CSI Ephemeral Inline Volumes

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). The mechanism for requesting and defining such volumes in Kubernetes are Persistent Volume Claim (PVC) and Persistent Volume (PV) 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 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 manages persistent memory (PMEM), a new kind of local storage that is provided by Intel® Optane™ DC Persistent Memory. 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.
• Drivers can get information about the volume mode by enabling the "pod info on mount" 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 and the original design document.

What follows in this blog post are usage examples based on real drivers and a summary at the end.

Examples

PMEM-CSI

Support for ephemeral inline volumes was added in release v0.6.0. The driver can be used on hosts with real Intel® Optane™ DC Persistent Memory, on special machines in GCE or with hardware emulated by QEMU. The latter is fully integrated into the makefile and only needs Go, Docker and KVM, so that approach was used for this example:

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:

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

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

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:

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:

kubectl create -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: nginx
    image: nginx:1.16-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

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

cert-manager-csi works together with 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 is an attempt to address this problem.

A related KEP introduces a standardized size parameter.

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 and can be reached via Slack and a mailing list.

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