Virtualization and Cloud Computing · Topic 1 of 4
Virtual Machines and Hypervisors
Understand virtual machines, hypervisor types, host and guest roles, snapshots, virtual networking and the hardware resources needed to run virtualization.
- 5 min
- Includes a diagram
- A VM runs on a host under a hypervisor; the VM is the guest.
- Type 1 runs directly on hardware; type 2 runs on top of an operating system.
- Guests share the host's processor, memory, storage and network.
What it is
Virtualization lets one physical computer run several separate virtual computers. Each virtual computer, called a virtual machine (VM), has its own operating system and apps, and believes it has its own hardware.
The software that creates and manages virtual machines is called a hypervisor. The physical computer is the host, and each virtual machine is a guest.
How it works
The hypervisor divides the host's resources among the guests and keeps them apart. A guest sees virtual hardware: a virtual processor, virtual memory, a virtual disk and a virtual network adapter.
Two hypervisor types
| Type | Also called | Runs on | Typical use |
|---|---|---|---|
| Type 1 | Bare metal | Directly on the hardware | Data centres and servers; efficient and secure |
| Type 2 | Hosted | On top of a normal operating system | Desktops and labs; easy to install like any app |
A type 1 hypervisor has no general purpose operating system under it, which means less overhead. A type 2 hypervisor is simpler for a technician or student who wants a few virtual machines on a laptop.
Resources a VM needs
Every guest takes a share of the host. The host must have enough of each resource for all guests that run at once.
| Resource | What to consider |
|---|---|
| Processor | The central processing unit (CPU) must support virtualization features, and you assign cores to each VM |
| Memory | Random access memory (RAM) is split among guests, and the host also needs some for itself |
| Storage | Each VM has a virtual disk file on the host, so total disk space and speed matter |
| Network | Each VM has a virtual network adapter attached to a virtual switch |
If the combined demand of guests is greater than the host can supply, all guests slow down. A solid state drive (SSD) and plenty of memory help most.
Virtual networking
A virtual switch connects the guests to each other and to the physical network. Common choices are:
- Bridged: the VM appears on the real network with its own address, like a separate computer.
- Network address translation (NAT): the VM shares the host's address and can reach out, but outside devices cannot easily reach it.
- Internal or host only: guests can talk only to each other or to the host.
Pick internal networking for a safe test lab and bridged networking when other computers must reach the VM.
Virtual disks and snapshots
A virtual disk is a file or set of files that acts as a hard drive for the VM. This makes it easy to copy, move or back up a whole machine.
A snapshot saves the state of a VM at a moment in time, so you can return to it later. Snapshots are useful before a risky change such as an update. They are not a replacement for backups, because they depend on the original disk file.
Starting out
- Check that the CPU and firmware support virtualization and that the feature is turned on.
- Install a hypervisor.
- Create a VM and give it cores, memory and a virtual disk.
- Attach an operating system installer and install it.
- Install the guest tools or drivers that improve speed and integration.
If a hypervisor will not start a 64 bit guest, check that hardware virtualization is enabled in the firmware settings.
Security of virtual machines
Each guest needs updates, antimalware and a firewall like a real computer. The hypervisor needs updates too, because a flaw there could expose all guests. Keep test and production guests apart.
What to remember for the exam
- A VM runs on a host under a hypervisor; the VM is the guest.
- Type 1 runs directly on hardware; type 2 runs on top of an operating system.
- Guests share the host's processor, memory, storage and network.
- Virtualization support in the CPU may need to be enabled in firmware.
- Bridged networking gives the VM its own address on the real network.
- NAT networking lets the VM go out through the host's address.
- A snapshot saves a VM state; it is not a backup.
- Guests need patches and security software.
Common mistakes
Giving all host memory to guests. The host needs some for itself, so overcommitting slows everything.
Treating snapshots as backups. A lost or damaged disk file loses the snapshots too.
Forgetting to enable virtualization in firmware. Without it a hypervisor may not run or may run slowly.
Mixing up type 1 and type 2. Type 1 sits on hardware, while type 2 needs a host operating system.
Ignoring guest security. A VM that is unpatched is as exposed as a real computer.
Check yourself
What is the difference between a type 1 and a type 2 hypervisor?
A type 1 runs directly on the hardware, while a type 2 runs as an application on a host operating system.
What does a snapshot do?
It saves the state of a virtual machine so you can return to that point later.
What setting often needs enabling before a hypervisor works well?
Hardware virtualization support in the firmware settings.
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