Ondat CSI Allowed Topologies


💡 This feature is available in release v2.9.0 or greater.

Ondat allows use of CSI’s Allowed Topology feature, for ensuring volumes are located in specified lists of topologies, where ‘topology’ is taken to mean a description of a node’s location, some sub-division of a cluster.

In Kubernetes topologies are defined by node labels. As it stands, the only label Ondat supports for this purpose is topology.kubernetes.io/zone, the Kubernetes' default label for representing “a logical failure domain”.

When to use CSI Allowed Topologies

CSI Allowed Topologies allows you to configure the “topologies” that a volume is allowed to be placed in, ensuring localised access for given workloads and availibility zones.

As such, an example use case would be for a workload that has an affinity for a given region of a cluster, ensuring that a deployment of that workload’s volume is always available in that same region for lowered latency and fault-tolerance.

Detailed CSI Allowed Topologies Behaviour

Where T is the set of allowed topologies, D is the set of volume deployments (primary & replicas) and |T| and |D| are the sizes of those respective sets, Ondat’s provisioning behaviour is this:

  • When |T|>|D| - All deployments are created on nodes within |D| unique topologies within T.
  • When |T|=|D| - All topologies in T contain exactly one deployment.
  • When |T|<|D| - All topologies in T contain at least one deployment.

So all allowed topologies will be populated with deployments if able, and if there are more deployments than allowed topologies those deployments can be placed on other toplogies.

How to use

You can enable by applying a storageos.com/fixed-topology: "true" label to any PVC that uses a StorageClass with an allowedTopologies block.