Kubernetes Volumes

There will be always cases where we need to save data on to a drive or disk. Containers

when stopped will destroy the data stored inside the container. Kubernetes provides us a way to manage storage for containers. Kubernetes uses volumes for adding additional storage to the pods. In this article we will see how we can create volumes and attach them to the pods. For the demo we will use NFS as our additional drive.

Important elements for running a state ful containers are,

PersistentVolume - Low level representation of the storage volume

PersistentVolumeClaim - Binding between a Pod and Persistent Volume

Volume Driver - code that will be used to communicate with the back end storage

StorageClass - Allows dynamic provisioning of Persistent volumes.

I used this link for creating nfs share and access that from the worker nodes where our pods

will run.

Persistent Volume - lets create a persistent volume for our Pod.

[root@manja17-I13330 kubenetes-config]# cat nfs-pv.yml

apiVersion: v1

kind: PersistentVolume

metadata:

 name: test-nfs

spec:

 capacity:

   storage: 1Mi

 accessModes:

   - ReadWriteMany

 nfs:

   server: 10.131.175.138

   path: "/nfsshare"

In the above config, we are actually using a nfs volume from the ip address “10.131.175.138” where nfs shares where already created under the /nfsshare. Run this and see how it works

[root@manja17-I13330 kubenetes-config]# kubectl create -f nfs-pv.yml

persistentvolume "test-nfs" created

If we check the mount command from our worker nodes we can see the below output,

[root@manja17-I14021 nfsshare]# mount | grep nfsshare

10.131.175.138:/nfsshare on /mnt/nfsshare type nfs4 (rw,relatime,vers=4.1,rsize=1048576,wsize=1048576,namlen=255,

hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=10.131.36.181,

local_lock=none,addr=10.131.175.138)

We can see that the /nfsshare from 10.131.165.138 is mounted onto the /mnt/nfssshare on

the worker nodes. In the above case we are creating a persistent volume with 1MI size

PersistentVolumeClaim -

[root@manja17-I13330 kubenetes-config]# cat nfs-pvc.yml

kind: PersistentVolumeClaim

apiVersion: v1

metadata:

 name: test-nfs

spec:

 accessModes:

   - ReadWriteMany

 resources:

   requests:

     storage: 1Mi

The Persistent volume claim works in different way. In the above configuration we are asking
K8 to provide us a volume which has storage of 1MI. We are not calling the persistent
volume directly rather we are creating a persistent Volume claim and giving K8 the authority of identifying the volumes with the size that we requested. if that finds with a pv with that volume it allocates that to the pod.

Sample Pod Configuration -

[root@manja17-I13330 kubenetes-config]# cat nfs-pod.yml

apiVersion: v1

kind: ReplicationController

metadata:

 name: nfs-web

spec:

 replicas: 1

 selector:

   role: web-frontend

 template:

   metadata:

     labels:

       role: web-frontend

   spec:

     containers:

     - name: web

       image: nginx

       ports:

         - name: web

           containerPort: 80

       volumeMounts:

           # name must match the volume name below

           - name: test-nfs

             mountPath: "/usr/share/nginx/html"

     volumes:

     - name: test-nfs

       persistentVolumeClaim:

         claimName: test-nfs

In the above pod configuration, the main elements are Volumes and Volume mounts. The volume element contains the Persistent Volume claim details that we created earlier. The Volume mount is the place where if volume with our requirement is available it will mount on the location /usr/share/nginx/html inside the container.

Run the Container and see how it works -

[root@manja17-I13330 kubenetes-config]# kubectl get pods

NAME              READY      STATUS             RESTARTS AGE

mysql             1/1        Running            0 1d

nfs-web-qk2qn     1/1        Running            0 4m

Now log in to the nfs container and see if we can access the nfs drives,

[root@manja17-I13330 kubenetes-config]# kubectl exec -it nfs-web-qk2qn -- bash

root@nfs-web-qk2qn:/# cd /usr/share/nginx/html/

root@nfs-web-qk2qn:/usr/share/nginx/html# touch container.txt

root@nfs-web-qk2qn:/usr/share/nginx/html# cat container.txt

root@nfs-web-qk2qn:/usr/share/nginx/html# echo "this is from container" >> container.txt

root@nfs-web-qk2qn:/usr/share/nginx/html# cat container.txt

this is from container

root@nfs-web-qk2qn:/usr/share/nginx/html# exit

More to Come, Happy learning :-)