LINSTOR Affinity Controller

The LINSTOR Affinity Controller keeps the affinity of your volumes in sync between Kubernetes and LINSTOR.

Affinity is used by Kubernetes to track on which node a specific resource can be accessed. For example, you can use affinity to restrict access to a volume to a specific zone. While this is all supported by Piraeus and LINSTOR, and you could tune your volumes to support almost any cluster topology, there was one important thing missing: updating affinity after volume migration.

After the initial PersistentVolume (PV) object in Kubernetes is created, it is not possible to alter the affinity later¹. This becomes a problem if your volumes need to migrate, for example if using ephemeral infrastructure, where nodes are created and discard on demand. Using a strict affinity setting could mean that your volume is not accessible from where you want it to: the LINSTOR resource might be there, but Kubernetes will see the volume as only accessible on some other nodes. So you had to specify a rather relaxed affinity setting for your volumes, at the cost of less optimal workload placement.

There is one other solution (or rather workaround): recreating your PersistentVolume whenever the backing LINSTOR resource changed. This is where the LINSTOR Affinity Controller comes in: it automates these required steps, so that using strict affinity just works. With strict affinity, the Kubernetes scheduler can place workloads on the same nodes as the volumes they are using, benefiting from local data access for increased read performance.

It also enables strict affinity settings should you use ephemeral infrastructure: even if you rotate out all nodes, your PV affinity will always match the actual volume placement in LINSTOR.

Deployment

The best way to deploy the LINSTOR Affinity Controller is by helm charm. If deployed to the same namespace as our operator this is a simple process:

helm repo add piraeus-charts https://piraeus.io/helm-charts/
helm install linstor-affinity-controller piraeus-charts/linstor-affinity-controller

If deploying to a different namespace, ensure that linstor.endpoint and linstor.clientSecret are set appropriately. For more information on the available options, see below.

Options

The following options can be set on the chart:

Option	Usage	Default
replicaCount	Number of replicas to deploy.	1
options.v	Set verbosity for controller	2
options.reconcileRate	Set the reconcile rate, i.e. how often the cluster state will be checked and updated	15s
options.resyncRate	How often the controller will resync it's internal cache of Kubernetes resources	15m
options.propertyNamespace	Namespace used by LINSTOR CSI to store Kubernetes Node Labels	"" (auto-detected based on existing node labels on startup
linstor.Endpoint	URL of the LINSTOR Controller API.	"" (auto-detected when using Piraeus-Operator)
linstor.clientSecret	TLS secret to use to authenticate with the LINSTOR API	"" (auto-detected when using Piraeus-Operator)
image.repository	Repository to pull the linstor-affinity-controller image from.	quay.io/piraeusdatastore/linstor-affinity-controller
image.pullPolicy	Pull policy to use. Possible values: IfNotPresent, Always, Never	IfNotPresent
image.tag	Override the tag to pull. If not given, defaults to charts AppVersion.	""
resources	Resources to request and limit on the container.	{requests: {cpu: 50m, mem: 100Mi}}
securityContext	Configure container security context.	{capabilities: {drop: [ALL]}, readOnlyRootFilesystem: true}
podSecurityContext	Security context to set on the pod.	{runAsNonRoot: true, runAsUser: 1000}
imagePullSecrets	Image pull secrets to add to the deployment.	[]
podAnnotations	Annotations to add to every pod in the deployment.	{}
nodeSelector	Node selector to add to a pod.	{}
tolerations	Tolerations to add to a pod.	[]
affinity	Affinity to set on a pod.	{}
rbac.create	Create the necessary roles and bindings for the controller.	true
serviceAccount.create	Create the service account resource	true
serviceAccount.name	Sets the name of the service account. If left empty, will use the release name as default	""
podDisruptionBudget.enabled	Enable creation of a pod disruption budget to protect the availability of the scheduler	true
autoscaling.enabled	Enable creation of a horizontal pod autoscaler to ensure availability in case of high usage`	"false

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Footnotes

1. That is not 100% true: you can add affinity if it was previously unset, but once set, it can't be modified. ↩