1.5

Virtualization abstracts computer hardware (CPU, memory, disks, network cards) of a single computer into separate computing environments.
Provides an illusion of running a separate complete computer on top of another.
Each environment is complete with its own operating system, including the kernel.
Virtualization allows operating systems to run as if they are applications within other operating systems.
Made possible by cheap multicore CPUs, cheap memory, and embedded virtualization technology in modern CPUs.
Virtualization is different from emulation:
- Emulation translates every application command at runtime, making it slower.
- Virtualization is for the entire operating system; an OS natively compiled for a specific CPU architecture can run on that CPU on top of another OS.
- Virtualization doesn't require real-time translation, making it much faster than emulation.

Hypervisor is software that creates and runs virtual machines, managing abstract hardware for the operating system.
The computer running the hypervisor is the host machine.
The virtual machine running on the host machine is the guest machine.
Nested virtualization: running virtual machines inside another virtual machine.
Different operating systems (Linux, Windows, macOS) can run simultaneously if compiled for the same architecture, each with its own kernel.
Modern CPUs have special instructions exclusively for hypervisors to enable virtualization; older machines may need to activate these instructions.
Management tools with hypervisors allow starting and stopping of virtual machines.

Runs directly on the host hardware, controlling the hardware to manage guest operating systems.
No intervening operating system; the hypervisor acts as the host OS.
All hosts on the host machine run as virtual machines.
One virtual machine on the host is used as a management tool.
Forms the basis of cloud computing platforms like Microsoft Azure and Amazon Web Services.

Software that runs on personal computers.
Allows end-users to run virtual machines on their own operating systems (Windows, macOS, Linux).
Useful for creating multiple environments (development, testing, acceptance).
Guest OS may need slight modifications to work with shared resources from the host OS.
KVM (Kernel Virtualization Module) in Linux converts the host OS into a hypervisor, a hybrid between types 1 and 2.
Common type 2 hypervisors: VirtualBox, VMware Workstation, Hyper-V.

Using hypervisor software for a direct console connection.
Connecting via network protocols for VMs on networks or in the cloud.
Linux guest OS (headless):
- Connect via SSH.
Linux guest OS (desktop environment):
- Client-server protocols such as X2Go (used in this course), VNC, Spice, and Steam viewers.
Windows guest OS:
- Connect via RDP (Remote Desktop Protocol), which is typically pre-installed.

Virtualization technology optimized for Windows.
Allows access to a Linux VM, including the full kernel for Linux.
Integrates Linux into the Windows ecosystem.
Install Linux as an app from the Microsoft Store rather than creating a full VM.
WSL2 boots faster than traditional VMs but may have slower file system operations.

Application-level virtualization, leaner and faster than full OS virtualization.
A container holds a single application (or small set of related applications) and the system resources needed to run it.
Containers reuse the host computer's kernel, so most are Linux containers.
Applications inside a container believe they are in a full, isolated system.
Isolation improves security, prevents crossover effects, and allows more efficient use of the host computer.
System independence: containers have their own system environments independent of the host OS distribution.
Hardware independence: containers can easily be moved from one computer to another.
Requires granting specific access to host resources.

Bare Metal:
- Hardware -> Host OS (Kernel) -> Applications
- Traditional way of computing; less flexible and potential security issues.
Virtualization:
- Hardware -> Host OS -> Hypervisor -> Guest OS (with individual kernels) -> Applications
- Increased flexibility, OS isolation, and hardware efficiency.
Containerization:
- Hardware -> Host OS (Kernel) -> Container Engine (e.g., Docker) -> Containers (single applications)
- More efficient by reusing the host OS kernel; faster boot times.

Docker:
- Provides an environment for a single application.
- Uses a Dockerfile: a declarative description of the application's setup inside the container, enabling repeatable creation.
- Layered architecture with immutable resourcing: containers are created once and replaced for modifications.
LXC (Linux Containers):
- Provides a full Linux installation environment similar to a VM but without its own kernel.
- Only takes about 3% of the time to boot compared to a full VM.
- Docker was originally built on top of LXC.

Containers:
- Complete, isolated environment running on the host kernel.
Packages:
- Share library files with the host OS, potentially leading to conflicts or effects from upgrades.
Alternatives to packages (Flatpak, Snap, AppImage) offer container-like isolation on Linux.

Automating the creation and configuration of virtual resources using code.
Replaces manual management with automated processes for large-scale cloud computing.
Modern declarative tools are preferred over procedural shell scripts.

Provisioning (creating virtual resources):
- Terraform: for large deployments across multiple cloud providers.
- Vagrant: for consistently creating development, testing, and staging environments.
Configuration (modifying systems over time):
- Ansible: configures resources in a consistent and repeatable manner.