Type              Status
  Initialized       True
  Ready             True
  ContainersReady   True
  PodScheduled      True
Volumes:
  my-pv-storage:
    Type:       PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:  my-pv-claim
    ReadOnly:   false
  kube-api-access-xk6kh:
    Type:                    Projected (a volume that contains injected data from multiple sources)
    TokenExpirationSeconds:  3607
    ConfigMapName:           kube-root-ca.crt
    ConfigMapOptional:       <nil>
    DownwardAPI:             true
QoS Class:                   BestEffort
Node-Selectors:              <none>
Tolerations:                 node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                             node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  19s   default-scheduler  Successfully assigned default/my-pv-pod to k8s-m02
  Normal  Pulling    18s   kubelet            Pulling image "nginx"
  Normal  Pulled     15s   kubelet            Successfully pulled image "nginx" in 2.858388422s
  Normal  Created    15s   kubelet            Created container my-pv-container
  Normal  Started    15s   kubelet            Started container my-pv-container

It looks like no issues, but let us verify the nginx application by logging into the pod and issuing the curl localhost command.

pradeep@learnk8s$ kubectl exec -it my-pv-pod -- /bin/sh
# curl localhost
<html>
<head><title>403 Forbidden</title></head>
<body>
<center><h1>403 Forbidden</h1></center>
<hr><center>nginx/1.21.6</center>
</body>
</html>
# exit

Hmm! This seems to be not working. It is forbidden. But this is expected right, we did not have the volume (hostPath) on this k8s-m02 node. We created it on k8s node only.

To solve this, we can add a nodeName spec to the Pod definition. Delete this pod and after adding the nodeName recreate the Pod.

pradeep@learnk8s$ kubectl delete pod my-pv-pod
pod "my-pv-pod" deleted

pradeep@learnk8s$ vi my-pv-pod.yaml

pradeep@learnk8s$ cat my-pv-pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: my-pv-pod
spec:
  nodeName: k8s
  volumes:
    - name: my-pv-storage
      persistentVolumeClaim:
        claimName: my-pv-claim
  containers:
    - name: my-pv-container
      image: nginx
      ports:
        - containerPort: 80
          name: "http-server"
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: my-pv-storage


pradeep@learnk8s$ kubectl create -f my-pv-pod.yaml
pod/my-pv-pod created

:warning: There seems to be some issue with this. Even after scheduling this pod on the node k8s, nginx container is reporting Forbidden. There are couple of issues already reported, but could not find any resolution to this problem yet.

https://github.com/kubernetes/website/issues/9523

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