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Stateful applications create a demand for persistent storage. All storage within OpenShift Container Platform 4.10 is managed separately from compute resources and from all networking and connectivity infrastructure facilities. The CSI API is designed to abstract storage use and enable storage portability.
This solution applies the following Kubernetes storage concepts:
These resources are logical constructs that the Kubernetes container infrastructure uses to maintain storage for all the container ecosystem components that depend on storage. Developers and operators can deploy applications and provision or deprovision 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 tenants of the cluster.
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.
With static persistent storage provisioning, an administrator preprovisions PVs for 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 shows 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 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 parameter. The storage capacity is 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 matures.
A resource provider can determine how the PV is created and 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 access modes of the PV, 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.
The use of generic NFS or generic iSCSI is functional and stable. However, NFS and iSCSI do not contain a mechanism to provide service continuity if access to the storage subsystem fails, and generic NFS and iSCSI do not provide the advanced storage protection support that is available using CSI drivers. The Dell storage engineering team validated the functionality and capability of suitable storage drivers, as described in the following table:
Storage type | ReadWriteOnce | ReadOnlyMany | ReadWriteMany |
HostPath (local disk) | Yes | N/A | N/A |
iSCSI (generic) | Yes | Yes | N/A |
NFS (generic) | Yes | Yes | Yes |