Persistent Storage - Additional Concepts | Architecture

Overview

Managing storage is a distinct problem from managing compute resources. OKD uses the Kubernetes persistent volume (PV) framework to allow cluster administrators to provision persistent storage for a cluster. Developers can use persistent volume claims (PVCs) to request PV resources without having specific knowledge of the underlying storage infrastructure.

PVCs are specific to a project and are created and used by developers as a means to use a PV. PV resources on their own are not scoped to any single project; they can be shared across the entire OKD cluster and claimed from any project. After a PV is bound to a PVC, however, that PV cannot then be bound to additional PVCs. This has the effect of scoping a bound PV to a single namespace (that of the binding project).

PVs are defined by a PersistentVolume API object, which represents a piece of existing, networked storage in the cluster that was provisioned by the cluster administrator. It is a resource in the cluster just like a node is a cluster resource. PVs are volume plug-ins like Volumes but have a lifecycle that is independent of any individual pod that uses the PV. PV objects capture the details of the implementation of the storage, be that NFS, iSCSI, or a cloud-provider-specific storage system.

High availability of storage in the infrastructure is left to the underlying storage provider.

PVCs are defined by a PersistentVolumeClaim API object, which represents a request for storage by a developer. It is similar to a pod in that pods consume node resources and PVCs consume PV resources. For example, pods can request specific levels of resources (e.g., CPU and memory), while PVCs can request specific storage capacity and access modes (e.g., they can be mounted once read/write or many times read-only).

Lifecycle of a volume and claim

PVs are resources in the cluster. PVCs are requests for those resources and also act as claim checks to the resource. The interaction between PVs and PVCs have the following lifecycle.

Provision 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.

Bind claims

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 PV volume might exceed your requested volume. This is especially true with manually provisioned PVs. To minimize the excess, OKD 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.

Use pods and claimed PVs

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. See below for syntax details.

PVC protection

PVC protection is enabled by default.

Release volumes

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.

Reclaim volumes

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

Retain reclaim policy allows manual reclamation of the resource for those volume plug-ins that support it.
Delete reclaim policy deletes both the PersistentVolume object from OKD and the associated storage asset in external infrastructure, such as AWS EBS, GCE PD, or Cinder volume.

Dynamically provisioned volumes have a default ReclaimPolicy value of Delete. Manually provisioned volumes have a default ReclaimPolicy value of Retain.

Reclaim a PersistentVolume Manually

When a PersistentVolumeClaim is deleted, the PersistentVolume still exists and is considered "released". However, the PV is not yet available for another claim because the previous claimant’s data remains on the volume.

To manually reclaim the PV as a cluster administrator:

Delete the PV.
```
$ oc delete <pv-name>
```
The associated storage asset in the external infrastructure, such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume, still exists after the PV is deleted.
Clean up the data on the associated storage asset.
Delete the associated storage asset. Alternately, to reuse the same storage asset, create a new PV with the storage asset definition.

The reclaimed PV is now available for use by another PVC.

Change the reclaim policy

To change the reclaim policy of a PV:

List the PVs in your cluster:

$ oc get pv

Example Output

NAME                                       CAPACITY   ACCESSMODES   RECLAIMPOLICY   STATUS    CLAIM             STORAGECLASS     REASON    AGE
 pvc-b6efd8da-b7b5-11e6-9d58-0ed433a7dd94   4Gi        RWO           Delete          Bound     default/claim1    manual                     10s
 pvc-b95650f8-b7b5-11e6-9d58-0ed433a7dd94   4Gi        RWO           Delete          Bound     default/claim2    manual                     6s
 pvc-bb3ca71d-b7b5-11e6-9d58-0ed433a7dd94   4Gi        RWO           Delete          Bound     default/claim3    manual                     3s

Choose one of your PVs and change its reclaim policy:

$ oc patch pv <your-pv-name> -p '{"spec":{"persistentVolumeReclaimPolicy":"Retain"}}'

Verify that your chosen PV has the right policy:

$ oc get pv

Example Output

NAME                                       CAPACITY   ACCESSMODES   RECLAIMPOLICY   STATUS    CLAIM             STORAGECLASS     REASON    AGE
 pvc-b6efd8da-b7b5-11e6-9d58-0ed433a7dd94   4Gi        RWO           Delete          Bound     default/claim1    manual                     10s
 pvc-b95650f8-b7b5-11e6-9d58-0ed433a7dd94   4Gi        RWO           Delete          Bound     default/claim2    manual                     6s
 pvc-bb3ca71d-b7b5-11e6-9d58-0ed433a7dd94   4Gi        RWO           Retain          Bound     default/claim3    manual                     3s

In the preceding output, the PV bound to claim default/claim3 now has a Retain reclaim policy. The PV will not be automatically deleted when a user deletes claim default/claim3.

Persistent volumes

Each PV contains a spec and status, which is the specification and status of the volume, for example:

PV object definition example

  apiVersion: v1
  kind: PersistentVolume
  metadata:
    name: pv0003
  spec:
    capacity:
      storage: 5Gi
    accessModes:
      - ReadWriteOnce
    persistentVolumeReclaimPolicy: Retain
    nfs:
      path: /tmp
      server: 172.17.0.2

Types of PVs

OKD supports the following PersistentVolume plug-ins:

Capacity

Generally, a PV has a specific storage capacity. This is set by using the PV’s capacity attribute.

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

Access modes

A PersistentVolume can be mounted on a host in any way supported by the resource provider. Providers will 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 (NFS and iSCSI, for example) 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:

Table 1. Access modes
Access Mode	CLI abbreviation	Description
ReadWriteOnce	`RWO`	The volume can be mounted as read-write by a single node.
ReadOnlyMany	`ROX`	The volume can be mounted read-only by many nodes.
ReadWriteMany	`RWX`	The volume can be mounted as read-write by many nodes.

A volume’s AccessModes are descriptors of the volume’s capabilities. They are not enforced constraints. The storage provider is responsible for runtime errors resulting from invalid use of the resource.

For example, Ceph offers ReadWriteOnce access mode. You must mark the claims as read-only if you want to use the volume’s ROX capability. Errors in the provider show up at runtime as mount errors.

iSCSI and Fibre Channel volumes do not currently have any fencing mechanisms. You must ensure the volumes are only used by one node at a time. In certain situations, such as draining a node, the volumes can be used simultaneously by two nodes. Before draining the node, first ensure the pods that use these volumes are deleted.

The following table lists the access modes supported by different PVs:

Table 2. Supported access modes for PVs
Volume Plug-in	ReadWriteOnce	ReadOnlyMany	ReadWriteMany
AWS EBS	✅	-	-
Azure File	✅	✅	✅
Azure Disk	✅	-	-
Ceph RBD	✅	✅	-
Fibre Channel	✅	✅	-
GCE Persistent Disk	✅	-	-
GlusterFS	✅	✅	✅
gluster-block	✅	-	-
HostPath	✅	-	-
iSCSI	✅	✅	-
NFS	✅	✅	✅
Openstack Cinder	✅	-	-
VMWare vSphere	✅	-	-
Local	✅	-	-

Use a recreate deployment strategy for pods that rely on AWS EBS, GCE Persistent Disks, or Openstack Cinder PVs.

Reclaim policy

The following table lists current reclaim policies:

Table 3. Current reclaim policies
Reclaim policy	Description
Retain	Allows manual reclamation.
Delete	Deletes both PV and associated external storage asset.

If you do not want to retain all pods, use dynamic provisioning.

Phase

Volumes can be found in one of the following phases:

Table 4. Volume phases
Phase	Description
Available	A free resource not yet bound to a claim.
Bound	The volume is bound to a claim.
Released	The claim was deleted, but the resource is not yet reclaimed by the cluster.
Failed	The volume has failed its automatic reclamation.

The CLI shows the name of the PVC bound to the PV.

Mount options

You can specify mount options while mounting a persistent volume by using the annotation volume.beta.kubernetes.io/mount-options.

For example:

Mount options example

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv0001
  annotations:
    volume.beta.kubernetes.io/mount-options: rw,nfsvers=4,noexec (1)
spec:
  capacity:
    storage: 1Gi
  accessModes:
  - ReadWriteOnce
  nfs:
    path: /tmp
    server: 172.17.0.2
  persistentVolumeReclaimPolicy: Retain
  claimRef:
    name: claim1
    namespace: default

1	Specified mount options are used while mounting the PV to the disk.

The following persistent volume types support mount options:

NFS
GlusterFS
Ceph RBD
OpenStack Cinder
AWS Elastic Block Store (EBS)
GCE Persistent Disk
iSCSI
Azure Disk
Azure File
VMWare vSphere

Fibre Channel and HostPath persistent volumes do not support mount options.

Recursive chown

Each time a PV is mounted to a Pod, or when a Pod restart occurs, the volume’s ownership and permissions are recursively changed to match those of the Pod. Group read/write access is added to the volume when chown is performed on all files and directories in the PV. This enables the process running inside the Pod to access the PV file system. A user can prevent these recursive changes to ownership by not specifying the fsGroup in their Pods and security context constraints (SCCs).

SELinux labels are also changed recursively. A user cannot prevent recursive changes to SELinux labels.

If a user has a large number of files (e.g., > 100,000), a significant delay can occur before the PV is mounted to a Pod.

Persistent volume claims

Each PVC contains a spec and status, which is the specification and status of the claim, for example:

PVC object definition example

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: myclaim
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 8Gi
  storageClassName: gold

Storage classes

Claims can optionally request a specific storage class by specifying the storage class’s name in the storageClassName attribute. Only PVs of the requested class, ones with the same storageClassName as the PVC, can be bound to the PVC. The cluster administrator can configure dynamic provisioners to service one or more storage classes. The cluster administrator can create a PV on demand that matches the specifications in the PVC.

The cluster administrator can also set a default storage class for all PVCs. When a default storage class is configured, the PVC must explicitly ask for StorageClass or storageClassName annotations set to "" to be bound to a PV without a storage class.

Access modes

Claims use the same conventions as volumes when requesting storage with specific access modes.

Resources

Claims, such as pods, can request specific quantities of a resource. In this case, the request is for storage. The same resource model applies to volumes and claims.

Claims as volumes

Pods access storage by using the claim as a volume. Claims must exist in the same namespace as the pod by using the claim. The cluster finds the claim in the pod’s namespace and uses it to get the PersistentVolume backing the claim. The volume is mounted to the host and into the pod, for example:

Mount volume to the host and into the pod example

kind: Pod
apiVersion: v1
metadata:
  name: mypod
spec:
  containers:
    - name: myfrontend
      image: dockerfile/nginx
      volumeMounts:
      - mountPath: "/var/www/html"
        name: mypd
  volumes:
    - name: mypd
      persistentVolumeClaim:
        claimName: myclaim

Block volume support

Block volume support is a Technology Preview feature and it is only available for manually provisioned PVs.

You can statically provision raw block volumes by including API fields in your PV and PVC specifications.

To use block volume, you must first enable the BlockVolume feature gate. To enable the feature gates for master(s), add feature-gates to apiServerArguments and controllerArguments. To enable the feature gates for node(s), add feature-gates to kubeletArguments. For example:

kubeletArguments:
   feature-gates:
     - BlockVolume=true

PV example

apiVersion: v1
kind: PersistentVolume
metadata:
  name: block-pv
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  volumeMode: Block (1)
  persistentVolumeReclaimPolicy: Retain
  fc:
    targetWWNs: ["50060e801049cfd1"]
    lun: 0
    readOnly: false

1	`volumeMode` field indicating that this PV is a raw block volume.

PVC example

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: block-pvc
spec:
  accessModes:
    - ReadWriteOnce
  volumeMode: Block (1)
  resources:
    requests:
      storage: 10Gi

1	`volumeMode` field indicating that a raw block persistent volume is requested.

Pod specification example

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:  (1)
        - name: data
          devicePath: /dev/xvda (2)
  volumes:
    - name: data
      persistentVolumeClaim:
        claimName: block-pvc (3)

1	`volumeDevices` (similar to `volumeMounts`) is used for block devices and can only be used with `PersistentVolumeClaim` sources.
2	`devicePath` (similar to `mountPath`) represents the path to the physical device.
3	The volume source must be of type `persistentVolumeClaim` and must match the name of the PVC as expected.

Table 5. Accepted values for `VolumeMode`
Value	Default
Filesystem	Yes
Block	No

Table 6. Binding scenarios for block volumes
PV VolumeMode	PVC VolumeMode	Binding Result
Filesystem	Filesystem	Bind
Unspecified	Unspecified	Bind
Filesystem	Unspecified	Bind
Unspecified	Filesystem	Bind
Block	Block	Bind
Unspecified	Block	No Bind
Block	Unspecified	No Bind
Filesystem	Block	No Bind
Block	Filesystem	No Bind

Unspecified values result in the default value of Filesystem.