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blog: introduce CSI support for ephemeral inline volumes (kubernetes#…
…16832) * csi-ephemeral-inline-volumes: introduce CSI support for ephemeral inline volumes This was alpha in Kubernetes 1.15 and became beta in 1.16. Several CSI drivers already support it (soon...). * csi-ephemeral-inline-volumes: bump date and address feedback (NodeUnpublishVolume) * csi-ephemeral-inline-volumes: add examples and next steps * csi-ephemeral-inline-volumes: rename file, minor edits * csi-ephemeral-inline-volumes: include Docker example
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content/en/blog/_posts/2020-01-21-csi-ephemeral-inline-volumes.md
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| --- | ||
| title: CSI Ephemeral Inline Volumes | ||
| date: 2020-01-21 | ||
| --- | ||
|
|
||
| **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 two | ||
| existing CSI gRPC calls are used (`NodePublishVolume` and `NodeUnpublishVolume`), | ||
| the way how they are | ||
| used is different and not covered by the CSI spec: for ephemeral | ||
| volumes, only `NodePublishVolume` is invoked by `kubelet` when asking | ||
| the CSI driver for a volume. 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. Likewise, only | ||
| `NodeUnpublishVolume` gets called after the pod has terminated and the | ||
| volume needs to be removed. | ||
|
|
||
| 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 | ||
| memory is useful both as persistent data storage (faster than normal SSDs) | ||
| and as ephemeral scratch space (higher capacity than DRAM). | ||
|
|
||
| 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 | ||
| and a summary at the end. | ||
|
|
||
| # Examples | ||
|
|
||
| ## [PMEM-CSI](https://github.com/intel/pmem-csi) | ||
|
|
||
| Support for ephemeral inline volumes was added in [release | ||
| v0.6.0](https://github.com/intel/pmem-csi/releases/tag/v0.6.0). The | ||
| driver can be used on hosts with real Intel® Optane™ DC Persistent | ||
| Memory, on [special machines in | ||
| GCE](https://github.com/intel/pmem-csi/blob/v0.6.0/examples/gce.md) or | ||
| with hardware emulated by QEMU. The latter is fully [integrated into | ||
| the | ||
| makefile](https://github.com/intel/pmem-csi/tree/v0.6.0#qemu-and-kubernetes) | ||
| and only needs Go, Docker and KVM, so that approach was used for this | ||
| example: | ||
|
|
||
| ```sh | ||
| git clone --branch release-0.6 https://github.com/intel/pmem-csi | ||
| cd pmem-csi | ||
| TEST_DISTRO=clear TEST_DISTRO_VERSION=32080 TEST_PMEM_REGISTRY=intel make start | ||
| ``` | ||
|
|
||
| Bringing up the four-node cluster can take a while but eventually should end with: | ||
|
|
||
| ``` | ||
| The test cluster is ready. Log in with /work/pmem-csi/_work/pmem-govm/ssh-pmem-govm, run kubectl once logged in. | ||
| Alternatively, KUBECONFIG=/work/pmem-csi/_work/pmem-govm/kube.config can also be used directly. | ||
| To try out the pmem-csi driver persistent volumes: | ||
| ... | ||
| To try out the pmem-csi driver ephemeral volumes: | ||
| cat deploy/kubernetes-1.17/pmem-app-ephemeral.yaml | /work/pmem-csi/_work/pmem-govm/ssh-pmem-govm kubectl create -f - | ||
| ``` | ||
|
|
||
| `deploy/kubernetes-1.17/pmem-app-ephemeral.yaml` specifies one volume: | ||
|
|
||
| ``` | ||
| kind: Pod | ||
| apiVersion: v1 | ||
| metadata: | ||
| name: my-csi-app-inline-volume | ||
| spec: | ||
| containers: | ||
| - name: my-frontend | ||
| image: busybox | ||
| command: [ "sleep", "100000" ] | ||
| volumeMounts: | ||
| - mountPath: "/data" | ||
| name: my-csi-volume | ||
| volumes: | ||
| - name: my-csi-volume | ||
| csi: | ||
| driver: pmem-csi.intel.com | ||
| fsType: "xfs" | ||
| volumeAttributes: | ||
| size: "2Gi" | ||
| nsmode: "fsdax" | ||
| ``` | ||
|
|
||
| Once we have created that pod, we can inspect the result: | ||
|
|
||
| ```sh | ||
| kubectl describe pods/my-csi-app-inline-volume | ||
| ``` | ||
|
|
||
| ``` | ||
| Name: my-csi-app-inline-volume | ||
| ... | ||
| Volumes: | ||
| my-csi-volume: | ||
| Type: CSI (a Container Storage Interface (CSI) volume source) | ||
| Driver: pmem-csi.intel.com | ||
| FSType: xfs | ||
| ReadOnly: false | ||
| VolumeAttributes: nsmode=fsdax | ||
| size=2Gi | ||
| ``` | ||
|
|
||
| ```sh | ||
| kubectl exec my-csi-app-inline-volume -- df -h /data | ||
| ``` | ||
|
|
||
| ``` | ||
| Filesystem Size Used Available Use% Mounted on | ||
| /dev/ndbus0region0fsdax/d7eb073f2ab1937b88531fce28e19aa385e93696 | ||
| 1.9G 34.2M 1.8G 2% /data | ||
| ``` | ||
|
|
||
|
|
||
| ## [Image Populator](https://github.com/kubernetes-csi/csi-driver-image-populator) | ||
|
|
||
| The image populator automatically unpacks a container image and makes | ||
| its content available as an ephemeral volume. It's still in | ||
| development, but canary images are already available which can be | ||
| installed with: | ||
|
|
||
| ```sh | ||
| kubectl create -f https://github.com/kubernetes-csi/csi-driver-image-populator/raw/master/deploy/kubernetes-1.16/csi-image-csidriverinfo.yaml | ||
| kubectl create -f https://github.com/kubernetes-csi/csi-driver-image-populator/raw/master/deploy/kubernetes-1.16/csi-image-daemonset.yaml | ||
| ``` | ||
|
|
||
| This example pod will run nginx and have it serve data that | ||
| comes from the `kfox1111/misc:test` image: | ||
|
|
||
| ```sh | ||
| kubectl create -f - <<EOF | ||
| apiVersion: v1 | ||
| kind: Pod | ||
| metadata: | ||
| name: nginx | ||
| spec: | ||
| containers: | ||
| - name: nginx | ||
| image: nginx:1.13-alpine | ||
| ports: | ||
| - containerPort: 80 | ||
| volumeMounts: | ||
| - name: data | ||
| mountPath: /usr/share/nginx/html | ||
| volumes: | ||
| - name: data | ||
| csi: | ||
| driver: image.csi.k8s.io | ||
| volumeAttributes: | ||
| image: kfox1111/misc:test | ||
| EOF | ||
| ``` | ||
|
|
||
| ```sh | ||
| kubectl exec nginx -- cat /usr/share/nginx/html/test | ||
| ``` | ||
|
|
||
| That `test` file just contains a single word: | ||
| ``` | ||
| testing | ||
| ``` | ||
|
|
||
| Such data containers can be built with Dockerfiles such as: | ||
| ``` | ||
| FROM scratch | ||
| COPY index.html /index.html | ||
| ``` | ||
|
|
||
| ## [cert-manager-csi](https://github.com/jetstack/cert-manager-csi) | ||
|
|
||
| cert-manager-csi works together with | ||
| [cert-manager](https://github.com/jetstack/cert-manager). The goal for | ||
| this driver is to facilitate requesting and mounting certificate key | ||
| pairs to pods seamlessly. This is useful for facilitating mTLS, or | ||
| otherwise securing connections of pods with guaranteed present | ||
| certificates whilst having all of the features that cert-manager | ||
| provides. This project is experimental. | ||
|
|
||
|
|
||
| # Next steps | ||
|
|
||
| One of the issues with ephemeral inline volumes is that pods get | ||
| scheduled by Kubernetes onto nodes without knowing anything about the | ||
| currently available storage on that node. Once the pod has been | ||
| scheduled, the CSI driver must make the volume available one that | ||
| node. If that is currently not possible, the pod cannot start. This | ||
| will be retried until eventually the volume becomes ready. The | ||
| [storage capacity tracking | ||
| KEP](https://github.com/kubernetes/enhancements/pull/1353) is an | ||
| attempt to address this problem. | ||
|
|
||
| A related KEP introduces a [standardized size | ||
| parameter](https://github.com/kubernetes/enhancements/pull/1409). | ||
|
|
||
| Currently, CSI ephemeral inline volumes stay in beta while issues like | ||
| these are getting discussed. Your feedback is needed to decide how to | ||
| proceed with this feature. For the KEPs, the two PRs linked to above | ||
| is a good place to comment. The SIG Storage also [meets | ||
| regularly](https://github.com/kubernetes/community/tree/master/sig-storage#meetings) | ||
| and can be reached via [Slack and a mailing | ||
| list](https://github.com/kubernetes/community/tree/master/sig-storage#contact). |