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).
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:1.13-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

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.