What is a Virtual Machine (VM)?

What is a Virtual Machine in cloud hosting?

A Virtual Machine (VM) is a software-based computer that runs on physical hardware through a hypervisor. The hypervisor is a layer of software that sits between the physical server and the VMs, dividing the server's CPU, memory, and storage among multiple isolated virtual environments. Each VM operates as if it were a standalone physical computer, complete with its own operating system, applications, and network configuration.

In cloud hosting, VMs are the foundation of compute resources. When you launch a VM in a cloud environment, the provider allocates a portion of physical server resources to your VM based on the flavor (predefined resource template) you select. Your VM runs independently of other VMs on the same physical host, and you have full administrative control over its operating system and software.

Related Terms

Instance: The running copy of a Virtual Machine in a cloud environment, such as when you launch a VM from an image in OpenStack.
Flavor: A predefined resource template that specifies how much CPU, RAM, and storage a VM receives, such as a "m1.medium" flavor with 2 vCPUs and 4GB RAM.
Volume: A persistent block storage device that can be attached to a VM, such as a 100GB disk that survives VM deletion.
Image: A template containing an operating system and preconfigured software used to create new VMs, such as an Ubuntu 22.04 server image.

Why Virtual Machines Exist

Before virtualization, running multiple applications with different requirements meant purchasing separate physical servers. A database server, a web server, and a development environment each required dedicated hardware, even if each server only used a fraction of its capacity. This led to wasted resources, higher costs, and complex hardware management.

Virtual Machines solve this problem by allowing a single physical server to run multiple isolated operating systems simultaneously. Each VM gets a dedicated slice of the hardware resources, but the underlying physical server can be fully utilized. Without VMs, cloud providers would need to assign entire physical servers to each customer, making cloud computing economically impractical.

VMs also provide isolation. If one VM crashes or is compromised, other VMs on the same physical host continue operating normally. This isolation enables multi-tenant cloud environments where different customers share physical infrastructure without security or stability concerns.

What Do Virtual Machines Actually Do?

Provide an isolated computing environment where you can install any operating system and run any compatible software
Allow you to resize compute resources by stopping the VM and changing to a different flavor
Enable you to create point-in-time backups called snapshots that capture the VM's entire state
Let you attach and detach storage volumes without affecting other VMs
Allow you to assign network interfaces and IP addresses independently of the physical network
Support live migration, where a running VM moves from one physical host to another without downtime
Provide console access so you can interact with the VM as if you were physically at the machine

When Would I Use a Virtual Machine?

Use a VM when you need a complete operating system environment with full administrative control. This includes:

Running a web server, database, or application that requires specific OS-level configurations
Hosting legacy applications that cannot run in containers
Running Windows workloads in a Linux-based cloud environment
Creating isolated development or testing environments that mirror production
Running workloads with specific compliance requirements that mandate OS-level isolation
Deploying applications that need to listen on multiple network ports or require custom kernel modules

When Would I NOT Use a Virtual Machine?

VMs carry overhead because each one runs its own operating system kernel, consuming memory and CPU cycles. Consider alternatives when:

You have many small, stateless applications that can share an OS kernel. Containers provide better resource efficiency for microservices architectures.
Your application only needs to run code in response to events. Serverless functions eliminate VM management entirely.
You need maximum performance with no virtualization overhead. Bare-metal servers provide direct hardware access.
Your workload is temporary and measured in seconds. Spinning up a VM takes longer than starting a container.

VMs also require you to manage the operating system, including security patches, updates, and configuration. If you prefer managed services, consider platform-as-a-service options where the provider handles the underlying infrastructure.

Real-World Example

Company A runs an e-commerce platform with three components: a web application, a PostgreSQL database, and a Redis cache. They deploy each component on a separate VM within their VPC.

The web application VM uses a flavor with 4 vCPUs and 8GB RAM to handle customer traffic. The database VM uses 8 vCPUs, 32GB RAM, and an attached 500GB volume for persistent data storage. The Redis cache VM uses a memory-optimized flavor with 2 vCPUs and 16GB RAM.

Each VM runs its own operating system. The web application runs on Ubuntu with custom Nginx configurations. The database runs on a hardened CentOS image with PostgreSQL optimizations. By using separate VMs, Company A isolates each component so that a crash in the web application does not affect the database, and security updates can be applied to each VM on different schedules.

Frequently Asked Questions

What is the difference between a VM and a container? A VM includes its own operating system kernel and simulates complete hardware. A container shares the host's OS kernel and packages only the application and its dependencies. VMs provide stronger isolation but use more resources. Containers start faster and use less memory but require the same OS family as the host. Choose VMs when you need different operating systems or full OS isolation.

Can I resize a VM after creating it? Yes. You stop the VM, change its flavor to one with different CPU, RAM, or disk specifications, and restart it. Some cloud providers support live resizing for certain resource types, but stopping the VM is the most reliable method. Plan for brief downtime during resize operations.

What happens to my data if the physical server hosting my VM fails? VMs running on failed hardware will stop. Data stored on the VM's root disk may be lost if the disk was on the failed server. Data stored on attached volumes typically survives because volumes are stored on redundant storage systems. Always use volumes for important data and maintain backups.

How many VMs can run on one physical server? It depends on the server's resources and the VM sizes. A physical server with 128GB RAM and 32 CPU cores might run dozens of small VMs or only a few large ones. Cloud providers manage this density automatically, and you do not control which physical server hosts your VM.

Is a VM the same as an instance? In cloud terminology, instance refers to a running VM. When you create a VM from an image and start it, you have an instance. The terms are often used interchangeably, but instance emphasizes the running state while VM describes the technology.

Summary

A Virtual Machine is a software-based computer that runs on physical hardware through a hypervisor, providing an isolated environment with its own operating system
VMs enable multiple workloads to share physical servers efficiently while maintaining isolation between tenants
Each VM can be independently configured, resized, backed up, and migrated without affecting other VMs
VMs are ideal when you need full OS control, strong isolation, or must run different operating systems
Cloud providers use VMs as the fundamental compute unit, with flavors defining the resource allocation for each VM