What Is Virtualization?
Virtualization Defined: One Physical Machine, Many Systems
What is virtualization? It is the technique of running several isolated computer systems on one physical machine. Each system sees its own processors, memory, storage and network ports, even though all of them are slices of the same hardware.
The physical machine is called the host. Each isolated system running on it is called a guest, or a virtual machine. A layer of software, known as a hypervisor, creates the guests and decides how the host's resources are shared among them. You can read more about that software in the hypervisor explainer.
The key idea is simple: the guests do not need to know they share hardware with others. From inside a guest, the system looks like a normal computer.
How Virtualization Divides Hardware Between Guests
Physical hardware is a fixed pool of resources. A server might offer 32 CPU threads, 128 GB of memory and several disks. Virtualization cuts that pool into smaller portions. One guest may receive 4 threads and 8 GB of memory, while another receives 2 threads and 4 GB.
The host keeps track of which portion belongs to which guest. When a guest reads from its disk or sends a network packet, the virtualization layer translates that request into an operation on the real device. Modern processors include features that make this translation fast, so most guests run at close to normal speed.
Isolation is the other half of the design. A crash or a runaway process inside one guest should not corrupt the memory of another. Strong isolation is what makes it practical to place unrelated customers on the same server.
Why Hosting Companies Rely on Virtualization
Imagine a hosting company with one powerful server. Without virtualization, each customer would need a separate physical machine, and most of those machines would sit mostly idle. With virtualization, the company runs many small guests on the same server. Each customer gets a private operating system, its own files and a fixed share of resources.
When a customer needs more capacity, the provider can often adjust that guest's resources in software rather than shipping new hardware. This flexibility is why virtual private servers (VPS) are so common. A VPS is, in practice, a virtual machine sold to one customer.
Frequently asked questions
Is virtualization the same as cloud computing?
No. Virtualization is a technology that splits one physical machine into several isolated systems. Cloud computing is a way of buying computing resources on demand, and it usually relies on virtualization behind the scenes. A company can use virtualization on its own servers without using any cloud service.
Does virtualization slow down a server?
It can add a small amount of overhead, because the virtualization layer has work to do. Modern hardware features reduce this cost, so for many everyday workloads the difference is small. Heavy disk or network use, or a host that is overloaded, makes the slowdown easier to notice. Lightweight alternatives such as containers share one operating system kernel and often have less overhead.