08. Spark MLlib
Virtualization is a foundational technology in modern computing that enables multiple operating systems and applications to run on a single physical machine. This lecture explores the core concepts, history, and types of virtualization.
What is Virtualization?
General Definition
Virtualization refers to creating a virtual (rather than physical) version of computing resources. In the context of this course:
- A machine implemented in software, rather than hardware
- A self-contained environment that acts like a computer
- An abstract specification for a computing device (instruction set, memory, etc.)
Common Distinction
There are two main categories of virtual machines:
1. Language-Based Virtual Machines
- Instruction set usually does not resemble any existing architecture
- Examples: Java VM, .NET CLR, Python VM
- Designed for platform independence and security
2. Virtual Machine Monitors (VMM) or Hypervisors
- Instruction set fully or partially taken from a real architecture
- Virtualizes complete hardware systems
- Examples: VMware, Xen, KVM, Hyper-V
This course focuses primarily on hypervisor-based virtualization used in cloud computing.
History of Virtualization
The evolution of virtualization technology:
1972 → IBM VM/370
First VM architecture for mainframe machines
1997 → Virtual PC for Mac
Connectix brings virtualization to personal computers
1999 → VMware Virtual Platform
Commercial virtualization for x86 architecture
2003 → Xen Hypervisor
Open-source hypervisor project launched
2005 → VMware Player
Free VM player for end users
2007 → VirtualBox
Cross-platform virtualization software
2010s → Cloud Era
AWS, Azure, GCP built on virtualization
Virtualization Terminology
Understanding the key terms:
Guest OS vs Host OS
- Guest OS: The operating system running inside the virtual machine
- Host OS: The operating system running on the physical machine (for Type 2 hypervisors)
- Physical Machine (PM): The actual hardware
- Virtual Machine (VM): The virtualized environment
┌─────────────────────────────────────┐
│ Virtual Machine 1 │ VM 2 │
│ ┌──────────────┐ ┌──────────────┐ │
│ │ Guest OS 1 │ │ Guest OS 2 │ │
│ └──────────────┘ └──────────────┘ │
├─────────────────────────────────────┤
│ Type 1 Hypervisor │
├─────────────────────────────────────┤
│ Hardware (CPU/RAM/Disk) │
└─────────────────────────────────────┘
- Runs directly on hardware
- No need for a host operating system
- Better performance and efficiency
- Examples: VMware ESXi, Xen, Hyper-V, KVM
- Used in: Data centers, cloud providers
Type 2 Hypervisor (Hosted)
┌─────────────────────────────────────┐
│ Virtual Machine 1 │ VM 2 │
│ ┌──────────────┐ ┌──────────────┐ │
│ │ Guest OS 1 │ │ Guest OS 2 │ │
│ └──────────────┘ └──────────────┘ │
├─────────────────────────────────────┤
│ Type 2 Hypervisor │
├─────────────────────────────────────┤
│ Host OS │
├─────────────────────────────────────┤
│ Hardware (CPU/RAM/Disk) │
└─────────────────────────────────────┘
- Runs as an application on top of a host OS
- Easier to set up and use
- Slightly lower performance due to host OS overhead
- Examples: VMware Workstation, VirtualBox, Parallels Desktop
- Used in: Development, testing, desktop virtualization
Properties of Virtual Machines
Virtual machines provide four key properties that make them valuable:
1. Partitioning
- Run multiple operating systems on one physical machine
- Divide system resources between virtual machines
- Each VM gets allocated CPU, memory, disk, and network resources
Example:
Physical Server: 64 GB RAM, 16 CPU cores
├── VM1 (Web Server): 16 GB RAM, 4 cores
├── VM2 (Database): 32 GB RAM, 8 cores
└── VM3 (App Server): 16 GB RAM, 4 cores
2. Isolation
- Fault isolation: If one VM crashes, others continue running
- Security isolation: VMs are separated at the hardware level
- Performance isolation: Resource controls prevent “noisy neighbor” problems
Benefits:
- Secure multi-tenancy in cloud environments
- Contain security breaches
- Predictable performance
3. Encapsulation
- Save entire VM state to files
- Move and copy VMs as easily as files
- Snapshot and restore VM states
Use Cases:
# VM files typically include:
- .vmdk / .vdi → Virtual disk files
- .vmx / .vbox → VM configuration
- .nvram → BIOS settings
- .vmem → Memory snapshot
This enables:
- Easy backup and disaster recovery
- VM migration between hosts
- Template-based deployment
4. Hardware Independence
- Provision or migrate any VM to any physical server
- Abstract hardware from the operating system
- Standardized virtual hardware regardless of physical hardware
Advantages:
- Workload mobility across different hardware
- Simplified hardware upgrades
- Cloud provider flexibility
How Virtualization Works
The Illusion
The VM gives users an illusion of running on a physical machine:
- Applications run normally without modification
- OS believes it has direct hardware access
- Users interact with VM like a real computer
The Reality
Behind the scenes:
- Operating systems normally run in privileged mode
- Direct access to hardware
- Can execute privileged instructions
- VM OSs run in user mode
- No direct hardware access
- Privileged instructions are trapped
- Most instructions execute directly
- Hardware executes them without hypervisor intervention
- Provides near-native performance
- Resource management handled by hypervisor
- Memory allocation
- Peripheral access
- CPU scheduling
- Privileged instructions are “trapped”
- Hypervisor intercepts them
- Emulates the instruction
- Returns control to VM
Hardware Assistance
Modern CPUs include virtualization support:
- Intel VT-x (Intel Virtualization Technology)
- AMD-V (AMD Virtualization)
These provide:
- Hardware-assisted virtualization
- Improved performance
- Better security isolation
VM Components
A virtual machine virtualizes several key components:
1. CPU Virtualization
- Virtual CPUs (vCPUs) mapped to physical CPUs
- CPU scheduling by hypervisor
- Support for multiple CPU architectures
2. Memory Virtualization
- Virtual memory presented to guest OS
- Memory management by hypervisor
- Techniques like memory ballooning and page sharing
3. Network Virtualization
- Virtual network interfaces
- Virtual switches and routers
- Network isolation and bridging
4. Disk Virtualization
- Virtual disks stored as files
- Thin provisioning and snapshots
- Multiple disk formats (VMDK, VHD, QCOW2)
Storage and Migration
- VM is stored as a file → Easy to save and migrate
- Can be moved along with apps and configuration
- Enables cloud elasticity and disaster recovery
Paravirtualization
An alternative approach to full virtualization:
Concept
- VM OS knows about virtualization
- Makes specific hypercalls instead of privileged instructions
- Requires modified guest OS
Advantages
- Better performance than full virtualization
- More efficient resource usage
- Lower overhead
Example: Xen
Xen hypervisor uses paravirtualization:
- Guest OS modified to make hypercalls
- Direct communication with hypervisor
- Used by early AWS EC2 instances
Modern Trend
- Hardware-assisted virtualization has reduced the need for paravirtualization
- Most modern systems use full virtualization with hardware support
- Paravirtualization still used for specific optimizations (e.g., virtio drivers)
Comparison: Type 1 vs Type 2
| Aspect |
Type 1 (Bare Metal) |
Type 2 (Hosted) |
| Installation |
Directly on hardware |
On top of host OS |
| Performance |
Higher (direct hardware access) |
Lower (host OS overhead) |
| Use Case |
Production servers, cloud |
Development, testing |
| Examples |
ESXi, Xen, Hyper-V |
VirtualBox, VMware Workstation |
| Management |
More complex |
Easier to use |
| Cost |
Often enterprise licensing |
Often free or low-cost |
Real-World Applications
Cloud Computing
All major cloud providers use virtualization:
- AWS: Xen, KVM (Nitro), bare metal options
- Azure: Hyper-V
- Google Cloud: KVM
Enterprise Data Centers
- Server consolidation (reduce physical servers)
- Disaster recovery and business continuity
- Test and development environments
Desktop Virtualization (VDI)
- Virtual desktops for remote workers
- Centralized management
- Enhanced security
Summary
Virtualization is a cornerstone technology that:
✓ Enables multiple VMs on a single physical machine
✓ Provides isolation for security and fault tolerance
✓ Allows easy migration and backup through encapsulation
✓ Abstracts hardware for flexibility and portability
✓ Powers modern cloud computing platforms
In the next lecture, we’ll dive deeper into how virtualization works internally, including binary translation and dynamic translation techniques.