The Container Storage Interface (CSI) allows OpenShift Container Platform to consume storage from storage back ends that implement the CSI interface as persistent storage.

CSI Architecture

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 back end 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.

Architecture of CSI components

It is possible to run multiple CSI drivers for different storage back ends. Each driver needs its own external controllers deployment and daemon set with the driver and CSI registrar.

External CSI controllers

External CSI Controllers is a deployment that deploys one or more pods with three containers:

  • An external CSI attacher container translates attach and detach calls from OpenShift Container Platform to respective ControllerPublish and ControllerUnpublish calls to the CSI driver.

  • An external CSI provisioner container that translates provision and delete calls from OpenShift Container Platform to respective CreateVolume and DeleteVolume calls to the CSI driver.

  • A CSI driver container

The CSI attacher and CSI provisioner containers communicate with the CSI driver container using UNIX Domain Sockets, ensuring that no CSI communication leaves the pod. The CSI driver is not accessible from outside of the pod.

attach, detach, provision, and delete operations typically require the CSI driver to use credentials to the storage backend. Run the CSI controller pods on infrastructure nodes so the credentials are never leaked to user processes, even in the event of a catastrophic security breach on a compute node.

The external attacher must also run for CSI drivers that do not support third-party attach or detach operations. The external attacher will not issue any ControllerPublish or ControllerUnpublish operations to the CSI driver. However, it still must run to implement the necessary OpenShift Container Platform attachment API.

CSI driver daemon set

The CSI driver daemon set runs a pod on every node that allows OpenShift Container Platform to mount storage provided by the CSI driver to the node and use it in user workloads (pods) as persistent volumes (PVs). The pod with the CSI driver installed contains the following containers:

  • A CSI driver registrar, which registers the CSI driver into the openshift-node service running on the node. The openshift-node process running on the node then directly connects with the CSI driver using the UNIX Domain Socket available on the node.

  • A CSI driver.

The CSI driver deployed on the node should have as few credentials to the storage back end as possible. OpenShift Container Platform will only use the node plug-in set of CSI calls such as NodePublish/NodeUnpublish and NodeStage/NodeUnstage, if these calls are implemented.

CSI drivers supported by OpenShift Container Platform

OpenShift Container Platform installs certain CSI drivers by default, giving users storage options that are not possible with in-tree volume plug-ins.

To create CSI-provisioned persistent volumes that mount to these supported storage assets, OpenShift Container Platform installs the necessary CSI driver Operator, the CSI driver, and the required storage class by default. For more details about the default namespace of the Operator and driver, see the documentation for the specific CSI Driver Operator.

The following table describes the CSI drivers that are installed with OpenShift Container Platform and which CSI features they support, such as volume snapshots, cloning, and resize.

Table 1. Supported CSI drivers and features in OpenShift Container Platform
CSI driver CSI volume snapshots CSI cloning CSI resize

AWS EBS (Tech Preview)

-

Google Cloud Platform (GCP) persistent disk (PD) (Tech Preview)

-

OpenStack Cinder

OpenStack Manila

Red Hat Virtualization (oVirt)

-

-

-

If your CSI driver is not listed in the preceding table, you must follow the installation instructions provided by your CSI storage vendor to use their supported CSI features.

Dynamic provisioning

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

The created storage class can be configured to enable dynamic provisioning.

Procedure
  • Create a default storage class that ensures all PVCs that do not require any special storage class are provisioned by the installed CSI driver.

    # oc create -f - << EOF
    apiVersion: storage.k8s.io/v1
    kind: StorageClass
    metadata:
      name: <storage-class> (1)
      annotations:
        storageclass.kubernetes.io/is-default-class: "true"
    provisioner: <provisioner-name> (2)
    parameters:
    EOF
    1 The name of the storage class that will be created.
    2 The name of the CSI driver that has been installed

Example using the CSI driver

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

Prerequisites
  • The CSI driver has been deployed.

  • A storage class has been created for dynamic provisioning.

Procedure
  • Create the MySQL template:

    # oc new-app mysql-persistent
    Example output
    --> Deploying template "openshift/mysql-persistent" to project default
    ...
    # oc get pvc
    Example output
    NAME              STATUS    VOLUME                                   CAPACITY
    ACCESS MODES   STORAGECLASS   AGE
    mysql             Bound     kubernetes-dynamic-pv-3271ffcb4e1811e8   1Gi
    RWO            cinder         3s