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Topology Aware StorageClasses

There can be a use case where we have a certain set of nodes that we want to dedicate as storage Nodes and wants to deploy the CSI Node driver on that nodes and later we want a particular type of application to use those Nodes.

We can create a storage class with allowedTopologies and mention all the storage nodes :

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: cstor-sparse-auto
provisioner: cstor.csi.openebs.io
allowVolumeExpansion: true
volumeBindingMode: WaitForFirstConsumer
parameters:
  replicaCount: "3"
  cstorPoolCluster: "cspc-ssd"
  cas-type: "cstor"
allowedTopologies:
- matchLabelExpressions:
  - key: "topology.cstor.openebs.io/nodeName"
    values:
      - node-1
      - node-2
      - node-3

The CStor CSI driver will create the Volume in the CStor Pool Instances present on this list of nodes. We can use volumeBindingMode: WaitForFirstConsumer to let the k8s select the node where the volume should be provisioned.

With volumeBindingMode: Immediate driver creates the volume without any topology awareness. Volume binding and dynamic provisioning are handled when the PVC is created. This is the default VolumeBindingMode and is suited for clusters that do not enforce topology constraints. in such cases nodeAffinity can be used in the application pod to select the desire nodes.

The problem with the above StorageClass is that it works fine if the number of nodes is less, but if the number of nodes is huge, it is cumbersome to list all the nodes like this. In that case, we can label all the similar nodes using the same key value and use that label as allowedTopologies to create the StorageClass.

$ kubectl label node node-1 node-2 node-3 company/nodegroup=storage

Now, restart the CStor csi-node Driver pods (if already deployed, otherwise please ignore) so that it can update the new node label as the supported topology.

$ kubectl delete pods -n openebs -l role=openebs-cstor-csi

Now, we can create the StorageClass like this:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: cstor-sparse-auto
provisioner: cstor.csi.openebs.io
allowVolumeExpansion: true
volumeBindingMode: WaitForFirstConsumer
parameters:
  replicaCount: "3"
  cstorPoolCluster: "cspc-ssd"
  cas-type: "cstor"
allowedTopologies:
- matchLabelExpressions:
  - key: "company/nodegroup"
    values:
      - storage