what are the advantages of running docker on a vm?

Question

Docker is an abstraction of OS (kernal) and below, VM is abstraction of Hardware. What is the point of running a Docker on an VM (like Azure) (apart from app portability)? should they not be directly hosting docker on the hardware?

Answer 1

Docker doesn't provide effective isolation for kernel-level security exploits (there's only one ring 0, and it's shared across all containers). Thus, one could reasonably wish to have the additional isolation provided by a virtualization mechanism.

Keep in mind that much of Docker's value is not about security, but about containerization -- building and distributing portable applications in such a way as to ensure that coupling between layers occurs only where and how intended.

Answer 2

The advantage of a cloud system like Azure is that you can go online with your credit card and get a machine up and running in a few minutes. This is enabled by that machine being virtual. Also VMs let you share hardware across multiple users with hardware-level isolation.

If everything else was equal, ie you didn't need any of the features of a VM, then you would be correct that a physical machine should be used, as it will run more efficiently.