Note: This chapter discusses various storage concepts related to the Dell EMC Ready Stack for Red Hat OpenShift 4.3, including the use of generic NFS and iSCSI for persistent storage and the use of the Container Storage Interface (CSI) and related CSI drivers for Dell EMC storage products. The CSI information in this guide is provided for background and planning purposes. At the time of publication, the CSI drivers that support OpenShift Container Platform 4.3 were not available. We therefore performed the laboratory and validation work for this release using Dell EMC Unity XT380-based generic NFS and generic iSCSI. The reflects that choice. The CSI drivers that support OpenShift Container Platform 4.3 are expected to be released in the near future, at which time we will provide guidance for using CSI-based storage.
Stateful applications create a demand for persistent storage. All storage within OpenShift Container Platform 4.3 is managed separately from compute (worker node) resources and from all networking and connectivity infrastructure facilities. The CSI API is designed to abstract storage use and enable storage portability.
The following Kubernetes storage concepts apply to this solution:
These resources are logical constructs that are used within the Kubernetes container infrastructure to maintain storage for all the components of the container ecosystem that depend on storage. Developers and operators can deploy applications and provision or de-provision persistent storage without having any specific technical knowledge of the underlying storage technology.
The OpenShift Container Platform administrator is responsible for provisioning storage classes and making them available to the cluster’s tenants.
Storage using PVCs is consumed or used in two ways: statically or dynamically. Static storage can be attached to one or more pods by static assignment of a PV to a PVC and then to a specific pod or pods.
Static persistent storage provisioning involves an administrator preprovisioning PVs to be used by Kubernetes tenants. When a user makes a persistent storage request by creating a PVC, Kubernetes finds the closest matching available PV. Static provisioning is not the most efficient method for using storage, but it might be preferred when it is necessary to restrict users from PV provisioning.
The following figure illustrates the static storage provisioning workflow in this solution:
Dynamic persistent storage provisioning, the most flexible provisioning method, enables Kubernetes users to secure PV provisioning on demand. Dynamic provisioning has fully automated LUN export provisioning.
The following figure shows the dynamic storage provisioning workflow in this solution:
After a PV is bound to a PVC, that PV cannot be bound to another PVC. This restriction binds the PV to a single namespace, that of the binding project. A PV that has been created for dynamic use is a storage class object that functions as, and is automatically consumed as, a cluster resource.
OpenShift Container Platform natively supports the following PV types:
The CSI API extends the storage types that can be used within an OpenShift Container Platform solution.
Each PV has a predetermined storage capacity that is set in its capacity definition parameter. The storage capacity can be set or requested by a pod that is launched within the container platform. Expect the choice of control parameters to expand as the CSI API is extended and as it matures.
A resource provider can determine how the PV is created and can set the storage control parameters. Access mode support is specific to the type of storage volume that is provisioned as a PV. Provider capabilities determine the PV’s access modes, while the capabilities of each PV determine the modes which that volume supports. For example, NFS can support multiple read/write clients, but a specific NFS PV might be configured as read-only.
Pod claims are matched to volumes with compatible access modes based on two matching criteria: access modes and size. A pod claim’s access modes represent a request.