Storage Basics

Volumes

On-disk files in a container are ephemeral, which presents the following problems to important applications running in the container:

1. When a container is rebuilt, files in the container will be lost.
2. When multiple containers run in a pod at the same time, files need to be shared among the containers.

Kubernetes volumes resolve both of these problems. Volumes, as part of a pod, cannot be created independently and can only be defined in pods. All containers in a pod can access its volumes, but the volumes must have been mounted to any directory in a container.

The following figure shows how a storage volume is used between containers in a pod.

The basic principles for using volumes are as follows:

• Multiple volumes can be mounted to a pod. However, do not mount too many volumes to a pod.
• Multiple types of volumes can be mounted to a pod.
• Each volume mounted to a pod can be shared among containers in the pod.
• You are advised to use PVCs and PVs to mount volumes for Kubernetes.

The lifecycle of a volume is the same as that of the pod to which the volume is mounted. When the pod is deleted, the volume is also deleted. However, files in the volume may outlive the volume, depending on the volume type.

Kubernetes provides various volume types, which can be classified as in-tree and out-of-tree.

PV and PVC

Kubernetes provides PersistentVolumes (PVs) and PersistentVolumeClaims (PVCs) to abstract details of how storage is provided from how it is consumed. You can request specific size of storage when needed, just like pods can request specific levels of resources (CPU and memory).

• PV: describes a persistent storage volume in a cluster. A PV is a cluster-level resource just like a node. It applies to the entire Kubernetes cluster. A PV has a lifecycle independent of any individual Pod that uses the PV.
• PVC: describes a request for storage by a user. When configuring storage for an application, claim a storage request (that is, PVC). Kubernetes selects a PV that best meets the request and binds the PV to the PVC. A PVC to PV binding is a one-to-one mapping. When creating a PVC, describe the attributes of the requested persistent storage, such as the storage size and read/write permission.

You can bind PVCs to PVs in a pod so that the pod can use storage resources. The following figure shows the relationship between PVs and PVCs.

Figure 1 PVC-to-PV binding

CSI

CSI is a standard for container storage interfaces and a storage plugin implementation solution recommended by the Kubernetes community. Everest is a storage add-on developed based on CSI. It provides different types of persistent storage for containers.

Volume Access Modes

Storage volumes can be mounted to the host system only in the mode supported by underlying storage resources. For example, a file storage system can be read and written by multiple nodes, but an EVS disk can be read and written by only one node.

• ReadWriteOnce: A storage volume can be mounted to a single node in read-write mode.
• ReadWriteMany: A storage volume can be mounted to multiple nodes in read-write mode.

Mounting a Storage Volume

You can mount volumes in the following ways:

Use PVs to describe existing storage resources, and then create PVCs to use the storage resources in pods. You can also use the dynamic creation mode. That is, specify the StorageClass when creating a PVC and use the provisioner in the StorageClass to automatically create a PV and bind the PV to the PVC.

PV Reclaim Policy

A PV reclaim policy is used to delete or reclaim underlying volumes when a PVC is deleted. The value can be Delete or Retain.

• Delete: Deleting a PVC will remove the PV from Kubernetes, so the associated underlying storage assets from the external infrastructure.
• Retain: When a PVC is deleted, the PV and underlying storage resources are not deleted. Instead, you must manually delete these resources. After that, the PV resources are in the Released state and cannot be directly bound to the PVC.

  You can manually delete and reclaim volumes by performing the following operations:

  1. Delete the PV.
  2. Clear data on the associated underlying storage resources as required.
  3. Delete the associated underlying storage resources.

  To reuse the underlying storage resources, create a PV.

CCE also allows you to delete a PVC without deleting underlying storage resources. This function can be achieved only by using a YAML file: Set the PV reclaim policy to Delete and add everest.io/reclaim-policy: retain-volume-only to annotations. In this way, when the PVC is deleted, the PV is deleted, but the underlying storage resources are retained.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: test
  namespace: default
  annotations:
    volume.beta.kubernetes.io/storage-provisioner: everest-csi-provisioner
    everest.io/disk-volume-type: SAS
  labels:
    failure-domain.beta.kubernetes.io/region: <your_region>   # Region of the node where the application is to be deployed
    failure-domain.beta.kubernetes.io/zone: <your_zone>       # AZ of the node where the application is to be deployed
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi
  storageClassName: csi-disk
  volumeName: pv-evs-test

---
apiVersion: v1
kind: PersistentVolume
metadata:
  annotations:
    pv.kubernetes.io/provisioned-by: everest-csi-provisioner
    everest.io/reclaim-policy: retain-volume-only
  name: pv-evs-test
  labels:
    failure-domain.beta.kubernetes.io/region: <your_region>   # Region of the node where the application is to be deployed
    failure-domain.beta.kubernetes.io/zone: <your_zone>       # AZ of the node where the application is to be deployed
spec:
  accessModes:
    - ReadWriteOnce
  capacity:
    storage: 10Gi
  csi:
    driver: disk.csi.everest.io
    fsType: ext4
    volumeHandle: 2af98016-6082-4ad6-bedc-1a9c673aef20
    volumeAttributes:
      storage.kubernetes.io/csiProvisionerIdentity: everest-csi-provisioner
      everest.io/disk-mode: SCSI
      everest.io/disk-volume-type: SAS
  persistentVolumeReclaimPolicy: Delete
  storageClassName: csi-disk

Documentation

• For more information about Kubernetes storage, see Storage.
• For more information about CCE container storage, see Overview.