Virtualization is the foundation that enables cloud computing by abstracting physical resources into logical units that can be provisioned on-demand.
Definition
According to NIST Special Publication 800-125:
“Virtualization is the simulation of the software and/or hardware upon which other software runs. This simulated environment is called a virtual machine (VM).”
In other words, virtualization creates an abstraction layer that transforms a real (physical) system so it appears as a different virtual system or as multiple virtual systems.
Key Concepts
- Host System: The physical hardware and software on which virtualization is implemented
- Guest System: The virtual system that runs on the host
- Hypervisor/VMM (Virtual Machine Monitor): Software that creates and manages virtual machines
Formal Definition
Virtualization can be formally defined through an isomorphism V that maps the guest state to the host state:
- For each sequence of operations e that modifies the guest’s state from Si to Sj
- There exists a corresponding sequence of operations e’ that performs an equivalent modification between the host’s states (S’i to S’j)
Categories of Virtualization
Virtualization technologies can be categorized into three main types:
1. Process Virtualization
- Creates a virtual environment for individual applications
- Examples: Java Virtual Machine (JVM), Common Language Runtime (.NET/Mono)
- Used for platform independence and sandboxing
2. OS-Level Virtualization
- Creates isolated environments (containers) within an operating system
- Examples: Linux Containment Features, Docker, FreeBSD Jails
- Used for application isolation and packaging
3. System Virtualization
Creates complete virtual machines with virtualized hardware
- Emulation: Complete software emulation of hardware (e.g., QEMU, Bochs)
- Full Virtualization: Virtualization where the guest OS is unmodified (e.g., VMware Workstation, VirtualBox)
- OS-Assisted Virtualization: Virtualization where the guest OS is modified to cooperate with the hypervisor (e.g., Xen)
- Hardware-Assisted Virtualization: Virtualization leveraging special CPU features (e.g., KVM, Hyper-V)
Types of Hypervisors
Type 1 (Bare-Metal Hypervisors)
- Run directly on hardware
- Examples: VMware ESXi, Xen, Microsoft Hyper-V, KVM
- More efficient, better performance
- Require special device drivers
Type 2 (Hosted Hypervisors)
- Run as an application on a host operating system
- Examples: VMware Workstation, Oracle VirtualBox, QEMU
- Less efficient but more flexible
- Can use the host OS device drivers
Importance in Cloud Computing
Virtualization is critical for cloud computing because it enables:
- Resource pooling: Physical resources can be shared among multiple users
- Isolation: Different users’ workloads can run on the same hardware without interfering with each other
- Rapid provisioning: Virtual resources can be created, modified, or deleted quickly
- Elasticity: The ability to scale resources up or down based on demand
- Efficient resource utilization: Higher utilization rates of physical hardware
Challenges
- Performance overhead: Virtualization introduces some performance penalties
- Security concerns: Potential for VM escape vulnerabilities
- Resource management: Allocation and scheduling of resources among VMs
- Complexity: Additional layer in the system architecture