[Kubernetes] Volumes

Volumes provide storage spaces for containers.

Concepts

Volumes

• specify the details of where and how data can be stored
• defined in the Pod spec section.

VolumeMounts

• map (attache) the volume to a specific container with a path
• defined in the container spec section

Volume Types

hostPath

• Data is stored directly in a specific location on the host file system.
• The path is on the node where the Pod is running.
• Not a good choice in the multi-node cluster.
• hostPath volume types:
  • Directory
  • DirectoryOrCreate
  • File
  • FileOrCreate

emptyDir

• Data is stored on the host file system only while the Pod is running.
• It is the temporary storage and will be deleted when the Pod is deleted.

persistentVolumeClaim

• Data is stored using a PersistentVolume.

Tutorial – Using hostPath

• In all nodes, create a path and a file

sudo mkdir /etc/mydir

echo "This is node {your node name}!" | sudo tee /etc/mydir/data.txt

• Create a pod yaml file

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: host-path-test
  name: host-path-test
spec:
  containers:
  - image: busybox
    name: host-path-test
    command: ['sh', '-c', 'cat /mydata/data.txt']
    volumeMounts:
    - name: host-data
      mountPath: /mydata
  volumes:
  - name: host-data
    hostPath:
      path: /etc/mydir
      type: DirectoryOrCreate
  restartPolicy: Never

• Create a pod and check the logs

kubectl apply -f pod.yaml

kubectl logs host-path-test

Tutorial – Using emptyDir

• Create a pod yaml file

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: empty-dir-test
  name: empty-dir-test
spec:
  containers:
  - image: busybox
    name: empty-dir-test
    command: ['sh', '-c', 'echo "Hello, empty dir!" > /data/data.txt; cat /data/data.txt']
    volumeMounts:
    - name: empty-dir-vol
      mountPath: /data
  volumes:
  - name: empty-dir-vol
    emptyDir: {}
  restartPolicy: Never

• Create a pod and check the logs

kubectl apply -f pod.yaml

kubectl logs empty-dir-test

PersistentVolumes

Using PersistentVolumes, you can decouple the volume details from Pods and treat storage like consumable resources.

PersistentVolume (PV)

• PV defines details of storage – type, capacity, and access mode.

• PV is an abstract component, and the actual physical storage must come from somewhere.
  • csi: Container Storage Interface (CSI) (for example, Amazon EFS, Amazon EBS, Amazon FSx, etc.)
    • The CSI is an abstraction designed to facilitate using different storage solutions with Kubernetes.
    • AWS has CSI plugins for Amazon EBS, Amazon EFS, and Amazon FSx for Lustre.
  • iscsi: iSCSI (SCSI over IP) storage
  • local: Local storage devices mounted on nodes
  • nfs: Network File System (NFS) storage

PV Access mode

• ReadOnlyMany
  • allows read-only mode by many nodes at the same time
• ReadWriteOnce
  • allows read/write by only one node at the same time
• ReadWriteMany
  • allows read/write by multiple nodes at the same time

Reclaim Policy

You can set the “persistentVolumeReclaimPolicy” for the PV.

• Retain (Default)
  • manual reclamation
• Recycle
  • basic scrub (rm -rf /thevolume/*)
• Delete
  • associated storage volume is deleted

PersistentVolumeClaim (PVC)

• a request for the type of storage needed
• PVC automatically binds to an available PV that meets the requirements.
• You can mount a PVC to a container just like a regular volume.

Tutorial – using PV and PVC

• Create a yaml file for a PV

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 20Mi
  accessModes:
  - ReadWriteOnce
  storageClassName: small-pv
  hostPath:
    path: /etc/mydir
    type: DirectoryOrCreate

• Create a PV and check it

kubectl apply -f pv.yaml

kubectl get pv

• Create a yaml file for a PVC

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-pvc
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 20Mi
  storageClassName: small-pv

• Create a PVC and check it

kubectl apply -f pvc.yaml

kubectl get pvc

You can see the claim is automatically bound.

• Create a pod yaml file

apiVersion: v1
kind: Pod
metadata:
  name: pv-pvc-test
spec:
  containers:
  - image: busybox
    name: empty-dir-test
    command: ['sh', '-c', 'echo "Hello, Persisent Volume!" > /data/data.txt; cat /data/data.txt']
    volumeMounts:
    - name: pv-host-path
      mountPath: /data
  volumes:
  - name: pv-host-path
    persistentVolumeClaim:
      claimName: my-pvc
  restartPolicy: Never

Check the pod and logs

kubectl apply -f pod.yaml

kubectl get pods

kubectl logs pv-pvc-test