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 PV volume might exceed your requested volume. 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 can not 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.

Once 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 PersistentVolume tells the cluster what to do with the volume after it is released. Volumes reclaim policy can either be Retain, Recycle, or Delete.

OpenShift Container Platform supports the following PersistentVolume plug-ins:

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.

A PersistentVolume 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:

You can view the name of the PVC bound to the PV by running:

$ oc get pv <pv-claim>

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

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.

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

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.

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:

kind: Pod
apiVersion: v1
metadata:
  name: mypod
spec:
  containers:
    - name: myfrontend
      image: dockerfile/nginx
      volumeMounts:
      - mountPath: "/var/www/html" 1
        name: mypd 2
  volumes:
    - name: mypd
      persistentVolumeClaim:
        claimName: myclaim 3

StorageClasses are used to differentiate and delineate storage levels and usages. By defining a storage class, users can obtain dynamically provisioned persistent volumes.

Before OpenShift Container Platform mounts the volume and passes it to a container, it checks that it contains a file system as specified by the fsType parameter in the persistent volume definition. If the device is not formatted with the file system, all data from the device is erased and the device is automatically formatted with the given file system.

This allows using unformatted AWS volumes as persistent volumes, because OpenShift Container Platform formats them before the first use.

By default, OpenShift Container Platform supports a maximum of 39 EBS volumes attached to one node. This limit is consistent with the AWS volume limits.

OpenShift Container Platform can be configured to have a higher limit by setting the environment variable KUBE_MAX_PD_VOLS. However, AWS requires a particular naming scheme (AWS Device Naming) for attached devices, which only supports a maximum of 52 volumes. This limits the number of volumes that can be attached to a node via OpenShift Container Platform to 52.

To provision Fibre Channel volumes using the PersistentVolume API the following must be available:

A PersistentVolume and a LUN have a one-to-one mapping between them.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv0001
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  fc:
    targetWWNs: ['500a0981891b8dc5', '500a0981991b8dc5'] 1
    lun: 2
    fsType: ext4

Storage must exist in the underlying infrastructure before it can be mounted as a volume in OpenShift Container Platform. To provision NFS volumes, a list of NFS servers and export paths are all that is required.

You can use disk partitions to enforce disk quotas and size constraints. Each partition can be its own export. Each export is one PV. OpenShift Container Platform enforces unique names for PVs, but the uniqueness of the NFS volume’s server and path is up to the administrator.

Enforcing quotas in this way allows the developer to request persistent storage by a specific amount, such as 10Gi, and be matched with a corresponding volume of equal or greater capacity.

This section covers NFS volume security, including matching permissions and SELinux considerations. The user is expected to understand the basics of POSIX permissions, process UIDs, supplemental groups, and SELinux.

Developers request NFS storage by referencing either a PVC by name or the NFS volume plug-in directly in the volumes section of their Pod definition.

The /etc/exports file on the NFS server contains the accessible NFS directories. The target NFS directory has POSIX owner and group IDs. The OpenShift Container Platform NFS plug-in mounts the container’s NFS directory with the same POSIX ownership and permissions found on the exported NFS directory. However, the container is not run with its effective UID equal to the owner of the NFS mount, which is the desired behavior.

As an example, if the target NFS directory appears on the NFS server as:

$ ls -lZ /opt/nfs -d
drwxrws---. nfsnobody 5555 unconfined_u:object_r:usr_t:s0   /opt/nfs

$ id nfsnobody
uid=65534(nfsnobody) gid=65534(nfsnobody) groups=65534(nfsnobody)

Then the container must match SELinux labels, and either run with a UID of 65534, the nfsnobody owner, or with 5555 in its supplemental groups in order to access the directory.

spec:
  containers:
    - name:
    ...
  securityContext: 1
    supplementalGroups: [5555] 2

spec:
  containers: 1
  - name:
  ...
    securityContext:
      runAsUser: 65534 2

NFS implements the OpenShift Container Platform Recyclable plug-in interface. Automatic processes handle reclamation tasks based on policies set on each persistent volume.

By default, PVs are set to Retain.

Once claim to a PVC is deleted, and the PV is released, the PV object should not be reused. Instead, a new PV should be created with the same basic volume details as the original.

For example, the administrator creates a PV named nfs1:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: nfs1
spec:
  capacity:
    storage: 1Mi
  accessModes:
    - ReadWriteMany
  nfs:
    server: 192.168.1.1
    path: "/"

The user creates PVC1, which binds to nfs1. The user then deletes PVC1, releasing claim to nfs1. This results in nfs1 being Released. If the administrator wants to make the same NFS share available, they should create a new PV with the same NFS server details, but a different PV name:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: nfs2
spec:
  capacity:
    storage: 1Mi
  accessModes:
    - ReadWriteMany
  nfs:
    server: 192.168.1.1
    path: "/"

Deleting the original PV and re-creating it with the same name is discouraged. Attempting to manually change the status of a PV from Released to Available causes errors and potential data loss.

Depending on what version of NFS is being used and how it is configured, there may be additional configuration steps needed for proper export and security mapping. The following are some that may apply:

OpenShift Container Platform supports hostPath mounting for development and testing on a single-node cluster.

In a production cluster, you would not use hostPath. Instead, a cluster administrator would provision a network resource, such as a GCE Persistent Disk volume, an NFS share, or an Amazon EBS volume. Network resources support the use of StorageClasses to set up dynamic provisioning.

A hostPath volume must be provisioned statically.

A Pod that uses a hostPath volume must be referenced by manual (static) provisioning.

After the PersistentVolumeClaim has been created, it can be used inside by an application. The following example demonstrates mounting this share inside of a Pod.

Verify that the storage exists in the underlying infrastructure before mounting it as a volume in OpenShift Container Platform. All that is required for the iSCSI is the iSCSI target portal, a valid iSCSI Qualified Name (IQN), a valid LUN number, the filesystem type, and the PersistentVolume API.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: iscsi-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  iscsi:
     targetPortal: 10.16.154.81:3260
     iqn: iqn.2014-12.example.server:storage.target00
     lun: 0
     fsType: 'ext4'

Use LUN partitions to enforce disk quotas and size constraints. Each LUN is one persistent volume. Kubernetes enforces unique names for persistent volumes.

Enforcing quotas in this way allows the end user to request persistent storage by a specific amount (e.g, 10Gi) and be matched with a corresponding volume of equal or greater capacity.

Users request storage with a PersistentVolumeClaim. This claim only lives in the user’s namespace and can only be referenced by a pod within that same namespace. Any attempt to access a persistent volume claim across a namespace causes the pod to fail.

Each iSCSI LUN must be accessible by all nodes in the cluster.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: iscsi-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  iscsi:
    targetPortal: 10.0.0.1:3260
    iqn: iqn.2016-04.test.com:storage.target00
    lun: 0
    fsType: ext4
    chapAuthDiscovery: true 1
    chapAuthSession: true 2
    secretRef:
      name: chap-secret 3

For iSCSI-based storage, you can configure multiple paths by using the same IQN for more than one target portal IP address. Multipathing ensures access to the persistent volume when one or more of the components in a path fail.

To specify multi-paths in the pod specification use the portals field. For example:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: iscsi-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  iscsi:
    targetPortal: 10.0.0.1:3260
    portals: ['10.0.2.16:3260', '10.0.2.17:3260', '10.0.2.18:3260'] 1
    iqn: iqn.2016-04.test.com:storage.target00
    lun: 0
    fsType: ext4
    readOnly: false

Configure the custom initiator iSCSI Qualified Name (IQN) if the iSCSI targets are restricted to certain IQNs, but the nodes that the iSCSI PVs are attached to are not guaranteed to have these IQNs.

To specify a custom initiator IQN, use initiatorName field.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: iscsi-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  iscsi:
    targetPortal: 10.0.0.1:3260
    portals: ['10.0.2.16:3260', '10.0.2.17:3260', '10.0.2.18:3260']
    iqn: iqn.2016-04.test.com:storage.target00
    lun: 0
    initiatorName: iqn.2016-04.test.com:custom.iqn 1
    fsType: ext4
    readOnly: false

CSI drivers are typically shipped as container images. These containers are not aware of OpenShift Container Platform where they run. To use CSI-compatible storage backend in OpenShift Container Platform, the cluster administrator must deploy several components that serve as a bridge between OpenShift Container Platform and the storage driver.

The following diagram provides a high-level overview about the components running in pods in the OpenShift Container Platform cluster.

It is possible to run multiple CSI drivers for different storage backends. Each driver needs its own external controllers' deployment and DaemonSet with the driver and CSI registrar.

Dynamic provisioning of persistent storage depends on the capabilities of the CSI driver and underlying storage backend. The provider of the CSI driver should document how to create a StorageClass in OpenShift Container Platform and the parameters available for configuration.

As seen in the OpenStack Cinder example, you can deploy this StorageClass to enable dynamic provisioning.

The following example installs a default MySQL template without any changes to the template.

Dynamically provisioning VMware vSphere volumes is the recommended method.

You can use either of the following procedures to dynamically provision these volumes using the default StorageClass.

To statically provision VMware vSphere volumes you must create the virtual machine disks for reference by the persistent volume framework.

A volume snapshot is a snapshot taken from a storage volume in a cluster. The external snapshot controller and provisioner enable use of the feature in the OpenShift Container Platform cluster and handle volume snapshots through the OpenShift Container Platform API.

With volume snapshots, a cluster administrator can:

Supported PersistentVolume types:

The controller and provisioner provide volume snapshotting. These external components run in the cluster.

There are two external components that provide volume snapshotting:

The external controller and provisioner services are distributed as container images and can be run in the OpenShift Container Platform cluster as usual.

apiVersion: v1
kind: Secret
metadata:
  name: awskeys
type: Opaque
data:
  access-key-id: <base64 encoded AWS_ACCESS_KEY_ID>
  secret-access-key: <base64 encoded AWS_SECRET_ACCESS_KEY>

$ echo -n "<aws_access_key_id>" | base64
$ echo -n "<aws_secret_access_key>" | base64

kind: Deployment
apiVersion: extensions/v1beta1
metadata:
  name: snapshot-controller
spec:
  replicas: 1
  strategy:
    type: Recreate
  template:
    metadata:
      labels:
        app: snapshot-controller
    spec:
      serviceAccountName: snapshot-controller-runner
      hostNetwork: true
      containers:
        - name: snapshot-controller
          image: "registry.redhat.io/openshift3/snapshot-controller:latest"
          imagePullPolicy: "IfNotPresent"
          args: ["-cloudprovider", "aws"]
          env:
            - name: AWS_ACCESS_KEY_ID
              valueFrom:
                secretKeyRef:
                  name: awskeys
                  key: access-key-id
            - name: AWS_SECRET_ACCESS_KEY
              valueFrom:
                secretKeyRef:
                  name: awskeys
                  key: secret-access-key
        - name: snapshot-provisioner
          image: "registry.redhat.io/openshift3/snapshot-provisioner:latest"
          imagePullPolicy: "IfNotPresent"
          args: ["-cloudprovider", "aws"]
          env:
            - name: AWS_ACCESS_KEY_ID
              valueFrom:
                secretKeyRef:
                  name: awskeys
                  key: access-key-id
            - name: AWS_SECRET_ACCESS_KEY
              valueFrom:
                secretKeyRef:
                  name: awskeys
                  key: secret-access-key

kind: Deployment
apiVersion: extensions/v1beta1
metadata:
  name: snapshot-controller
spec:
  replicas: 1
  strategy:
    type: Recreate
  template:
    metadata:
      labels:
        app: snapshot-controller
    spec:
      serviceAccountName: snapshot-controller-runner
      containers:
        - name: snapshot-controller
          image: "registry.redhat.io/openshift3/snapshot-controller:latest"
          imagePullPolicy: "IfNotPresent"
          args: ["-cloudprovider", "gce"]
        - name: snapshot-provisioner
          image: "registry.redhat.io/openshift3/snapshot-provisioner:latest"
          imagePullPolicy: "IfNotPresent"
          args: ["-cloudprovider", "gce"]

Similar to how a persistent volume claim (PVC) binds to a persistent volume (PV) to provision a volume, VolumeSnapshotData and VolumeSnapshot are used to create a volume snapshot.

Volume snapshots must use a supported PersistentVolume type.

apiVersion: volumesnapshot.external-storage.k8s.io/v1
kind: VolumeSnapshot 1
metadata:
  name: snapshot-demo
spec:
  persistentVolumeClaimName: ebs-pvc 2

$ oc get volumesnapshot -o yaml

apiVersion: volumesnapshot.external-storage.k8s.io/v1
kind: VolumeSnapshot
metadata:
  clusterName: ""
  creationTimestamp: 2017-09-19T13:58:28Z
  generation: 0
  labels:
    Timestamp: "1505829508178510973"
  name: snapshot-demo
  namespace: default 1
  resourceVersion: "780"
  selfLink: /apis/volumesnapshot.external-storage.k8s.io/v1/namespaces/default/volumesnapshots/snapshot-demo
  uid: 9cc5da57-9d42-11e7-9b25-90b11c132b3f
spec:
  persistentVolumeClaimName: ebs-pvc
  snapshotDataName: k8s-volume-snapshot-9cc8813e-9d42-11e7-8bed-90b11c132b3f
status:
  conditions:
  - lastTransitionTime: null
    message: Snapshot created successfully
    reason: ""
    status: "True"
    type: Ready
  creationTimestamp: null

$ oc delete volumesnapshot/<snapshot-name>

The following resource shows the parameters and default values that you use to configure a StorageClass. This example uses the AWS ElasticBlockStore (EBS) object definition.

kind: StorageClass 1
apiVersion: storage.k8s.io/v1 2
metadata:
  name: gp2 3
  annotations: 4
    storageclass.kubernetes.io/is-default-class: 'true'
    ...
provisioner: kubernetes.io/aws-ebs 5
parameters: 6
  type: gp2
...

To set a StorageClass as the cluster-wide default, add the following annotation to your StorageClass’s metadata:

storageclass.kubernetes.io/is-default-class: "true"

For example:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  annotations:
    storageclass.kubernetes.io/is-default-class: "true"
...

This enables any Persistent Volume Claim (PVC) that does not specify a specific volume to automatically be provisioned through the default StorageClass.

To set a StorageClass description, add the following annotation to your StorageClass’s metadata:

kubernetes.io/description: My StorageClass Description

For example:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  annotations:
    kubernetes.io/description: My StorageClass Description
...