In this article, I want to show how Kubernetes cluster can use 
an external NFS server for storage.

The code for this article is here

https://github.com/upinder-sujlana/K8S-Volumes/blob/master/README.md

Topology
--------------
kmaster  - 192.168.1.80
knode1   - 192.168.1.81
knode2   - 192.168.1.82

The three form a K8S cluster:-
kmaster@kmaster:~$ kubectl get nodes
NAME      STATUS   ROLES    AGE    VERSION
kmaster   Ready    master   233d   v1.14.2
knode1    Ready    <none>   233d   v1.14.2
knode2    Ready    <none>   233d   v1.14.2
kmaster@kmaster:~$

The 3-nodes are OS details are the same:-
kmaster@kmaster:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04 LTS
Release:        18.04
Codename:       bionic
kmaster@kmaster:~$


Additionally, I have a 4th node outside the cluster, but in the same
LAN that I am using as a NFS Server :-
minikube - 192.168.1.85 (NFS Server running here)

On the NFS Server, I have exposed three directories to the above
cluster (permit all) directory names are gold, silver, bronze.

Time to create a Persistent volume.

kmaster@kmaster:~/dockerimagemaker/NFS$ cat nfs-pv.yml
apiVersion: v1
kind: PersistentVolume
metadata:
  # any PV name
  name: nfs-pv
  labels:
    volume: nfs-pv-volume
spec:
  capacity:
    # storage size
    storage: 5Gi
  accessModes:
    # ReadWriteMany(RW from multi nodes), ReadWriteOnce(RW from a node), ReadOnlyMany(R from multi nodes)
    - ReadWriteMany
  persistentVolumeReclaimPolicy:
    # retain even if pods terminate
    Retain
  nfs:
    # NFS server's definition
    path: /home/minikube/NFSShare/gold
    server: 192.168.1.85
    readOnly: false
kmaster@kmaster:~/dockerimagemaker/NFS$


kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl create -f nfs-pv.yml
persistentvolume/nfs-pv created
kmaster@kmaster:~/dockerimagemaker/NFS$

kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl get pv --show-labels -o wide
NAME     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS   REASON   AGE    LABELS
nfs-pv   5Gi        RWX            Retain           Available                                   100s   volume=nfs-pv-volume
kmaster@kmaster:~/dockerimagemaker/NFS$


kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl describe pv nfs-pv
Name:            nfs-pv
Labels:          volume=nfs-pv-volume
Annotations:     <none>
Finalizers:      [kubernetes.io/pv-protection]
StorageClass:
Status:          Available
Claim:
Reclaim Policy:  Retain
Access Modes:    RWX
VolumeMode:      Filesystem
Capacity:        5Gi
Node Affinity:   <none>
Message:
Source:
    Type:      NFS (an NFS mount that lasts the lifetime of a pod)
    Server:    192.168.1.85
    Path:      /home/minikube/NFSShare/gold
    ReadOnly:  false
Events:        <none>
kmaster@kmaster:~/dockerimagemaker/NFS$


Creating a persistent volume claim.

kmaster@kmaster:~/dockerimagemaker/NFS$ cat nfs-pvc.yml
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  # any PVC name
  name: nfs-pvc
spec:
  selector:
    matchLabels:
      volume: nfs-pv-volume
  accessModes:
  # ReadWriteMany(RW from multi nodes), ReadWriteOnce(RW from a node), ReadOnlyMany(R from multi nodes)
  - ReadWriteMany
  resources:
     requests:
       # storage size to use
       storage: 1Gi
kmaster@kmaster:~/dockerimagemaker/NFS$

kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl create -f nfs-pvc.yml
persistentvolumeclaim/nfs-pvc created
kmaster@kmaster:~/dockerimagemaker/NFS$

kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl get pvc --show-labels
NAME      STATUS   VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS   AGE   LABELS
nfs-pvc   Bound    nfs-pv   5Gi        RWX                           32s   <none>
kmaster@kmaster:~/dockerimagemaker/NFS$

kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl describe pvc nfs-pvc
Name:          nfs-pvc
Namespace:     default
StorageClass:
Status:        Bound
Volume:        nfs-pv
Labels:        <none>
Annotations:   pv.kubernetes.io/bind-completed: yes
               pv.kubernetes.io/bound-by-controller: yes
Finalizers:    [kubernetes.io/pvc-protection]
Capacity:      5Gi
Access Modes:  RWX
VolumeMode:    Filesystem
Events:        <none>
Mounted By:    <none>
kmaster@kmaster:~/dockerimagemaker/NFS$

Time to test, going to create a test deployment (busybox) and
see if it will work. The Pod shall mount the gold directory to
its /tmp folder and I shall just send output of date command to the folder.



kmaster@kmaster:~/dockerimagemaker/NFS$ cat nfstester.yml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nfstester
  labels:
    type: nfstester
spec:
  replicas: 1
  selector:
    matchLabels:
      type: nfstester
  strategy:
    type: Recreate
  template:
    metadata:
      labels:
        type: nfstester
    spec:
      volumes:
        - name: nfstester
          persistentVolumeClaim:
            claimName: nfs-pvc
      containers:
      - name: nfstester
        image: busybox
        command: [ 'sh', '-c', 'while true; do date;sleep 10; done >> /tmp/hellopod.txt']
        volumeMounts:
          - name: nfstester
            mountPath: /tmp
kmaster@kmaster:~/dockerimagemaker/NFS$

All this test pod does is every 10 sec wakes up and dumps the "date" to the mounted NFS share.


kmaster@kmaster:~$ kubectl create -f nfstester.yml
deployment.apps/nfstester created
kmaster@kmaster:~$


Went to the NFS server directory and started the tail on the newly created file:
minikube@ubuntu:~/NFSShare/gold$ tail -f hellopod.txt
Wed Jan 15 20:50:56 UTC 2020
Wed Jan 15 20:51:06 UTC 2020
Wed Jan 15 20:51:16 UTC 2020
Wed Jan 15 20:51:26 UTC 2020


kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl get pvc
NAME      STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS         AGE
nfs-pvc   Bound     nfs-pv   5Gi        RWX            manualstorageclass   24d
kmaster@kmaster:~/dockerimagemaker/NFS$
kmaster@kmaster:~/dockerimagemaker/NFS$ kubectl get pv
NAME     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM             STORAGECLASS         REASON   AGE
nfs-pv   5Gi        RWX            Retain           Bound    default/nfs-pvc   manualstorageclass            24d
kmaster@kmaster:~/dockerimagemaker/NFS$