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Ephemeral volumes with storage capacity tracking: EmptyDir on steroids

Tuesday, September 01, 2020

Author: Patrick Ohly (Intel)

Some applications need additional storage but don't care whether that data is stored persistently across restarts. For example, caching services are often limited by memory size and can move infrequently used data into storage that is slower than memory with little impact on overall performance. Other applications expect some read-only input data to be present in files, like configuration data or secret keys.

Kubernetes already supports several kinds of such ephemeral volumes, but the functionality of those is limited to what is implemented inside Kubernetes.

CSI ephemeral volumes made it possible to extend Kubernetes with CSI drivers that provide light-weight, local volumes. These inject arbitrary states, such as configuration, secrets, identity, variables or similar information. CSI drivers must be modified to support this Kubernetes feature, i.e. normal, standard-compliant CSI drivers will not work, and by design such volumes are supposed to be usable on whatever node is chosen for a pod.

This is problematic for volumes which consume significant resources on a node or for special storage that is only available on some nodes. Therefore, Kubernetes 1.19 introduces two new alpha features for volumes that are conceptually more like the EmptyDir volumes:

The advantages of the new approach are:

Storage can be local or network-attached.
Volumes can have a fixed size that applications are never able to exceed.
Works with any CSI driver that supports provisioning of persistent volumes and (for capacity tracking) implements the CSI GetCapacity call.
Volumes may have some initial data, depending on the driver and parameters.
All of the typical volume operations (snapshotting, resizing, the future storage capacity tracking, etc.) are supported.
The volumes are usable with any app controller that accepts a Pod or volume specification.
The Kubernetes scheduler itself picks suitable nodes, i.e. there is no need anymore to implement and configure scheduler extenders and mutating webhooks.

This makes generic ephemeral volumes a suitable solution for several use cases:

Use cases

Persistent Memory as DRAM replacement for memcached

Recent releases of memcached added support for using Persistent Memory (PMEM) instead of standard DRAM. When deploying memcached through one of the app controllers, generic ephemeral volumes make it possible to request a PMEM volume of a certain size from a CSI driver like PMEM-CSI.

Local LVM storage as scratch space

Applications working with data sets that exceed the RAM size can request local storage with performance characteristics or size that is not met by the normal Kubernetes EmptyDir volumes. For example, TopoLVM was written for that purpose.

Read-only access to volumes with data

Provisioning a volume might result in a non-empty volume:

Such volumes can be mounted read-only.

How it works

Generic ephemeral volumes

The key idea behind generic ephemeral volumes is that a new volume source, the so-called EphemeralVolumeSource contains all fields that are needed to created a volume claim (historically called persistent volume claim, PVC). A new controller in the kube-controller-manager waits for Pods which embed such a volume source and then creates a PVC for that pod. To a CSI driver deployment, that PVC looks like any other, so no special support is needed.

As long as these PVCs exist, they can be used like any other volume claim. In particular, they can be referenced as data source in volume cloning or snapshotting. The PVC object also holds the current status of the volume.

Naming of the automatically created PVCs is deterministic: the name is a combination of Pod name and volume name, with a hyphen (-) in the middle. This deterministic naming makes it easier to interact with the PVC because one does not have to search for it once the Pod name and volume name are known. The downside is that the name might be in use already. This is detected by Kubernetes and then blocks Pod startup.

To ensure that the volume gets deleted together with the pod, the controller makes the Pod the owner of the volume claim. When the Pod gets deleted, the normal garbage-collection mechanism also removes the claim and thus the volume.

Claims select the storage driver through the normal storage class mechanism. Although storage classes with both immediate and late binding (aka WaitForFirstConsumer) are supported, for ephemeral volumes it makes more sense to use WaitForFirstConsumer: then Pod scheduling can take into account both node utilization and availability of storage when choosing a node. This is where the other new feature comes in.

Storage capacity tracking

Normally, the Kubernetes scheduler has no information about where a CSI driver might be able to create a volume. It also has no way of talking directly to a CSI driver to retrieve that information. It therefore tries different nodes until it finds one where all volumes can be made available (late binding) or leaves it entirely to the driver to choose a location (immediate binding).

The new CSIStorageCapacity alpha API allows storing the necessary information in etcd where it is available to the scheduler. In contrast to support for generic ephemeral volumes, storage capacity tracking must be enabled when deploying a CSI driver: the external-provisioner must be told to publish capacity information that it then retrieves from the CSI driver through the normal GetCapacity call.

When the Kubernetes scheduler needs to choose a node for a Pod with an unbound volume that uses late binding and the CSI driver deployment has opted into the feature by setting the CSIDriver.storageCapacity flag flag, the scheduler automatically filters out nodes that do not have access to enough storage capacity. This works for generic ephemeral and persistent volumes but not for CSI ephemeral volumes because the parameters of those are opaque for Kubernetes.

As usual, volumes with immediate binding get created before scheduling pods, with their location chosen by the storage driver. Therefore, the external-provisioner's default configuration skips storage classes with immediate binding as the information wouldn't be used anyway.

Because the Kubernetes scheduler must act on potentially outdated information, it cannot be ensured that the capacity is still available when a volume is to be created. Still, the chances that it can be created without retries should be higher.

Security

CSIStorageCapacity

CSIStorageCapacity objects are namespaced. When deploying each CSI drivers in its own namespace and, as recommended, limiting the RBAC permissions for CSIStorageCapacity to that namespace, it is always obvious where the data came from. However, Kubernetes does not check that and typically drivers get installed in the same namespace anyway, so ultimately drivers are expected to behave and not publish incorrect data.

Generic ephemeral volumes

If users have permission to create a Pod (directly or indirectly), then they can also create generic ephemeral volumes even when they do not have permission to create a volume claim. That's because RBAC permission checks are applied to the controller which creates the PVC, not the original user. This is a fundamental change that must be taken into account before enabling the feature in clusters where untrusted users are not supposed to have permission to create volumes.

Example

A special branch in PMEM-CSI contains all the necessary changes to bring up a Kubernetes 1.19 cluster inside QEMU VMs with both alpha features enabled. The PMEM-CSI driver code is used unchanged, only the deployment was updated.

On a suitable machine (Linux, non-root user can use Docker - see the QEMU and Kubernetes section in the PMEM-CSI documentation), the following commands bring up a cluster and install the PMEM-CSI driver:

git clone --branch=kubernetes-1-19-blog-post https://github.com/intel/pmem-csi.git
cd pmem-csi
export TEST_KUBERNETES_VERSION=1.19 TEST_FEATURE_GATES=CSIStorageCapacity=true,GenericEphemeralVolume=true TEST_PMEM_REGISTRY=intel
make start && echo && test/setup-deployment.sh

If all goes well, the output contains the following usage instructions:

The test cluster is ready. Log in with [...]/pmem-csi/_work/pmem-govm/ssh.0, run
kubectl once logged in.  Alternatively, use kubectl directly with the
following env variable:
   KUBECONFIG=[...]/pmem-csi/_work/pmem-govm/kube.config

secret/pmem-csi-registry-secrets created
secret/pmem-csi-node-secrets created
serviceaccount/pmem-csi-controller created
...
To try out the pmem-csi driver ephemeral volumes:
   cat deploy/kubernetes-1.19/pmem-app-ephemeral.yaml |
   [...]/pmem-csi/_work/pmem-govm/ssh.0 kubectl create -f -

The CSIStorageCapacity objects are not meant to be human-readable, so some post-processing is needed. The following Golang template filters all objects by the storage class that the example uses and prints the name, topology and capacity:

kubectl get \
        -o go-template='{{range .items}}{{if eq .storageClassName "pmem-csi-sc-late-binding"}}{{.metadata.name}} {{.nodeTopology.matchLabels}} {{.capacity}}
{{end}}{{end}}' \
        csistoragecapacities

csisc-2js6n map[pmem-csi.intel.com/node:pmem-csi-pmem-govm-worker2] 30716Mi
csisc-sqdnt map[pmem-csi.intel.com/node:pmem-csi-pmem-govm-worker1] 30716Mi
csisc-ws4bv map[pmem-csi.intel.com/node:pmem-csi-pmem-govm-worker3] 30716Mi

One individual object has the following content:

kubectl describe csistoragecapacities/csisc-6cw8j

Name:         csisc-sqdnt
Namespace:    default
Labels:       <none>
Annotations:  <none>
API Version:  storage.k8s.io/v1alpha1
Capacity:     30716Mi
Kind:         CSIStorageCapacity
Metadata:
  Creation Timestamp:  2020-08-11T15:41:03Z
  Generate Name:       csisc-
  Managed Fields:
    ...
  Owner References:
    API Version:     apps/v1
    Controller:      true
    Kind:            StatefulSet
    Name:            pmem-csi-controller
    UID:             590237f9-1eb4-4208-b37b-5f7eab4597d1
  Resource Version:  2994
  Self Link:         /apis/storage.k8s.io/v1alpha1/namespaces/default/csistoragecapacities/csisc-sqdnt
  UID:               da36215b-3b9d-404a-a4c7-3f1c3502ab13
Node Topology:
  Match Labels:
    pmem-csi.intel.com/node:  pmem-csi-pmem-govm-worker1
Storage Class Name:           pmem-csi-sc-late-binding
Events:                       <none>

Now let's create the example app with one generic ephemeral volume. The pmem-app-ephemeral.yaml file contains:

# This example Pod definition demonstrates
# how to use generic ephemeral inline volumes
# with a PMEM-CSI storage class.
kind: Pod
apiVersion: v1
metadata:
  name: my-csi-app-inline-volume
spec:
  containers:
    - name: my-frontend
      image: intel/pmem-csi-driver-test:v0.7.14
      command: [ "sleep", "100000" ]
      volumeMounts:
      - mountPath: "/data"
        name: my-csi-volume
  volumes:
  - name: my-csi-volume
    ephemeral:
      volumeClaimTemplate:
        spec:
          accessModes:
          - ReadWriteOnce
          resources:
            requests:
              storage: 4Gi
          storageClassName: pmem-csi-sc-late-binding

After creating that as shown in the usage instructions above, we have one additional Pod and PVC:

kubectl get pods/my-csi-app-inline-volume -o wide

NAME                       READY   STATUS    RESTARTS   AGE     IP          NODE                         NOMINATED NODE   READINESS GATES
my-csi-app-inline-volume   1/1     Running   0          6m58s   10.36.0.2   pmem-csi-pmem-govm-worker1   <none>           <none>

kubectl get pvc/my-csi-app-inline-volume-my-csi-volume

NAME                                     STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS               AGE
my-csi-app-inline-volume-my-csi-volume   Bound    pvc-c11eb7ab-a4fa-46fe-b515-b366be908823   4Gi        RWO            pmem-csi-sc-late-binding   9m21s

That PVC is owned by the Pod:

kubectl get -o yaml pvc/my-csi-app-inline-volume-my-csi-volume

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  annotations:
    pv.kubernetes.io/bind-completed: "yes"
    pv.kubernetes.io/bound-by-controller: "yes"
    volume.beta.kubernetes.io/storage-provisioner: pmem-csi.intel.com
    volume.kubernetes.io/selected-node: pmem-csi-pmem-govm-worker1
  creationTimestamp: "2020-08-11T15:44:57Z"
  finalizers:
  - kubernetes.io/pvc-protection
  managedFields:
    ...
  name: my-csi-app-inline-volume-my-csi-volume
  namespace: default
  ownerReferences:
  - apiVersion: v1
    blockOwnerDeletion: true
    controller: true
    kind: Pod
    name: my-csi-app-inline-volume
    uid: 75c925bf-ca8e-441a-ac67-f190b7a2265f
...

Eventually, the storage capacity information for pmem-csi-pmem-govm-worker1 also gets updated:

csisc-2js6n map[pmem-csi.intel.com/node:pmem-csi-pmem-govm-worker2] 30716Mi
csisc-sqdnt map[pmem-csi.intel.com/node:pmem-csi-pmem-govm-worker1] 26620Mi
csisc-ws4bv map[pmem-csi.intel.com/node:pmem-csi-pmem-govm-worker3] 30716Mi

If another app needs more than 26620Mi, the Kubernetes scheduler will not pick pmem-csi-pmem-govm-worker1 anymore.

Next steps

Both features are under development. Several open questions were already raised during the alpha review process. The two enhancement proposals document the work that will be needed for migration to beta and what alternatives were already considered and rejected:

Your feedback is crucial for driving that development. SIG-Storage meets regularly and can be reached via Slack and a mailing list.

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