Chapter 3. Understanding persistent storage

In response to requests from a developer defined in a PVC, a cluster administrator configures one or more dynamic provisioners that provision storage and a matching PV.

Alternatively, a cluster administrator can create a number of PVs in advance that carry the details of the real storage that is available for use. PVs exist in the API and are available for use.

When you create a PVC, you request a specific amount of storage, specify the required access mode, and create a storage class to describe and classify the storage. The control loop in the master watches for new PVCs and binds the new PVC to an appropriate PV. If an appropriate PV does not exist, a provisioner for the storage class creates one.

The size of all PVs might exceed your PVC size. This is especially true with manually provisioned PVs. To minimize the excess,OpenShift Container Platform binds to the smallest PV that matches all other criteria.

Claims remain unbound indefinitely if a matching volume does not exist or cannot be created with any available provisioner servicing a storage class. Claims are bound as matching volumes become available. For example, a cluster with many manually provisioned 50Gi volumes would not match a PVC requesting 100Gi. The PVC can be bound when a 100Gi PV is added to the cluster.

Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a pod. For those volumes that support multiple access modes, you must specify which mode applies when you use the claim as a volume in a pod.

After you have a claim, and that claim is bound, the bound PV belongs to you for as long as you need it. You can schedule pods and access claimed PVs by including persistentVolumeClaim in the pod’s volumes block.

The Storage Object in Use Protection feature ensures that PVCs in active use by a pod and PVs that are bound to PVCs are not removed from the system, as this can result in data loss.

Storage Object in Use Protection is enabled by default.

If a user deletes a PVC that is in active use by a pod, the PVC is not removed immediately. PVC removal is postponed until the PVC is no longer actively used by any pods. Also, if a cluster admin deletes a PV that is bound to a PVC, the PV is not removed immediately. PV removal is postponed until the PV is no longer bound to a PVC.

When you are finished with a volume, you can delete the PVC object from the API, which allows reclamation of the resource. The volume is considered released when the claim is deleted, but it is not yet available for another claim. The previous claimant’s data remains on the volume and must be handled according to policy.

The reclaim policy of a persistent volume tells the cluster what to do with the volume after it is released. A volume’s reclaim policy can be Retain, Recycle, or Delete.

Manually reclaim released persistent volumes (PVs) to make them available for new claims or to properly clean up storage assets.

When a persistent volume claim (PVC) is deleted, the persistent volume (PV) still exists and is considered "released". However, the PV is not yet available for another claim because the data of the previous claimant remains on the volume.

Change a persistent volume’s reclaim policy to control whether storage is automatically deleted or retained when claims are removed. Switching from Delete to Retain protects data from accidental loss, while changing to Delete enables automatic cleanup of unused volumes.

+ OpenShift Container Platform supports the following persistent volume plugins:

Generally, a persistent volume (PV) has a specific storage capacity. This is set by using the capacity attribute of the PV.

Currently, storage capacity is the only resource that can be set or requested. Future attributes may include IOPS, throughput, and so on.

A persistent volume can be mounted on a host in any way supported by the resource provider. Providers have different capabilities and each PV’s access modes are set to the specific modes supported by that particular volume. For example, NFS can support multiple read/write clients, but a specific NFS PV might be exported on the server as read-only. Each PV gets its own set of access modes describing that specific PV’s capabilities.

Claims are matched to volumes with similar access modes. The only two matching criteria are access modes and size. A claim’s access modes represent a request. Therefore, you might be granted more, but never less. For example, if a claim requests RWO, but the only volume available is an NFS PV (RWO+ROX+RWX), the claim would then match NFS because it supports RWO.

Direct matches are always attempted first. The volume’s modes must match or contain more modes than you requested. The size must be greater than or equal to what is expected. If two types of volumes, such as NFS and iSCSI, have the same set of access modes, either of them can match a claim with those modes. There is no ordering between types of volumes and no way to choose one type over another.

All volumes with the same modes are grouped, and then sorted by size, smallest to largest. The binder gets the group with matching modes and iterates over each, in size order, until one size matches.

The following table lists the access modes:

Volumes can be found in one of the following phases:

+ .Volume phases

Volume Attributes Classes enable dynamic modification of storage performance, such as IOPS and throughput, on persistent volume claims without re-provisioning or downtime. Administrators define classes representing quality-of-service levels, and users apply them for on-demand performance tuning.

apiVersion: storage.k8s.io/v1
kind: VolumeAttributesClass
metadata:
  name: silver
driverName: ebs.csi.aws.com
parameters:
  iops: "300"
  throughput: "125"
  type: io2
  ...

apiVersion: storage.k8s.io/v1
kind: VolumeAttributesClass
metadata:
  name: silver
driverName: pd.csi.storage.gke.io
parameters:
  iops: "3000"
  throughput: "150Mi"
  ...

Raw block volumes are filesystem-free storage that applications access directly for improved performance. Specify volumeMode: Block in persistent volumes and claims, and configure privileged containers. Storage provider support varies: static only, dynamic only, both, or none.

The following table displays which volume plugins support block volumes.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: block-pv
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  volumeMode: Block
  persistentVolumeReclaimPolicy: Retain
  fc:
    targetWWNs: ["50060e801049cfd1"]
    lun: 0
    readOnly: false

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: block-pvc
spec:
  accessModes:
    - ReadWriteOnce
  volumeMode: Block
  resources:
    requests:
      storage: 10Gi

apiVersion: v1
kind: Pod
metadata:
  name: pod-with-block-volume
spec:
  containers:
    - name: fc-container
      image: fedora:26
      command: ["/bin/sh", "-c"]
      args: [ "tail -f /dev/null" ]
      volumeDevices:
        - name: data
          devicePath: /dev/xvda
  volumes:
    - name: data
      persistentVolumeClaim:
        claimName: block-pvc

To reduce pod timeouts when using a storage volume with many files, configure the fsGroup field. By specifying this field, you can manage how file ownership and permissions are applied, preventing delays caused by the default recursive permission changes on large volumes.

This can occur because, by default, OpenShift Container Platform recursively changes ownership and permissions for the contents of each volume to match the fsGroup specified in the securityContext of the pod when that volume is mounted. For volumes with many files, checking and changing ownership and permissions can be time consuming, slowing pod startup. You can use the fsGroupChangePolicy field inside a securityContext to control the way that OpenShift Container Platform checks and manages ownership and permissions for a volume.

fsGroupChangePolicy defines behavior for changing ownership and permission of the volume before being exposed inside a pod. This field only applies to volume types that support fsGroup-controlled ownership and permissions. This field has two possible values:

You can set fsGroupChangePolicy at either the namespace or pod level.

The SELinux mount option applies security contexts during mount without recursive relabeling, reducing pod startup times on volumes with many files. This optimization is enabled by default for ReadWriteOncePod volumes and will become the default for ReadWriteOnce and ReadWriteMany volumes.

SELinux (Security-Enhanced Linux) is a security mechanism that assigns security labels (contexts) to all objects (files, processes, network ports, and so on) on a system. These labels determine what a process can access. In OpenShift Container Platform, SELinux helps prevent containers from escaping and accessing the host system or other containers.

When a pod starts, the container runtime recursively relabels all files on a volume to match the pod’s SELinux context. For volumes with many files, this can significantly increase pod startup times.

Mount option specifies avoiding recursive relabeling of all files by attempting to mount the volume with the correct SELinux label directly using the -o context mount option, thus helping to avoid pod timeout problems.