OpenEBS: Update mount-points and capacity of Block Devices without restarting NDM

The Node Disk Manager (NDM) daemonset runs on every node in the Kubernetes cluster, discovers and monitors various storage devices connected to the node. It exports these devices as BlockDevice (BD) custom resources on the Kubernetes cluster, which are then used by other OpenEBS stack components. The NDM currently supports the detection of various storage devices connected to the node. However, it cannot detect specific changes that may happen to the block devices while connected to the node. Specifically, the NDM cannot detect changes in the mount-point(s) and the filesystem associated with a device. It also cannot detect a change in the capacity of the block device.

The mount-point(s) of a block device can easily be changed on a system. It is easy to change the disk size on the cloud too. This causes issues in the Kubernetes cluster using OpenEBS as the changes in the device properties will not be reflected immediately in the BD resource.

This proposal aims to solve the issue above by adding functionality in the NDM to detect changes in the mount-points and capacity of supported block devices and propagate them immediately to the corresponding BD resource.