Are containers virtual machines or not?
There’s a common analogy that VMs are like houses and containers are like apartments. And you are the application. When you live in a house, you have free rein to do as you please. When you live in an apartment, you have to share certain spaces, and parts of the building are off-limits. Interestingly, this analogy suggests that the difference between containers and VMs is not one of architecture but of implementation!
Apartment buildings and houses both have rooms, water, electric, roofs, and doors. Containers and VMs both virtualize compute, storage, networking, and memory. So what’s the difference, really?
Shared Almost Nothing
Those who say that a container is not a virtual machine are quick to note that all containers on a host share the same kernel. Not a copy of the same kernel, but the exact same kernel running on the host.
Virtual machines, on the other hand, have completely isolated kernels. If you’re running 100 identical Linux VMs, each VM has its own unique copy of the same kernel. You can upgrade the kernel in one, and it doesn’t affect any others. That seems like a pretty significant difference. But let’s look a little deeper.
Containers do use virtualization, but rather than fully virtualizing compute, network, storage, and memory, containers do something a little different. They fully virtualize the compute and networking portion. But storage and memory, on the other hand, are mostly but not completely virtualized. The kernel is stored on the host, and the container is given read-only access to it. The rest of a container’s storage and memory are virtualized.
In a container, the operating system is necessarily separated from the rest of the VM. But that hardly means containers aren’t VMs.
VMs that Look Like Containers?
With a little tweaking, a traditional virtual machine could meet the criteria of a container. One could, for example, create a shared, read-only filesystem containing a Linux kernel and have multiple VMware virtual machines booting from it. As those VMs boot, VMware could identify the duplicate virtual memory blocks and deduplicate them. Those VMs wouldn’t cease to be virtual machines just because they’re all sharing a kernel.
Let’s take a more realistic example: multiple virtual machines booting Kali Linux from a shared, read-only ISO. Each VM has its own virtual disk for persistence, but the operating system kernel is shared. Is that a container?
Containers Were Born On Linux
There’s a reason containers were born on Linux and not Windows. The Linux architecture lends itself to the clean separation of the kernel from everything else. Windows, on the other hand, just mashes it all together. That’s why Windows and *nix went into opposite directions. To get more efficient use of system resources, FreeBSD got jails, and Linux got OpenVZ, LXC, and eventually Docker. Windows, on the other hand, just got full-on x86 virtualized.
Containers Mimic Physical Machines
As I said in an earlier post, virtualization is mimicry. Containers present applications with compute, memory, storage, and networking, and control how applications can use those resources. Virtual machines do the exact same thing. Not only that, you can start, stop, pause, and shutdown containers, just like with VMs!
At this point, it’s starting to become clear that the earlier analogy – VMs being like houses – is flawed. Virtual machines aren’t like houses per se, but like buildings in general. Containers are a particular implementation of virtual machine, like an apartment is a particular instance of a building.
Containers Aren’t Those VMs
Let me be clear that I’m not suggesting that people who say containers aren’t VMs are wrong.
When people say containers aren’t VMs, they’re trying to explain that a container is not the kind of virtual machine you’d find in VMware or Hyper-V. It’s not the type of VM you attach an ISO to, boot up, and install an operating system on. It’s perfectly valid to insist that containers are not those types of virtual machines because, well, they’re not. They’re very different. My goal here is to highlight the similarities so that the differences become more apparent.
Also, I’m not saying containers are bad. Far from it. I’ve been using Docker since 2014 and I love it. And I’m excited to see how Docker for Windows Server will fare. Application virtualization has been a holy grail of Windows for a long time, and Docker just might finally deliver it.