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Server Virtualization
Virtualization has been around for quite a while - decades ago, big corporate mainframes were logically divided into different partitions running different operating systems. But server virtualization using Intel-based servers has only penetrated the enterprise data center in the last few years. Most industry sources agree that the "tipping point" was reached in 2008, when more than 50% of all new Windows servers placed in service were virtualized.
Server virtualization is
accomplished by installing a layer of abstraction software, generally referred to as a "hypervisor," onto the
server hardware. Each of the guest operating systems runs in its own protected environment, isolated from the
others. Each one thinks it is talking directly to the server hardware, but in fact its interactions
with the hardware are mediated by the hypervisor.A key breakthrough occurred a few years ago when Intel and AMD both introduced processors that had built-in hardware support for virtualization. Before that, complex emulation software was necessary to isolate the guest operating systems from the hardware, e.g., to prevent a processor "stop" command from actually reaching the processor, lest it halt execution for all the guests running on that processor.
The pioneer in this space was, of course, VMware, and they accomplished some truly magical things given the limitations of hardware and software at that point in time. But the new generation of processors made possible a new generation of hypervisors that were much "thinner," more efficient, and consumed fewer resources, making it possible for the guest systems to achieve performance levels very close to "bare metal" performance - in other words, the performance that could have been achieved had they been installed directly on the hardware.
One of the first companies to exploit the new hardware capabilities was XenSource, which was founded by some of the people who were involved in the development of the open source Xen hypervisor at Cambridge University. XenSource was acquired by Citrix in the fall of 2007, and the product they had developed became Citrix XenServerTM. XenServer is a native 64-bit hypervisor that supports both Windows and Linux guests, and has proven to be the best and most efficient platform for virtualizing XenApp (formerly Presentation Server) workloads. Recently, it was the best-performing hypervisor in 9 out of the 12 tests run in the recent Network World hypervisor "bake off."
Microsoft, of course, recently introduced Hyper-V, which has also proven to be an extremely efficient hypervisor for virtualizing Windows workloads. They also introduced the first "enlightened" Windows Server Operating System, Windows Server 2008, which is capable of recognizing when it is running in a virtual environment, and behaving accordingly.
Server virtualization is particularly powerful when it is combined with storage virtualization. From the viewpoint of a virtual host, a guest operating system is nothing but one big file. If that file resides, not on the local storage of the virtual host, but on shared storage that multiple virtual hosts can access, then it becomes relatively easy to move a guest operating system from one host to another. XenServer is capable of "live motion," where a guest operating system is moved from one host to another while it's running, and it happens so quickly that there is no discernable interruption in service. Hyper-V, in it's first release, features "quick motion," where the guest operating system is paused, copied to another host, then execution is resumed. Hyper-V is expected to support live motion with the Windows Server 2008 R2 release.
One of the biggest announcements in the virtualization space took place this past February, when Citrix decided to make XenServer free. This free version of XenServer includes live motion, full multi-server management for an unlimited number of virtual hosts and guests, physical-to-virtual and virtual-to-virtual migration tools, and no limit on the amount of RAM in a virtual host machine. In fact, you can simply click on the link in the upper right of this page, and download XenServer right now!
At the same time, Citrix announced Citrix EssentialsTM for XenServer and Citrix Essentials for Hyper-V. Citrix Essentials for XenServer and Hyper-V adds a set of management and automation capabilities to both XenServer and Hyper-V:
- Automated lab management streamlines the process of building, testing, and sharing virtual systems throughout the application lifecycle, from development labs into the production environment, including the ability to automatically delete development systems after a specified time period to reduce server sprawl.
- Advanced storage integration allows for managing your shared storage environment from within the Citrix management system.
- Dynamic provisioning services using Citrix Provisioning Server for the on-demand deployment of workloads to either physical servers or the virtual environment from a single shared "golden image."
- Workflow orchestration for simplified scripting of workflows to automate key management processes.
- High availability for the automatic restart and intelligent placement of virtual machines in the event that a physical server should fail.
