VMware ESXi and vSphere Cluster Management
What Is Virtualization?
Learn what virtualization is, how virtual machines share physical hardware, what hosts, guests, and hypervisors do, and how to prepare for VirtualBox.
Virtualization is a software-based technique that abstracts or emulates physical computing hardware. It separates the physical computer from the software environments that use it, allowing multiple independent computing environments to run on one physical machine.
This lesson introduces the host, guest operating system, virtual machine, resource allocation, and hypervisor. It also explains why virtualization is useful, what limitations to expect, and what to understand before creating a VM in Oracle VirtualBox.
What Is Virtualization?
A physical computer contains tangible components such as a processor, memory, storage, network devices, and display hardware. These components are called physical hardware.
Virtualization places a software layer between that hardware and one or more operating systems. The software layer presents each environment with virtual hardware. From the operating system's point of view, that virtual hardware behaves like the components of a computer.
Because the hardware is represented in software, one physical machine can support several isolated computing environments. Each environment can have its own operating system and applications, even though the environments share the same underlying hardware.
Host Machines, Guests, and Virtual Machines
The host machine
The host machine is the physical computer or server that supplies resources to virtual machines. Its resources can include CPU capacity, RAM, storage, network connectivity, display hardware, and other devices.
The host may run a normal operating system, as it commonly does on a desktop or laptop, or it may run virtualization software directly on server hardware.
The guest operating system
A guest operating system is an operating system installed and running inside a virtual machine. For example, a Linux installation running in VirtualBox on a Windows laptop is a guest operating system.
The guest does not normally control the host's physical devices directly. Instead, it uses virtual hardware assigned by the virtualization platform.
The virtual machine
A virtual machine (VM) is an independent, software-defined computer. It has virtual components such as a processor, memory, disk, network adapter, display, and other devices.
A VM is not the same thing as the physical desktop or server that hosts it. The host provides the physical resources; the VM provides an isolated environment in which a guest operating system and its applications run.
Virtual Hardware and Physical Hardware
Virtual hardware is a software representation of hardware devices or interfaces used by a virtual environment. The hypervisor connects these virtual devices to available host resources.
A virtual CPU (vCPU) is processor capacity presented to a VM and backed by the host's physical CPU. Virtual memory is RAM assigned to a VM from the host's available physical memory. A virtual disk is commonly a file or storage construct that acts as the VM's hard drive.
How Virtual Machines Share Resources
A virtualization platform organizes available physical resources into logical resources. This logical collection can be called a resource pool. Virtual machines receive portions of the pool according to their configuration and current workload.
For example, a host might have four physical CPU cores, 16 GB of RAM, and 500 GB of storage. Two VMs could be configured with two vCPUs and 4 GB of RAM each, while their virtual disks use 60 GB and 80 GB of the host's storage. The host operating system and virtualization platform still need resources, and the remaining capacity is not automatically free for unlimited use.
Virtual resources are backed by finite host resources. Assigning four vCPUs to a VM does not give it four additional physical processors. Assigning 8 GB of virtual memory reserves or makes available part of the host's physical memory. Good VM sizing leaves enough CPU, RAM, storage capacity, and disk performance for the host and other workloads.
One Physical Machine Versus Multiple VMs
Traditional physical-server setup
In a traditional deployment, one physical server runs one operating system and one primary workload. For example, a physical server might run a database system. If the server has substantial unused capacity, that capacity may remain idle.
Virtualized server setup
In a virtualized deployment, one physical server runs a hypervisor and several VMs. One VM might run a database operating system, another might run a web server, and a third might run a test environment. Each VM has its own guest operating system and workload, while all of them share the host's physical resources.
Different operating systems can run on the same host when the virtualization platform and host hardware support them. The VMs remain separate because the hypervisor controls their virtual hardware and access to shared resources.
What Is a Hypervisor?
A hypervisor is the software layer that creates, runs, and manages virtual machines. It presents virtual hardware to each VM and coordinates access to the host's CPU, memory, storage, networking, and devices.
The hypervisor schedules work from different VMs, keeps their virtual memory and devices organized, and applies the resource settings configured for each VM. This coordination allows multiple guests to run simultaneously without normally being able to control one another's virtual hardware.
Type 1: bare-metal hypervisors
A Type 1, or bare-metal, hypervisor runs directly on the physical server hardware. It is commonly used in data centers and server environments because the virtualization layer is the primary software layer managing the machine.
Type 2: hosted hypervisors
A Type 2, or hosted, hypervisor runs as an application within a conventional host operating system. The host operating system manages the physical computer, while the hosted hypervisor creates and runs VMs.
Oracle VirtualBox is an example of a hosted desktop virtualization platform. It can run on a desktop or laptop host and provide VMs for activities such as Linux command-line practice, software testing, and operating system learning.
Benefits and Appropriate Uses
More efficient hardware use
Server virtualization uses virtualization to run multiple server operating systems and workloads on one physical server. Consolidating workloads can make better use of CPU, RAM, storage, and networking capacity than dedicating a separate physical server to every small workload.
Isolation for testing and learning
A VM provides a separate operating system environment. A learner can install Linux in a VM without replacing the normal host operating system. A developer can test software in one guest, while another VM remains configured for a different project.
Installing or misconfiguring software in one test VM does not normally change the guest operating system of another VM. However, host resources remain shared, and features such as shared folders, clipboard integration, and network access intentionally exchange information across the boundary.
Flexibility and repeatability
VMs can be created with selected hardware settings and used for distinct purposes. A learner can create a repeatable environment with a chosen guest operating system, virtual disk size, memory amount, and network configuration without buying another physical computer.
Benefits have limits
Virtualization reduces the need for separate physical machines, but it does not eliminate physical resource requirements. Every running VM consumes some host CPU, memory, storage capacity, and disk performance. The host must also have enough capacity to run its own operating system and the virtualization platform.
Basic Limitations and Considerations
- Shared CPU: Several VMs compete for host processor capacity. A CPU-heavy guest can affect other guests and the host.
- Shared memory: Allocated VM memory comes from finite physical RAM. Assigning too much can cause severe slowdown or make the host unresponsive.
- Storage capacity and performance: Virtual disks consume host storage. Several guests performing disk operations at once can compete for storage performance.
- Networking: Virtual network adapters depend on the host's network connection and the selected virtual networking mode.
- Hardware virtualization support: The host processor and firmware may need hardware virtualization features enabled. Platform settings can also affect which guests can run.
- Host dependency: Guests are isolated from one another, but they still depend on the host system, host hardware, and virtualization platform. A host failure can make its VMs unavailable.
- No unlimited resources: A VM has only the virtual CPU, memory, storage, and devices configured for it. These should not be treated as unlimited or automatically fully dedicated physical resources.
Common Troubleshooting Situations
The VM is slow or the host becomes unresponsive
Likely causes include assigning too much memory or CPU capacity, running too many VMs at once, or using storage that cannot handle the combined disk workload.
Reduce VM allocations or the number of concurrent workloads, and leave adequate CPU and memory for the host operating system. Check whether the host has sufficient free storage and disk performance.
A 64-bit guest cannot start
Hardware virtualization support might be disabled in system firmware. Another virtualization or security feature may also be using the required platform capability, or the host hardware and software configuration may not meet the guest's requirements.
Check host compatibility, firmware virtualization settings, and platform-specific conflicts before changing the VM configuration.
The VM runs out of disk space
The virtual disk may have been created too small, or the guest operating system and applications may have consumed its available capacity. Plan storage before installation and expand the virtual disk or add virtual storage when both the platform and guest operating system support it.
Changes in the VM seem to affect the host
This usually reflects confusion between the host and guest environments. Shared folders, clipboard integration, and network access can intentionally exchange data. Review those integration features and remember that the VM's operating system is separate from the host operating system, while the underlying resources remain shared.
Preparing to Create a VM in VirtualBox
Before creating a virtual machine, identify the guest operating system you want to install and check its CPU, memory, storage, and platform requirements. Keep enough resources available for the host.
- Choose the guest operating system and select the appropriate operating system type in the virtualization tool.
- Decide how many virtual CPUs the guest needs without assigning most of the host's CPU capacity.
- Allocate virtual memory while leaving sufficient RAM for the host operating system.
- Create a virtual disk with enough capacity for the guest operating system, applications, updates, and user files.
- Choose a virtual network configuration appropriate for the guest's purpose.
- Start the VM and install the guest operating system from supported installation media.
Once these concepts are clear, creating a VM becomes a resource-planning task rather than a mysterious hardware operation. You are configuring a software-defined computer whose virtual hardware is supplied by the host through the hypervisor.
Continue with virtualization fundamentals as a reference before moving on to guest installation, virtual networking, snapshots, and host-guest integration.
Exam-Relevant Summary
- Virtualization abstracts physical computing resources so multiple independent environments can run on one machine.
- The host machine is the physical computer that supplies resources.
- A guest operating system runs inside a virtual machine.
- A VM uses virtual CPU, memory, disk, networking, display, and other devices.
- A hypervisor creates and manages VMs and coordinates their access to host resources.
- A Type 1 hypervisor runs directly on hardware; a Type 2 hypervisor runs inside a host operating system.
- Oracle VirtualBox is a hosted desktop virtualization platform.
- Virtual resources are backed by finite physical resources, so poor allocation can slow both guests and the host.