Use a local disk through Local Persistent Volumes in Kubernetes

2020-01-19 Kubernetes DevOps lapee79

The Local Persistent Volumes feature has been promoted to GA in Kubernetes 1.14. A local persistent volume represents a local disk directly-attached to a single Kubernetes Node.

In the Kubernetes system, local disks can be used through HostPath, LocalVolume.

HostPath: The volume itself does not contain scheduling information. If you want to fix each pod on a node, you need to configure scheduling information, such as nodeSelector, for the pod.
LocalVolume: The volume itself contains scheduling information, and the pods using this volume will be fixed on a specific node, which can ensure data continuity.

Local Persistent Volumes allow you to access local disk by using the standard PVC object.

Create StorageClass with WaitForFirstConsumer Binding Mode

First, a StorageClass should be created that sets volumeBindingMode: WaitForFirstConsumer to enable volume topology-aware scheduling. This mode instructs Kubernetes to wait to bind a PVC until a Pod using it is scheduled.

cat << EOF | kubectl apply -f -
kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer
EOF

Prepare volume on the host

# Execute these commands on the worker node where the POD will be located
mkdir -p /data/volumes/pv1
chmode 777 /data/volumes/pv1

Create Local PersistentVolume

Create PersistentVolume with a reference to local-storage StorageClass

cat << EOF | kubectl apply -f -
apiVersion: v1
kind: PersistentVolume
metadata:
  name: test-local-pv
spec:
  capacity:
    storage: 10Gi
  accessModes:
  - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  storageClassName: local-storage
  local:
    path: /data/volumes/pv1
  nodeAffinity:
    required:
      nodeSelectorTerms:
      - matchExpressions:
        - key: kubernetes.io/hostname
          operator: In
          values:
          - k8s-worker-01
EOF
persistentvolume/test-local-pv created

kubectl get pv
NAME            CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS    REASON   AGE
test-local-pv   10Gi       RWO            Retain           Available           local-storage            17s

Create a PersistentVolumeClaim

Create PersistentVolumeClaim with a reference to local-storage StorageClass

cat << EOF | kubectl apply -f -
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: test-pvc
spec:
  accessModes:
  - ReadWriteOnce
  storageClassName: local-storage
  resources:
    requests:
      storage: 10Gi
EOF
persistentvolumeclaim/test-pvc created

kubectl get pvc
NAME       STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS    AGE
test-pvc   Pending                                      local-storage   27s

kubectl get pv
NAME            CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS    REASON   AGE
test-local-pv   10Gi       RWO            Retain           Available           local-storage            33s

The status of test-local-pv is Available because the POD has not yet been created.

Create a POD with local persistent volume

cat << EOF | kubectl apply -f -
apiVersion: v1
kind: Pod
metadata:
  name: test-local-vol
  labels:
    name: test-local-vol
spec:
  containers:
  - name: app
    image: busybox
    command: ['sh', '-c', 'echo "The local volume is mounted!" > /mnt/test.txt && sleep 3600']
    volumeMounts:
      - name: local-persistent-storage
        mountPath: /mnt
  volumes:
    - name: local-persistent-storage
      persistentVolumeClaim:
        claimName: test-pvc
EOF

pod/test-local-vol created

kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
test-local-vol   1/1     Running   0          8s

Check the status of PersistentVolume and PersistentVolumeClaim

kubectl get pvc
NAME       STATUS   VOLUME          CAPACITY   ACCESS MODES   STORAGECLASS    AGE
test-pvc   Bound    test-local-pv   10Gi       RWO            local-storage   57s

kubectl get pv
NAME            CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM              STORAGECLASS    REASON   AGE
test-local-pv   10Gi       RWO            Retain           Bound    default/test-pvc   local-storage            104s

You can see the file on the host’s path.

cat /data/volumes/pv1/test.txt
The local volume is mounted!

Summary

The primary benefit of Local Persistent Volumes over remote persistent storage is performance: local disks usually offer higher IOPS and throughput and lower latency compared to remote storage systems. Kubernetes local volumes have following features:

PersistentVolumeClaim will wait for a POD to show up before a local persistent volume is bound
Once a local persistent volume is bound to a claim, it remains bound, even if the requesting POD has died or has been deleted
A new POD can attach to the existing data in a local volume by referencing the same PersistentVolumeClaim
Similar to NFS shares, Kubernetes persistent local volumes allow multiple PODs to have read/write access

Examples of good workloads include software defined storage systems and replicated databases. Other types of applications should continue to use highly available, remotely accessible, durable storage.