csi-ephemeral-inline-volumes: introduce CSI support for ephemeral inl…

…ine volumes

This was alpha in Kubernetes 1.15 and became beta in 1.16. Several CSI
drivers already support it (soon...).

content/en/blog/_posts/2019-10-20-csi-ephemeral-inline-volumes.md

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+---
+title: CSI Ephemeral Inline Volumes
+date: 2019-10-20
+---
+
+**Author:** Patrick Ohly (Intel)
+
+Typically, volumes provided by an external storage driver in
+Kubernetes are *persistent*, with a lifecycle that is completely
+independent of pods or (as a special case) loosely coupled to the
+first pod which uses a volume ([late binding
+mode](https://kubernetes.io/docs/concepts/storage/storage-classes/#volume-binding-mode)).
+The mechanism for requesting and defining such volumes in Kubernetes
+are [Persistent Volume Claim (PVC) and Persistent Volume
+(PV)](https://kubernetes.io/docs/concepts/storage/persistent-volumes/)
+objects. Originally, volumes that are backed by a Container Storage Interface
+(CSI) driver could only be used via this PVC/PV mechanism.
+
+But there are also use cases for data volumes whose content and
+lifecycle is tied to a pod. For example, a driver might populate a
+volume with dynamically created secrets that are specific to the
+application running in the pod. Such volumes need to be created
+together with a pod and can be deleted as part of pod termination
+(*ephemeral*). They get defined as part of the pod spec (*inline*).
+
+Since Kubernetes 1.15, CSI drivers can also be used for such
+*ephemeral inline* volumes. The [CSIInlineVolume feature
+gate](https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/)
+had to be set to enable it in 1.15 because support was still in alpha
+state. In 1.16, the feature reached beta state, which typically means
+that it is enabled in clusters by default.
+
+CSI drivers have to be adapted to support this because although an
+existing CSI gRPC call (`NodePublishVolume`) is used, the way how it
+is used is different and not covered by the CSI spec: for ephemeral
+volumes, only that call is invoked by `kubelet` and all other calls
+(like `CreateVolume`, `NodeStageVolume`, etc.) are skipped. The volume
+parameters are provided in the pod spec and from there copied into the
+`NodePublishVolumeRequest.volume_context` field. There are currently
+no standardized parameters; even common ones like size must be
+provided in a format that is defined by the CSI driver.
+
+Initially, the assumption was that CSI drivers would be specifically
+written to provide either persistent or ephemeral volumes. But there
+are also drivers which provide storage that is useful in both modes:
+for example, [PMEM-CSI](https://github.com/intel/pmem-csi) manages
+persistent memory (PMEM), a new kind of local storage that is provided
+by [Intel® Optane™ DC Persistent
+Memory](https://www.intel.com/content/www/us/en/architecture-and-technology/optane-dc-persistent-memory.html). Such
+storage is useful as persistent data storage as well as ephemeral
+scratch space.
+
+Therefore the support in Kubernetes 1.16 was extended:
+* Kubernetes and users can determine which kind of volumes a driver
+  supports via the `volumeLifecycleModes` field in the [`CSIDriver`
+  object](https://kubernetes-csi.github.io/docs/csi-driver-object.html#what-fields-does-the-csidriver-object-have).
+* Drivers can get information about the volume mode by enabling the
+  ["pod info on
+  mount"](https://kubernetes-csi.github.io/docs/pod-info.html) feature
+  which then will add the new `csi.storage.k8s.io/ephemeral` entry to
+  the `NodePublishRequest.volume_context`.
+
+For more information about implementing support of ephemeral inline
+volumes in a CSI driver, see the [Kubernetes-CSI
+documentation](https://kubernetes-csi.github.io/docs/ephemeral-local-volumes.html)
+and the [original design
+document](https://github.com/kubernetes/enhancements/blob/master/keps/sig-storage/20190122-csi-inline-volumes.md). What
+follows in this blog post are usage examples based on real drivers
+that support such volumes.
+
+## [PMEM-CSI](https://github.com/intel/pmem-csi)
+
+## [Image Populator](https://github.com/kubernetes-csi/csi-driver-image-populator)
+
+## cert-manager