fs-backup deprecate support for volumes provisioned by in-tree drivers

[Lyndon-Li]

In-tree drivers have been deprecated from k8s upstream for quite some time and users most probably have moved to CSI volumes. So it is proposed to deprecate the same support from Velero.

This brings below benefits to Velero:

1. Remove host-path mount from node-agent
2. Remove root privilege from node-agent
3. A single node-agent for both linux and Windows nodes

[Lyndon-Li]

@vmware-tanzu/velero-maintainers
Please vote or comment on this issue, so that we can push the deprecation forwards.

[sseago]

It's worth calling out that moving to csi-only volume support also requires deprecating support for PVCs that use static provisioning (i.e. pre-created NFS PVs that don't use any storage class at all). I have no idea how many people still rely on this functionality vs. using the NFS CSI driver, though. It may not be a use case that matters to anyone at this point.

[Lyndon-Li]

@sseago

1. Static provisioning doesn't mean non-csi necessarily. So if the target PV for the static provision is created by a CSI driver, the current deprecation doesn't affect it.
2. NFS volumes are special, in many cases, they are used for static provision and non-csi. Therefore, we can make a special treatment for this kind of volumes and support them --- the mount of these volumes are done by kubelet, and the prefix of the host path are a fix one, so we can have this fix path hardcoded in the code without visit the file system.

the prefix of the host path are a fix one, so we can have this fix path hardcoded in the code without visit the file system

Actually, we can do the same thing for all the in-tree drivers, merely, the fixed prefixes are different varying from drivers. However, we think it is better deprecate supporting of them comparing to make particular treatment.

The essence is that we don't access the file system of the host path from node-agent.

Therefore, to answer your question:

• Static provisioned volumes by CSI drivers are supported
• NFS volumes are supported, either CSI provisioned or non-CSI provisioned
• For other static provisioned non-CSI volumes, we don't support them in principle. But if some kind of volumes is as popular as NFS and we missed it, we can add the same special treatment.