Skip Kubernetes Attach and Detach

Status

Overview

Volume drivers, like NFS, for example, have no concept of an attach (ControllerPublishVolume). However, Kubernetes always executes Attach and Detach operations even if the CSI driver does not implement an attach operation (i.e. even if the CSI Driver does not implement a ControllerPublishVolume call).

This was problematic because it meant all CSI drivers had to handle Kubernetes attachment. CSI Drivers that did not implement the PUBLISH_UNPUBLISH_VOLUME controller capability could work around this by deploying an external-attacher and the external-attacher would responds to Kubernetes attach operations and simply do a noop (because the CSI driver did not advertise the PUBLISH_UNPUBLISH_VOLUME controller capability).

Although the workaround works, it adds an unnecessary operation (round-trip) in the preparation of a volume for a container, and requires CSI Drivers to deploy an unnecessary sidecar container (external-attacher).

Skip Attach with CSI Driver Object

The CSIDriver Object enables CSI Drivers to specify how Kubernetes should interact with it.

Specifically the attachRequired field instructs Kubernetes to skip any attach operation altogether.

For example, the existence of the following object would cause Kubernetes to skip attach operations for all CSI Driver testcsidriver.example.com volumes.

apiVersion: storage.k8s.io/v1
kind: CSIDriver
metadata:
  name: testcsidriver.example.com
spec:
  attachRequired: false

CSIDriver object should be manually included in the driver deployment manifests.

Previously, the cluster-driver-registrar sidecar container could be deployed to automatically create the object. Once the flags to this container are configured correctly, it will automatically create a CSIDriver Object when it starts with the correct fields set.

Alpha Functionality

In alpha, this feature was enabled via the CSIDriver Object CRD.

apiVersion: csi.storage.k8s.io/v1alpha1
kind: CSIDriver
metadata:
....