Kubernetes - Volumes

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Disk files for a container is ephemeral. To persist data over container restart, we need to use PersistentVolume

Kubernetes supports many Volume Types

  • emptyDir
  • azureDisk
  • awsElasticBlockStore
  • gcePersistentDisk
  • …

emptyDir

An emptyDir volume is first created when a Pod is assigned to a Node, and exists as long as that Pod is running on that node. It can be used for Pods to share files.

By default, emptyDir volumes are stored on whatever medium is backing the node - that might be disk or SSD or network storage, depending on your environment.

All containers in a pod can share an emptyDir. emptyDir volume can mount to the same or different path.

emptyDir volume is removed when a Pod is removed.

Example

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apiVersion: v1
kind: Pod
metadata:
  name: test-pd
spec:
  containers:
  - image: k8s.gcr.io/test-webserver
    name: test-container
    volumeMounts:
    - mountPath: /cache
      name: cache-volume
  volumes:
  - name: cache-volume
    emptyDir: {}

PersistentVolume

For persistent volume use PersistentVolume(PV) and PersistentVolumeClaim(PVC).

  • PersistentVolume is usually managed by administrators
  • PersistentVolume can be requested by PersistentVolumeClaim

There are three steps to use PersistentVolume.

  1. Administrator create PersistentVolume
  2. Developer create PersistentVolumeClaim that claims part of that PersistentVolume disk space
  3. Developer create pod that that uses PersistentVolumeClaim

Step1: Create PersistentVolume

mongo-pv.yml

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apiVersion: v1
kind: PersistentVolume
metadata: 
  labels: 
    type: local
  name: mongo-pv
spec: 
  storageClassName: mongo-demo
  accessModes: 
    - ReadWriteOnce
  capacity: 
    storage: 1Gi
  hostPath: 
    path: /data/mongo_data

use kubectl get pv to display PersistentVolume resources. storageClassName is very important. It is used to tie PersistentVolume and PersistentVolumeClaim. The status is Available at this time. If a PersistentVolumeClaim is bound, the status will be Bound.

3 Access Modes

  • ReadWriteOnce: The Volume can be mounted as read-write by a single node.
  • ReadOnlyMany: The Volume can be mounted read-only by many nodes.
  • ReadWriteMany: The Volume can be mounted as read-write by many nodes. PersistentVolumes that are backed by Compute Engine persistent disks don’t support this access mode.

use kubectl get pv command to display the PersistentVolumes.

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$ kubectl get pv
NAME       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS   REASON   AGE
mongo-pv   1Gi        RWO            Retain           Available           mongo-demo              6s

Step2: Create PersistentVolumeClaim

mongo-pvc.yml

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apiVersion: v1
kind: PersistentVolumeClaim
metadata: 
  name: mongo-pvc
spec: 
  storageClassName: mongo-demo
  accessModes: 
    - ReadWriteOnce
  resources:
    requests: 
      storage: 250Mi

use kubectl get pvc to display PersistentVolumeClaim

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$ kubectl get pvc
NAME             STATUS   VOLUME     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
mongo-pv-claim   Bound    mongo-pv   1Gi        RWO            mongo-demo     8s

Step3: Create Pod that uses the PersistentVolumeClaim

mongo.yml: defines a service and statefulset that uses PersistentVolume.

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apiVersion: v1
kind: Service
metadata:
 name: mongo
 labels:
  app: mongo
spec:
 ports:
  - port: 27017
    targetPort: 27017
 clusterIP: None
 selector:
  app: mongo
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mongo
spec:
  selector:
    matchLabels:
      app: mongo
  serviceName: "mongo"
  replicas: 1
  template:
    metadata:
      labels:
        app: mongo
    spec:
      terminationGracePeriodSeconds: 10
      containers:
      - name: mongo
        image: mongo
        command:
        - mongod
        - "--bind_ip"
        - 0.0.0.0
        ports:
        - containerPort: 27017
        volumeMounts:
        - name: mongo-volume
          mountPath: /data/db
      volumes:
      - name: mongo-volume
        persistentVolumeClaim:
          claimName: mongo-pvc

After the statefulset is created, use kubectl get statefulsets to check the statefulset status

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$ kubectl get statefulsets
NAME    READY   AGE
mongo   1/1     11m

Use kubectl exec -it mongo-0 /bin/bash to get into the container and use the mongodb database. The data will persiste over pod restart.

References