CSI volume snapshots - Using Container Storage Interface (CSI) | Storage

Overview of CSI volume snapshots
CSI snapshot controller and sidecar
- External controller
- External sidecar
About the CSI Snapshot Controller Operator
- Volume snapshot CRDs
Volume snapshot provisioning
- Dynamic provisioning
- Manual provisioning
Creating a volume snapshot
Deleting a volume snapshot
Restoring a volume snapshot

This document describes how to use volume snapshots with supported Container Storage Interface (CSI) drivers to help protect against data loss in OpenShift Container Platform. Familiarity with persistent volumes is suggested.

CSI volume snapshot is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see https://access.redhat.com/support/offerings/techpreview/.

Overview of CSI volume snapshots

A snapshot represents the state of the storage volume in a cluster at a particular point in time. Volume snapshots can be used to provision a new volume.

OpenShift Container Platform supports CSI volume snapshots by default. However, a specific CSI driver is required.

With CSI volume snapshots, a cluster administrator can:

Deploy a third-party CSI driver that supports snapshots.
Create a new persistent volume claim (PVC) from an existing volume snapshot.
Take a snapshot of an existing PVC.
Restore a snapshot as a different PVC.
Delete an existing volume snapshot.

With CSI volume snapshots, an app developer can:

Use volume snapshots as building blocks for developing application- or cluster-level storage backup solutions.
Rapidly rollback to a previous development version.
Use storage more efficiently by not having to make a full copy each time.

Be aware of the following when using volume snapshots:

Support is only available for CSI drivers. In-tree and FlexVolumes are not supported.
OpenShift Container Platform does not ship with any CSI drivers. It is recommended to use the CSI drivers provided by community or storage vendors. Follow the installation instructions provided by the CSI driver.
CSI drivers may or may not have implemented the volume snapshot functionality. CSI drivers that have provided support for volume snapshots will likely use the csi-external-snapshotter sidecar. See documentation provided by the CSI driver for details.
OpenShift Container Platform 4.4 supports version 1.1.0 of the CSI specification.

CSI snapshot controller and sidecar

OpenShift Container Platform provides a snapshot controller that is deployed into the control plane. In addition, your CSI driver vendor provides the CSI snapshot sidecar as a helper container that is installed during the CSI driver installation.

The CSI snapshot controller and sidecar provide volume snapshotting through the OpenShift Container Platform API. These external components run in the cluster.

The external controller is deployed by the CSI Snapshot Controller Operator.

External controller

The CSI snapshot controller binds VolumeSnapshot and VolumeSnapshotContent objects. The controller manages dynamic provisioning by creating and deleting VolumeSnapshotContent objects.

External sidecar

Your CSI driver vendor provides the csi-external-snapshotter sidecar. This is a separate helper container that is deployed with the CSI driver. The sidecar manages snapshots by triggering CreateSnapshot and DeleteSnapshot operations. Follow the installation instructions provided by your vendor.

About the CSI Snapshot Controller Operator

The CSI Snapshot Controller Operator runs in the openshift-cluster-storage-operator namespace. It is installed by the Cluster Version Operator (CVO) in all clusters by default.

The CSI Snapshot Controller Operator installs the CSI snapshot controller, which runs in the csi-snapshot-controller namespace.

Volume snapshot CRDs

During OpenShift Container Platform installation, the CSI Snapshot Controller Operator creates the following snapshot custom resource definitions (CRDs) in the snapshot.storage.k8s.io/ API group:

VolumeSnapshotContent: A snapshot taken of a volume in the cluster that has been provisioned by a cluster administrator.

Similar to the PersistentVolume CRD, the VolumeSnapshotContent CRD is a cluster resource that points to a real snapshot in the storage back end.

For manually pre-provisioned snapshots, a cluster administrator creates a number of VolumeSnapshotContent objects. These carry the details of the real volume snapshot in the storage system.

The VolumeSnapshotContent CRD is not namespaced and is for use by a cluster administrator.
VolumeSnapshot: Similar to the PersistentVolumeClaim CRD, the VolumeSnapshot CRD defines a developer request for a snapshot. The CSI Snapshot Controller Operator runs the CSI snapshot controller, which handles the binding of a VolumeSnapshot object with an appropriate VolumeSnapshotContent object. The binding is a one-to-one mapping.

The VolumeSnapshot CRD is namespaced. A developer uses the CRD as a distinct request for a snapshot.
VolumeSnapshotClass: Allows a cluster administrator to specify different attributes belonging to a VolumeSnapshot object. These attributes may differ among snapshots taken of the same volume on the storage system, in which case they would not be expressed by using the same storage class of a persistent volume claim.

The VolumeSnapshotClass CRD defines the parameters for the csi-external-snapshotter sidecar to use when creating a snapshot. This allows the storage back end to know what kind of snapshot to dynamically create if multiple options are supported.

Dynamically provisioned snapshots use the VolumeSnapshotClass CRD to specify storage-provider-specific parameters to use when creating a snapshot.

The VolumeSnapshotContentClass CRD is not namespaced and is for use by a cluster administrator to enable global configuration options for their storage back end.

Volume snapshot provisioning

There are two ways to provision snapshots: dynamically and manually.

Dynamic provisioning

Instead of using a preexisting snapshot, you can request that a snapshot be taken dynamically from a persistent volume claim. Parameters are specified using a VolumeSnapshotClass CRD.

Manual provisioning

As a cluster administrator, you can manually pre-provision a number of VolumeSnapshotContent objects. These carry the real volume snapshot details available to cluster users.

Creating a volume snapshot

When you create a VolumeSnapshot object, OpenShift Container Platform creates a volume snapshot.

Prerequisites

Logged in to a running OpenShift Container Platform cluster.
A PVC created using a CSI driver that supports VolumeSnapshot objects.
A storage class to provision the storage back end.

No pods are using the persistent volume claim (PVC) that you want to take a snapshot of.

Do not create a volume snapshot of a PVC if a pod is using it. Doing so might cause data corruption because the PVC is not quiesced (paused). Be sure to first tear down a running pod to ensure consistent snapshots.

Procedure

To dynamically create a volume snapshot:

Create a file with the VolumeSnapshotClass object described by the following YAML:
volumesnapshotclass.yaml
```
apiVersion: snapshot.storage.k8s.io
kind: VolumeSnapshotClass (1)
metadata:
  name: csi-hostpath-snap
driver: hostpath.csi.k8s.io
deletionPolicy: Delete
```
1 Allows you to specify different attributes belonging to a volume snapshot.
Create the object you saved in the previous step by entering the following command:
```
$ oc create -f volumesnapshotclass.yaml
```

Create a VolumeSnapshot object:

volumesnapshot-dynamic.yaml

apiVersion: snapshot.storage.k8s.io
kind: VolumeSnapshot
metadata:
  name: mysnap
spec:
  volumeSnapshotClassName: csi-hostpath-snap (1)
  source:
    persistentVolumeClaimName: myclaim (2)

1	The request for a particular class by the volume snapshot. If `volumeSnapshotClassName` is empty, then no snapshot is created.
2	The name of the `PersistentVolumeClaim` object bound to a persistent volume. This defines what you want to create a snapshot of. Required for dynamically provisioning a snapshot.

Create the object you saved in the previous step by entering the following command:
```
$ oc create -f volumesnapshot-dynamic.yaml
```

To manually provision a snapshot:

Provide a value for the volumeSnapshotContentName parameter as the source for the snapshot, in addition to defining volume snapshot class as shown above.
volumesnapshot-manual.yaml
```
apiVersion: snapshot.storage.k8s.io
kind: VolumeSnapshot
metadata:
  name: snapshot-demo
spec:
  source:
    volumeSnapshotContentName: mycontent (1)
```
1 The volumeSnapshotContentName parameter is required for pre-provisioned snapshots.
Create the object you saved in the previous step by entering the following command:
```
$ oc create -f volumesnapshot-manual.yaml
```

Verification steps

After the snapshot has been created in the cluster, additional details about the snapshot are available.

To display details about the volume snapshot that was created, enter the following command:

$ oc describe volumesnapshot mysnap

The following example displays details about the mysnap volume snapshot:

volumesnapshot.yaml

apiVersion: snapshot.storage.k8s.io
kind: VolumeSnapshot
metadata:
  name: mysnap
spec:
  source:
    persistentVolumeClaimName: myclaim
  volumeSnapshotClassName: csi-hostpath-snap
status:
  boundVolumeSnapshotContentName: snapcontent-1af4989e-a365-4286-96f8-d5dcd65d78d6 (1)
  creationTime: "2020-01-29T12:24:30Z" (2)
  readyToUse: true (3)
  restoreSize: 500Mi

1	The pointer to the actual storage content that was created by the controller.
2	The time when the snapshot was created. The snapshot contains the volume content that was available at this indicated time.
3	If the value is set to `true`, the snapshot can be used to restore as a new PVC. If the value is set to `false`, the snapshot was created. However, the storage back end needs to perform additional tasks to make the snapshot usable so that it can be restored as a new volume. For example, Amazon Elastic Block Store data might be moved to a different, less expensive location, which can take several minutes.

To verify that the volume snapshot was created, enter the following command:
```
$ oc get volumesnapshotcontent
```
The pointer to the actual content is displayed. If the boundVolumeSnapshotContentName field is populated, a VolumeSnapshotContent object exists and the snapshot was created.
To verify that the snapshot is ready, confirm that the VolumeSnapshot object has readyToUse: true.

Deleting a volume snapshot

You can configure how OpenShift Container Platform deletes volume snapshots.

Procedure

To enable deletion of a volume snapshot in a cluster:

Specify the deletion policy that you require in the VolumeSnapshotClass object, as shown in the following example:

volumesnapshot.yaml

apiVersion: snapshot.storage.k8s.io
kind: VolumeSnapshotClass
metadata:
  name: csi-hostpath-snap
driver: hostpath.csi.k8s.io
deletionPolicy: Delete (1)

1 If the Delete value is set, the underlying snapshot will be deleted, along with the VolumeSnapshotContent object. If the Retain value is set, both the underlying snapshot and VolumeSnapshotContent object remain.
If the Retain value is set, and the VolumeSnapshot object is deleted without deleting the corresponding VolumeSnapshotContent object, then the content will remain. The snapshot itself is also retained in the storage back end.

Restoring a volume snapshot

After your VolumeSnapshot object is bound, you can use that object to provision a new volume that is pre-populated with data from the snapshot.

The volume snapshot content object is used to restore the existing volume to a previous state.

Prerequisites

Logged in to a running OpenShift Container Platform cluster.
A persistent volume claim (PVC) created using a Container Storage Interface (CSI) driver that supports volume snapshots.
A storage class to provision the storage back end.

Procedure

Specify a VolumeSnapshot data source on a PVC as shown in the following:

pvc-restore.yaml

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: myclaim-restore
spec:
  storageClassName: csi-hostpath-sc
  dataSource:
    name: mysnap (1)
    kind: VolumeSnapshot (2)
    apiGroup: snapshot.storage.k8s.io (3)
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

1	Name of the `VolumeSnapshot` object representing the snapshot to use as source.
2	Must be set to the `VolumeSnapshot` value.
3	Must be set to the `snapshot.storage.k8s.io` value.

Create a PVC by entering the following command:
```
$ oc create -f pvc-restore.yaml
```
Verify that the restored PVC has been created by entering the following command:
```
$ oc get pvc
```
Two different PVCs are displayed.