IT 3300 : Virtualization

Virtualization Concepts

Driving forces

• Underutilized hardware — a physical server sat mostly idle
• Space — fewer physical boxes in the rack
• Energy — power and cooling cost real money
• Administration — manage images, not screwdrivers

Categories of virtualization

• Client — VMware Workstation, VirtualBox, Parallels
• Server — our focus this semester
• Storage — SAN, NAS
• Network — SDN (software-defined networking)

Types of virtualization

• OS-level virtualization (containers)
• Hardware emulation / full virtualization
• Paravirtualization

We'll define each, because the differences explain the whole course.

OS-level virtualization

• Isolated environments that each think they own the whole OS
• Each has its own file system, process tree, network, libraries
• They share the host kernel
• This is what containers are: LXC, Docker, jails

OS-level: pros

• Files shared by many containers are stored once
• Best performance of any virtualization approach
• Highest density (most environments per machine)
• Great for isolating applications; often cheaper to license

OS-level: cons

• Every container shares the host's kernel
  • Same kernel version, same patches, same OS family
  • Can't run a Windows container on a Linux kernel
• If the host kernel dies, every container goes with it

Hardware virtualization (full)

• A hypervisor emulates hardware for each guest
• Guests run on "virtual" hardware, not the physical machine
• The guest OS has no idea it is virtualized
• Images can migrate between hypervisors, even across machines

Hypervisors

• A.k.a. Virtual Machine Manager (VMM)
• Intercepts guest calls to hardware and mediates them
• "Hypervisor" = the supervisor of the supervisors (the kernels)

Hypervisor types

• Type 1 (bare metal) — runs directly on hardware
  • VMware ESXi, Microsoft Hyper-V, Proxmox/KVM, Xen
• Type 2 (hosted) — runs on top of a host OS
  • VirtualBox, VMware Workstation, QEMU

Hardware VZ: trade-offs

• Pro: run dissimilar OSes; excellent for server consolidation
• Con: slight overhead — everything passes through the hypervisor
• Con: hardware support limited to the hypervisor's drivers

Paravirtualization

• Hardware is not fully emulated
• The guest OS knows it is virtualized and cooperates
• Guest issues efficient API calls instead of trapped hardware calls
• Less overhead; drivers come from the guest, not the hypervisor
• Example: Xen (paravirtualized guests)

Putting it together

• Full VZ — guest is unaware; best isolation & portability; some overhead
• Paravirtualization — guest cooperates with the hypervisor; less overhead
• OS-level (containers) — least overhead, highest density, shared kernel

Where this course lives

• Proxmox gives us full/para virtualization (KVM) and OS-level (LXC)
• Docker is OS-level virtualization for applications
• Kubernetes orchestrates those containers at scale