What Is Virtualization?
Learn how virtualization separates software environments from physical hardware, how virtual machines share host resources, and how Oracle VirtualBox uses a hosted hypervisor.
Virtualization is a technology that uses software to create logical or virtual computing resources from physical hardware. It separates the hardware a computer actually contains from the software environments that use that hardware.
With virtualization, one physical computer can run multiple isolated computing environments at the same time. Each environment can have its own operating system, applications, storage, and network configuration.
How virtualization works
In a traditional computer, an operating system runs directly on physical hardware. Applications use services provided by that operating system, and the operating system manages the computer's processor, memory, disks, and other devices.
Virtualization inserts a software layer between physical hardware and one or more software-defined computers. This layer presents each virtual computer with hardware-like devices while coordinating access to the real hardware underneath.
The result is a separation between:
- Physical resources: The actual CPU, RAM, storage devices, network interfaces, and display hardware in the computer.
- Virtual resources: Logical CPU, memory, disks, network adapters, and other devices presented to a virtual machine.
- Operating systems and applications: Software that uses the virtual resources as though it were running on a separate computer.
Host computers and virtual machines
Host
A host is the physical computer or server that supplies the actual hardware resources. In hosted virtualization, the host also includes the operating system installed directly on that physical computer.
Virtual machine
A virtual machine (VM) is an isolated, software-defined computer. It has virtual hardware and can run an operating system and applications independently of other VMs.
The operating system installed inside a VM is called the guest operating system. For example, a desktop computer might run Windows as its host operating system while Oracle VirtualBox runs a VM containing a Linux guest operating system. The host can continue running its normal applications while the Linux guest runs inside the VM.
See What Is Guest Operating System for a focused explanation of the guest operating system.
Physical computer versus virtual machine
Resource abstraction and allocation
Resource abstraction means representing a physical resource as a logical resource that software can use. A VM does not normally receive ownership of an entire physical computer. Instead, the virtualization platform assigns it selected portions of the host's available capacity.
Common mappings include:
A virtualization platform can manage available CPU, memory, storage, and networking as a resource pool: a logical collection of resources that can be allocated to workloads. On a desktop, this pool is usually the host computer's finite capacity. On a server, the pool may span more substantial hardware managed by virtualization software.
Resource allocation is configurable for each VM. You can choose how much memory and how many vCPUs a VM receives, create a virtual disk of a selected size, and choose how its virtual network adapter connects. These assignments are limits or allocations backed by real host resources; they do not create additional physical capacity.
Running multiple operating systems
The traditional server model places one operating system on one physical server. If an organization needs several services, it may need several physical machines, even when each service uses only part of a server's capacity.
Server virtualization changes this arrangement. One physical server can run several VMs, with each VM running its own guest operating system and applications. For example, one VM might run a Linux server, another might run a different supported operating system, and a third might provide a test environment.
These VMs share the host's physical resources but remain separate environments. A process in one guest normally cannot directly read another guest's memory, and changing one guest's operating system does not normally change the files inside another guest. Isolation is a major reason virtualization is useful, although it is not a substitute for security controls.
Hypervisors
A hypervisor is the software layer that creates, runs, and manages virtual machines. It presents virtual hardware to each VM and coordinates the VM's access to physical CPU time, memory, storage, networking, and other devices.
There are two broad hypervisor categories:
Oracle VirtualBox is a hosted, or type 2, virtualization product. It runs on a supported desktop host operating system. VirtualBox then provides the virtual hardware and management functions used by guest operating systems.
The relationship can be summarized as:
- Physical hardware supplies CPU, memory, storage, networking, and other devices.
- The host operating system manages the physical computer.
- Oracle VirtualBox runs as an application on that host operating system.
- VirtualBox creates and operates a VM with virtual hardware.
- The guest operating system runs inside the VM and uses that virtual hardware.
For more detail about the product's role, see How Oracle VirtualBox Works and What Is a Hypervisor?
Virtual hardware inside a VM
A VM sees a collection of virtualized or emulated components. These components make the VM look like a computer to its guest operating system, even though the underlying devices belong to the host.
- Virtual processor: One or more vCPUs backed by physical CPU capability.
- Virtual RAM: A selected amount of host memory presented to the guest.
- Virtual disk: A file or storage object that acts as the VM's hard drive.
- Virtual disk controller: A virtual controller through which the guest accesses its virtual disk.
- Virtual network adapter: A software-defined adapter connected through a chosen networking mode.
- Virtual display adapter: A virtual graphics device used by the guest desktop.
- Firmware: Virtual firmware that helps the guest start and locate boot devices.
Emulation is software behavior that presents a hardware-like device or function to another system. Virtualization may use emulated devices, optimized virtual devices, or a combination of both.
Virtual hardware settings belong to an individual VM. Increasing a VM's virtual memory or changing its virtual disk controller changes that VM's configuration; it does not physically add RAM or replace the host computer's controller.
Benefits and common uses
- Workload consolidation: Several services can share fewer physical servers, improving hardware utilization.
- Testing: A developer or administrator can test software, updates, or operating systems in a separate environment.
- Different operating systems: An application that requires another operating system can run in a suitable guest VM.
- Learning: Students can practice installation, administration, networking, and troubleshooting without dedicating another physical computer.
- Isolation: Experiments and test changes can be separated from the primary desktop environment.
- Portability: A VM's configuration and virtual disks can often be moved to another compatible host.
- Repeatability: A known VM configuration can be copied or recreated for consistent labs and tests.
For a broader summary of these advantages, read Virtualization Benefits.
Limitations and important considerations
- VMs still depend on the host's available CPU, memory, disk capacity, disk speed, and network connection.
- Running many guests at once can reduce performance when resources are overcommitted.
- A VM with excessive memory or vCPU allocation can compete with the host operating system and other VMs.
- Virtualization provides separation, but it does not remove the need for operating system updates, application updates, security controls, backups, and careful network configuration.
- Some hardware-dependent software, specialized peripherals, timing-sensitive workloads, and high-performance tasks may not behave exactly like they would on a physical computer.
- Guest operating systems and VM configurations must be supported by the virtualization platform and the host hardware.
- Hardware virtualization features may need to be available and enabled in the computer's firmware settings.
Troubleshooting resource problems
A VM runs slowly
Likely causes include insufficient host memory or CPU capacity, too many VMs running at once, an oversized resource allocation competing with the host, or slow storage. Check host utilization and reduce simultaneous workloads or adjust the VM's allocation when appropriate.
The host becomes unresponsive after starting a VM
The VM may have been assigned too much RAM, leaving too little for the host operating system. It may also have too many vCPUs for the host's capacity or current workload. Leave sufficient resources for the host and avoid treating every available resource as safe to allocate.
A guest cannot install or start as expected
Check that the selected guest operating system type is correct, that the host supports required hardware virtualization features, and that those features are available in firmware. Also check the VM's memory, CPU, storage, firmware, and boot-device settings.
How the concepts map to VirtualBox
When creating a VM in VirtualBox, the practical choices correspond directly to the concepts in this lesson:
- Guest operating system type: Identifies the operating system the VM will install and helps VirtualBox choose compatible defaults.
- Base memory: Sets the amount of virtual memory assigned to the guest.
- Virtual processors: Sets the number of vCPUs presented to the guest.
- Virtual hard disk: Defines the VM's storage capacity and virtual disk format.
- Network adapter mode: Determines how the guest connects to the host, local network, or other VMs.
These are configuration decisions, not purchases of new physical hardware. To continue into the practical workflow, see Create Virtual Machine. You can also review Virtual Disk and Network Modes.
Key terms to remember
- Virtualization: Software-based creation of logical computing resources separated from physical hardware.
- Virtual machine: An isolated software-defined computer with virtual hardware.
- Host: The physical computer and, in hosted virtualization, its directly installed operating system.
- Host operating system: The operating system installed directly on the physical computer.
- Guest operating system: The operating system installed and run inside a VM.
- Hypervisor: Software that creates VMs and manages their access to physical resources.
- vCPU: A processor resource presented to a VM and backed by physical CPU capability.
- Virtual memory: Host RAM assigned to a VM.
- Virtual disk: A file or storage object that acts as a VM's hard drive.
- Isolation: Separation that allows VMs to function as distinct environments.