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Compare bare-metal hypervisors and hosted hypervisors

Bare-metal hypervisors are the most commonly deployed hypervisor, installed directly onto the computing hardware. Learn what a hosted hypervisor, which runs within the OS, can do.

Virtualization technology, which usually relies on a bare-metal hypervisor -- software that resides between the hardware and the operating system (OS) -- opens the door to new levels of hardware consolidation and workload flexibility. Simply stated, a hypervisor creates a layer of abstraction that isolates an OS and its associated applications from the underlying computing hardware. The isolation effectively mitigates software from its traditional reliance on hardware devices and their drivers.

The implications of this behavior are profound. A hypervisor allows OSes and their application workloads to run on a broader array of hardware. Similarly, multiple OSes and workloads, each a unique virtual machine (VM) or VM instance, can reside on the same system to simultaneously share computing resources.

You can migrate each VM between computing platforms on demand with little (if any) processing disruption. The result is better use of computing platforms with seamless workload migration and backup capabilities.

Introduction to Virtualization e-book
This article is excerpted from Chapter 7 of the Introduction to Virtualization e-book, which covers the basics of server virtualization technology. Learn about server consolidation, disaster recovery, high availability and more.

Hypervisors generally fall into two categories: bare-metal and hosted. Both offer distinct benefits and drawbacks; which type you choose for your enterprise can greatly affect performance and manageability.

Bare-metal hypervisors, the most commonly deployed type, install directly onto the computing hardware. The OS installs and runs above the hypervisor. Major virtualization products can be termed as bare-metal hypervisors, including Oracle VM, VMware ESX Server, Microsoft Hyper-V and Citrix XenServer.

Some bare-metal hypervisors can be embedded into the firmware suite of the computing platform -- the same level as the motherboard BIOS. Examples of this approach include Hitachi Virtage, VMware ESXi and Linux KVM, a kernel-based VM.

A hosted hypervisor, on the other hand, runs within the OS and allows additional OS and application instances to run on top of it. VMware Server and Microsoft Virtual Server, as well as numerous endpoint-based virtualization platforms like VMware Workstation, Microsoft Virtual PC and Parallels Workstation are hosted hypervisors.

Hypervisors don't simply manage virtual instances. Current hypervisors actively manage the computing platform for better control and resource allocation to the instances that depend on their unique processing needs.

"There's intelligence in the underlying hypervisor for performance balancing so that I can run applications inside virtual machines," said Chris Wolf, senior analyst at the Burton Group, an IT research and advisory firm headquartered in Midvale, Utah. This also allows administrators to properly allocate resources to applications that need it most, he added. Wolf said that ample security features are available in many hypervisors to support multi-tenancy while providing features like auditing and administrative validation.

Looking beyond the 'big three' hypervisors

Citrix, Microsoft and VMware have become entrenched as the "big three" bare-metal hypervisor vendors, leaving administrators to ask: What other hypervisors are out there? The answer is that there are few alternatives, and this is due, in part, to the economic climate.

It's extremely difficult to bring a new hypervisor to market. The fact that basic hypervisors are "free" means that new vendors must license premium features and tools that would also need to be perfected. "For someone to come out with a new hypervisor now, they would have to start reinventing the wheel with all of the virtual infrastructure that's built on top of it," said Tony Iams, vice president and senior analyst of system software research at Ideas International Ltd., a global IT analysis firm located in Rye Brook, N.Y.

Still, some competitors out there are worth a look. Red Hat KVM is one alternative to the big three hypervisors, though that ecosystem is more appropriate for existing Red Hat customers. It's difficult to convert current Microsoft, VMware or Citrix users over to the Red Hat platform.

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