Infrastructure & Operations › Linux & Servers

Virtual Machine

A fully emulated computer running on shared hardware.

Also known as: VM, virtual server, guest OS

A virtual machine (VM) is a complete computer simulated in software: its own operating system, virtual CPU, memory and disk, running on shared physical hardware. A program called a hypervisor divides the real machine among many VMs and keeps them separated.

This is what you rent when a cloud provider gives you a “server” or compute instance. You get a full OS, with root access, that behaves like a real machine.

Why it’s useful

• Isolation. Each VM has its own kernel, so one customer’s crash or breach doesn’t reach another’s.
• Efficiency. One powerful host runs many smaller machines.
• Flexibility. Create, resize, copy, snapshot and delete in minutes instead of ordering hardware.
• Any OS. Run a Linux guest on a Windows laptop, or the reverse. Developers also use local VMs to test a clean environment.

VM vs container

A VM includes a whole operating system, so it is heavier: usually gigabytes in size and slower to start. A container shares the host’s kernel and packages only the app and its libraries, so it is lighter and starts quickly. VMs give stronger isolation; containers give speed and density. Many systems use both, with containers running inside VMs. See containers vs VMs.

Things to keep in mind

• A VM is a machine you maintain. You patch the OS, manage users and watch the disk.
• Performance is shared. A “noisy neighbour” on the same host can occasionally affect you, depending on the provider and instance type.
• Compared with bare metal, you give up a small amount of raw performance for flexibility.