Containers vs Virtual Machines: A Complete Guide for Virtual Solutions

Containers vs virtual machines — what is the difference between these two virtualization solutions? On one side, you have containerization, which virtualizes an operating system (OS) and allows multiple workloads to run on top of it. On the other, you have virtual machine-based virtualization that virtualizes the hardware and allows multiple OS instances to run on top of it.

This post starts by defining virtual machines and containers, proceeds with their most prominent differences, and then discusses how they can work together in one environment, including one managed with Parallels ® Remote Application Server (RAS).

What Are Virtual Machines?

Perhaps the easiest way to visualize what virtual machines are is by imagining computers running on top of a computer, which is purely software. In comparison, the computer on which those “software-based computers” are running on is just a regular computer, i.e., one that consists of both hardware and software. It has, for instance, physical CPU, memory, and storage—which this post refers to as resources—whereas those “software-based computers” do not.

In virtualization parlance, those “software-based computers” are called virtual machines or VMs. VMs have virtualized resources apportioned to them from the underlying physical resources. When a VM is created, a portion of the physical resources is reserved for that VM. In addition, each of these VMs has an operating system and one or more applications.

The software that enables one or more VMs to run on top of a physical computer and shares the physical resources to the VMs is called a hypervisor. Some hypervisors run on top of a host OS, while others run on bare metal (no-host OS), i.e., they are installed directly on a physical server.

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As pieces of software, each VM is encapsulated in a file. And being files, they’re much more portable than their physical counterparts. You can copy or clone them. You can transfer them over the network. You can discard or delete them when no longer needed. You can even create a “gold image” as a template and then create hundreds or even thousands of VMs off of that template programmatically (or through an administrator console).

Because of these properties, virtual machines have greatly simplified IT administrators’ lives. VMs have allowed IT administrators and staff to cut down provisioning and deployment times of servers and desktops from hours to minutes or, when it comes to vast numbers (100s or 1,000s of endpoints), from weeks or months to days. They can be used to create test environments, run legacy applications or non-native applications on a host OS, examine malware-infected files, and so on.

What Are Containers?

Containers are a bit similar. Like VMs, they employ some form of virtualization. But instead of virtualizing the underlying hardware, containers virtualize the host operating system, which is the OS the containers run on. Containers share the host OS kernel, but other elements of the host OS (e.g., system files or libraries running in user mode) are incorporated inside each container depending on what files/libraries a particular container needs.

Typically, a container includes an application, OS files required by that application, and other dependencies and relevant configuration files. Because the bulk of the OS (the kernel) is excluded from a container, containers are generally much smaller than VMs. If VMs can be gigabytes in size, containers can be just a few tens of megabytes.

Because of their tiny footprint, containers can be booted and deployed much faster than VMs—as in boot-up times of less than a second for containers vs. minutes for VMs. And because containers share a host OS, any patching/updating done on the host OS is inherited by the containers above it (This applies only to files running in the kernel. Other files in each individual container will have to be updated separately).

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Due to their extreme portability and the fact that they already contain all the code and information needed for containerized applications to run in isolation, containers are useful in automated DevOps pipelines. DevOps teams that implement continuous integration/deployment (CI/CD) often use containers to simplify and speed up these processes. This portability also makes containers suitable for multi-cloud environments.

Container images can be uploaded to a repository to be shared with other people who wish to use them. These images can be databases, operating systems, content management systems (CMSs), web servers, and just about any containerized application out there. Developers, IT staff, and other people can use these images to build their own applications.

Containers vs Virtual Machines: What’s the Difference?

Generally speaking, containers are more portable, while VMs have tighter isolation. But those aren’t the only distinguishing features between the two. Here’s more about those two characteristics and some other differences as well:

Portability. Both container and VM-based applications have everything they need to run. Thus, you can say they’re both portable. However, because containers don’t include an OS kernel but simply take advantage of the resources provided by the underlying host OS, they’re much smaller and, consequently, more portable than VMs.
Isolation. VMs are more isolated from their surrounding environment, whether it’s the host OS or neighboring VMs. This level of isolation is one of the reasons why it’s safe to host virtual servers of different organizations on the same physical server. While containers also have a high degree of isolation, they still access the host OS by default and aren’t as isolated as VMs.
Deployment. You need a hypervisor such as Microsoft Hyper-V, VMware ESXi, or Citrix Hypervisor to deploy a VM. On the other hand, to deploy a container, you need Docker or a similar solution (although Docker is currently the most widely used). You need to use scripts or a tool like System Center Virtual Machine Manager for large-scale VM deployments. For containers, the de facto solution for large-scale deployments and automation and orchestration is Kubernetes.
Persistent storage. Both VMs and containers can be stateless. All files created during runtime persist only within a VM or container’s lifetime in this scenario. However, there may be instances when you want to keep specific files even after a VM or container has been terminated/destroyed. For VMs, you can get persistent storage by creating a virtual hard disk (VHD). For containers, you can use volumes or bind mounts.

Containers vs Virtual Machines: Which to Choose?

You will need to use a virtual machine if your project has special hardware requirements or if you are building on one hardware platform and need to target another, such as Windows vs. MacOS. Containers can be used to satisfy the majority of other “software only” needs.

Containers vs Virtual Machines: Can They Work Together?

Although used separately, containers and virtual machines can coexist for the most part. And in some cases, it’s much better if they do. Here are some use cases when combining containers and virtual machines makes sense.

Improve Container Portability

So far, this post raves a lot about the ultra-portability of containers. Despite that perception, containers do have some limitations in that department. If you recall, containers use the OS kernel of their host. Because of this, you can’t have a Windows-based container running on top of a Linux host and vice versa.

This also means if you have a bunch of containers running on a Linux host, you can’t transport and run those containers on a Windows host. You can only run them on another Linux host. So much for container portability. That shouldn’t be a problem if you leverage virtual machines.

In the above scenario, you can install a hypervisor on your Windows host, set up a Linux VM on top of it, and then—voila —you already have an environment where you can run your Linux containers.

Take Advantage of VM Isolation

All containers running on the same host share that host’s kernel, which is a security risk. A bug in the host OS can theoretically allow malicious or malware-infected containers to reach other containers running on that same host. Also, if the kernel has a vulnerability, it can affect all containers running on top of it.

Because virtual machines have more robust isolation, containers running on a VM can take advantage of that security feature. You can, for example, group together containers that require stronger security and run them on a separate VM (perhaps with a hardened OS). In contrast, other containers with lower security requirements run on another VM.

Let Container-Based Apps communicate with Existing VM-Based Apps

If you already have applications running on VMs, you can let applications running on containers communicate with them. For example, you can let a containerized mobile app connect to a database running on a VM or a containerized client app connect to a server application running on a VM.

Take Advantage of Your Virtualization Potential with Parallels RAS

Parallels Remote Application Server (RAS) is an all-in-one virtualization solution that allows you to deliver virtual applications and desktops remotely to any device, anywhere. It supports a wide range of hypervisors that serve as virtual desktop infrastructure (VDI) hosts, including Microsoft Hyper-V, VMware ESXi, Scale Computing HC3, Nutanix AHV, and many others.

Parallels RAS can be deployed on private, public, and hybrid cloud environments and allows administrators to manage the entire infrastructure from a single plane of glass.

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