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Hypervisors and the workloads they virtualize allow organizations to make more effective use of hardware.
When organizations began to discover that they were running workloads on physical servers with less than 15% utilization, they began turning to server virtualization hypervisors to increase the utilization levels of their physical servers to 80% or more.
A physical server running multiple virtual workloads -- usually 10, 20, 30 or more virtual workloads per physical server -- provides a more efficient utilization model and supports the concept of a green data center. The magic that makes server virtualization possible is a small piece of code called a hypervisor -- a tool that exposes hardware resources to support the operation of multiple virtual machines (VMs), which are operating system instances that are designed to share resources such as CPU cores, RAM, network interface cards (NICs) and storage. These are x86-based operating systems and can run Windows or Linux in both 32-bit and 64-bit versions.
Getting the most out of your hypervisors
Hypervisors run workloads inside VMs, which are used to run the traditional networked services that organizations rely on to run their businesses. IT professionals continue to manage these workloads in the same way they always have -- a VM can behave and interact with the outside world in exactly the same way as a physical machine.
But physical server management has changed. This new operational model relegates server hardware to the same level as other hardware devices such as routers, switches and storage containers. Servers are resources that you can pool together into highly available clusters that ensure that the VMs running user-facing workloads are always available.
To obtain the most from your hypervisor, you must make sure you've selected the right one for your needs. Several types of hypervisors are on the market, and there are several manufacturers that offer hypervisors. To make the best decision, you should weigh the following factors and then determine which product best suits your organization.
Type 1 vs. Type 2 hypervisors
There are two types of hypervisors on the market: Type 1 (or bare-metal) hypervisors, which run directly on top of hardware, and Type 2, which run as applications on top of existing operating systems. Type 1 hypervisors support hardware virtualization, whereas Type 2 hypervisors support software virtualization. Check out this full virtualization hypervisor comparison.
Comparing hypervisors' performance metrics
Most hardware virtualization hypervisors -- VMware, Microsoft and Citrix Systems -- include similar basic features. One of the best ways to determine which hypervisor meets your needs is to compare their performance metrics. These include CPU overhead, amount of maximum host and guest memory and support for virtual processors.
But metrics alone should not support your choice. In addition to the capabilities of the hypervisor, you must also verify the guest operating systems that each hypervisor supports.
If you are running heterogeneous systems in your service network, then you must select the hypervisor that has support for the operating systems you currently run. If you run a homogeneous network based on Windows or Linux, then support for a smaller number of guest operating systems might fit your needs.
Additional selection criteria for hypervisors
The selection you make may well be based on other factors. For example, some organizations may opt for VMware just because it offers the oldest and most popular hardware virtualization hypervisor on the market.
Others may opt for Microsoft Hyper-V because they are already a Windows shop, or because it's free. Still others may opt for Citrix Systems XenServer because it is reputed to be a powerful virtualization platform, and they already have an existing relationship with Citrix through the use of its XenApp product. And there may be those who decide that the Big Three are not for them and go for a lesser-known hypervisor.
In the end, the choice you make will depend on a lot of factors. All hypervisors are not made equal, but they all offer similar features. Understanding the features they have as well as the guest operating systems each supports is an essential aspect of any hardware virtualization hypervisor selection process. Matching this data to your organization's requirements will be at the core of the decision you make.
About the experts
Danielle Ruest and Nelson Ruest are IT experts focused on continuous service availability and infrastructure optimization. They are authors of several books, including Virtualization: A Beginner's Guide and Windows Server 2008, The Complete Reference from McGraw Hill Osborne as well as the MCITP Self-Paced Training Kit (Exam 70-238): Deploying Messaging Solutions with Microsoft Exchange Server 2007 from MS Press. Their latest book is a training kit for Microsoft exam titled 70-652: Configuring Windows Server Virtualization with Hyper-V from MS Press. Contact them at firstname.lastname@example.org.
This was first published in September 2010