Description

There are two basic concepts, PV (PersistentVolume) and PVC (PersistentVolumeClaim)

Within the PaaS K8S of our platform, PV is a Cinder Volume, i.e. an ordinary disk in the terminology of our Cloud. Can be created manually by the cluster administrator, or dynamically via PVC.

PersistentVolumeClaim (PVC) is a request to create a PV, when it is created, a PV will be automatically created in the cluster (the corresponding disk will appear in the Disks section of the control panel).

The PersistentVolume mechanism allows you to connect an existing disk to the K8S cluster as a persistent data store.

Example

Let's look at an example. You have an ext4 file system disk with a test_file.txt file. Let's create a PV based on this disk, describing a manifest for it:

 ---
apiVersion: v1
kind: PersistentVolume
metadata:
name: pv-test
spec:
accessModes:
- ReadWriteOnce
capacity:
storage: 8Gi
cinder:
volumeID: 239e30a9-5a14-41a1-9245-17ace88076a4
fsType: ext4
persistentVolumeReclaimPolicy: Retain
storageClassName: dp1
volumeMode: Filesystem

accessModes: ReadWriteOnce - means that this disk can be connected to only one pod (multiple connections to different pods are available only for NFS)
capacity: storage: 8Gi - PV size field, is required and must be equal to the size of the used disk
volumeID - the field where the ID of the used disk is indicated
fsType: ext4 - type of filesystem located on disk
persistentVolumeReclaimPolicy: Retain - PV lifecycle parameter, if the value is Retain, the disk will remain in the project after removing PVC and PV (if the value is Delete, the disk will be deleted)
storageClassName: dp1 - must match the type of disk used

Let's create a PV based on the manifest and check it:

 $ kubectl apply -f pv.yaml
persistentvolume / pv-test created
$ kubectl get pv
NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
pv-test 8Gi RWO Retain Available 20m

Now we need to describe the PVC for this PV

 ---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: test-pvc
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 8Gi
storageClassName: dp1
volumeName: pv-test

Size (storage) and type (storageClassName) must match the specified parameters in PV
volumeName: pv-test - here you need to specify the name of the PV created in the previous step

Create PVC

 $ kubectl apply -f pvc.yaml
persistentvolumeclaim / test-pvc created
$ kubectl get pvc
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
test-pvc Bound pv-test 8Gi RWO dp1 22m

Now let's create a manifest for the pod using PVC

 ---
apiVersion: v1
kind: Pod
metadata:
name: test-pod
spec:
volumes:
- name: pvc
persistentVolumeClaim:
claimName: test-pvc
containers:
- image: nginx
name: nginx
volumeMounts:
- mountPath: / mnt
name: pvc

The spec: volumes section describes the disk that will be mounted in our container, here we set its name and through the persistentVolumeСlaim parameter we indicate our created PVC (by name)
The containers: volumeMounts section specifies the disk described in the previous paragraph, the mountPath: / mnt parameter specifies the path where the disk will be mounted.

Create under

 $ kubectl apply -f pod_test.yaml
pod / centos-pod created
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
test-pod 1/1 Running 0 24m

Now let's connect to the running pod and read the file from the mounted disk

 $ kubectl exec -it test-pod - / bin / bash
root @ test-pod: / # cat /mnt/test-file.txt
Hello World!
root @ test-pod: / #

The PersistentVolumeClaim mechanism allows automatic creation of disks if the volumeName parameter is not specified .

Available disc types for PVC can be viewed with the command

 $ kubectl get storageclasses

We recommend that you familiarize yourself with more detailed information about Persistent Volumes on the official Kubernetes website .