The main feature that really makes this solution impressive is the concept of live migration. Live migration is when a virtual operating system can be moved, while running, from one physical virtualization server to another. This can be done for purposes of load balancing, disaster testing or to survive a disaster itself. With some live migration solutions, generally sold as high availability, this migration can happen so quickly that it provides effectively "zero downtime." And even heavily used web servers could survive the loss of a physical server without customers ever knowing that a physical server had gone down. The transition between virtual machine host nodes is completely transparent to the end users.
There is one major caveat. Relying upon a SAN in a disaster recovery scenario, of course, creates another point of failure - the SAN system. So when planning to use SAN to increase the reliability of your virtual machines, be sure not to use a SAN that is not as redundant or more so than your servers themselves. Otherwise you may increase cost while accidentally lowering reliability and performance.
For the average small business it is not unlikely that it will make sense to not only virtualize some of the server infrastructure but virtualize all or nearly all of it. Virtualization's advantages are so many and its downsides so few and minor that it is a rare workload in the small business space that would justify dedicated hardware servers.
Unlike real desktops and servers, virtualized desktops often add a bit of complexity due to licensing requirements, especially with Microsoft Windows desktops.
Virtualizing desktops is also somewhat complicated because there are many modes for physically providing desktops. Obviously once we begin talking about virtualizing the desktop infrastructure we are actually talking about a range of solutions. This is because some device must always exist "on the desktop," providing a keyboard, mouse and monitor which cannot be virtualized. And the desktop operating system itself must be running elsewhere.
Even without virtualization this is done (and sometimes marketed as virtualization when, in fact it is simply remote access) very commonly through desktop blades, rackmount desktops or terminal servers. All these solutions move the desktop into the datacenter and provide access to it either from thin client front ends or simply via software to remote users existing machines (such as users at home logging in to the office).
We will start with the concept of the terminal server, as this is the most easily virtualized and the most straightforward. Whether we are talking about virtualizing the server on which we run Microsoft Terminal Server (now known as Remote Desktop Services), Citrix XenApp or simply a standard Linux remote desktop terminal server, we need do nothing more than install that server into a virtual environment rather than into a physical one. It is really a question of server virtualization, not of desktop virtualization it is only perceived by the end user as being related to their desktops.
The other method of desktop virtualization, "true desktop virtualization" as I will refer to it, is to actually run desktop operating system images on a virtual server just as if they were normal desktops dedicated to a user.
This means virtualizing operating systems like Windows XP, Windows Vista or Windows 7 with each image being dedicated to a single user just as if it was a physical desktop.
We could, theoretically, do the same thing with Linux or some other flavor of Unix. But since those systems do not have per user licensing or desktop specific versions and since they always run their desktops in a server mode, we can only differentiate between a true virtualized desktop and a Unix-based terminal server in its usage not by any strict technological means, as they are one and the same.
Only Windows truly offers a dedicated desktop model that allows this to happen in this particular manner without the concept of shared access to a single image simultaneously.
Due to licensing restrictions from Microsoft, Windows desktops must be installed one image per user, even if technologies exist to make this technologically unnecessary. But still there are benefits to this model. The big benefits to virtualized desktops definitely go to companies who have employees who roam either internally or even externally.
Using virtualized desktops provides far more control to the company than does providing laptops. Laptops can be stolen, lost or damaged. Laptops wear out and need to be replaced regularly. A virtual desktop that is made accessible from the outside of the company can be secured and protected in ways that a laptop cannot be. Upgrades are much simpler and there is no concern of the virtual desktop becoming cut off from the corporate network and being unable to be supported by the IT staff.
Almost any worker who uses a computer in the office already has one at home for personal use and often also has laptop in addition to high speed Internet access. Providing remote access to a virtual desktop at the office therefore potentially incurs no additional hardware expense for the company or staff while easing administrative burdens, lowering power consumption and increasing security. For workers still sitting at a traditional desk inside of the company's offices there is still a need for something physically sitting on the desk that will connect the keyboard, mouse and monitor to the newly virtualized desktop. This could be an old PC that was planned for retirement, a dedicated hardware thin client or even a laptop.
Internal staff can then move around the office or between offices and sit at any available desk with a thin client and log in to their own dedicated virtual desktop and work exactly as if they were at their own desk. They can then go home and work from there as well if this is allowed.
Like virtualized servers, desktops, if the need is warranted, can be easily backed up using either traditional means or by simply taking complete system images. The flexibility is there to do whatever makes the most sense in your environment.
Desktop virtualization is hardly the no-brainer that server virtualization is. Its less advantageous due to the complexity and surprise cost of licensing. And, except for remote users, hardware on the desktop must always remain.
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