Expert Tips on
Virtualization Selection
and Strategies
In this E-Guide, virtualization experts Andrew Kutz and Steve Shah weigh in with detailed technology comparisons for those considering a virtualization solution. You’ll hear industrial insight on the various options emerging—including hybrid virtualization—as well as what’s in store for the market in 2008 and what factors you need to consider when making a final decision.
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Expert Tips on Virtualization Selection and Strategies Table of Contents
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Weighing hosted, bare-metal, OS and hybrid server virtualization Citrix, Microsoft and VMware: Comparing virtualization strategies Resources from SAVVIS
Expert Tips on
Virtualization Selection
and Strategies
Weighing hosted, bare-metal, OS and hybrid server virtualization
Andrew Kutz, Site Expert, SearchServerVirtualization.com
In this tip, you’ll learn the details and differences among server virtualization, operating system (OS) virtualization, hosted virtualization and bare-metal virtualization. You’ll also get a glimpse of the new virtualization technology called hybrid virtualization, and learn how Microsoft’s plans for 2008 will affect the virtualization space.
Seven years have passed since VMware released ESX, and although VMware pioneered x86 server virtualization, they are no longer the only settlers headed toward the virtualized west. There are several caravans full of talented vendors creating their own brands of virtualization. To help you sort through the various offerings, this article reviews the four types of virtualization architectures currently in use on the market today and suggests what direction these models may take in the years ahead.
Hosted virtualization
The first type of virtualization is what most users are most familiar with—hosted virtualization. All of the desktop virtualization products, such as VMware Workstation, VMware Fusion, and Parallels Desktop for the Mac, implement hosted virtualization architecture.
As you can see, the hosted virtualization approach relies on having an existing operating system (OS) in place. The hypervisor sits on top of the OS, and the virtual machines (VM) are managed by the hypervisor.
There are many benefits to this type of virtualization. Users can install a virtualization product onto their desktop just as any other application, and continue to use their desktop OS. Hosted virtualization products also take advantage of the host OS’s device drivers resulting in the virtualization product supporting whatever hardware the host does. However, hosted virtualization also has its downsides. Notice that there is a memory manager and central process-ing unit (CPU) scheduler in both the hypervisor and the host OS. This arrangement creates a large amount of
overhead. The reason this approach was taken was due to necessity since when hosted virtualization products were created prior to hardware virtualization extensions.
Hosted virtualization products are still going strong today (as evidenced by VMware Workstation 6.0), but for how long this trend continues is unknown. The fact is that the fourth type of virtualization architecture, hybrid, has the ability to offer all of the advantages of hosted virtualization without any of the overhead.
Only time will tell if companies like VMware, Microsoft, and Parallels evolve their hosted products to use a hybrid model.
Bare-metal virtualization
The second virtualization architecture is the current enterprise data center leader—bare-metal virtualization. VMware ESX is easily the market leader in enterprise virtualization at the moment, and it utilizes bare-metal virtu-alization architecture.
Immediately apparent about the above architecture is the lack of an existing OS; the hypervisor sits directly on top of the hardware—hence the term “metal virtualization.” The reason so many data centers implement bare-metal products, such as ESX and Xen, is because of the speed it provides due to the decreased overhead from the OS that hosted virtualization uses.
Some readers may be wandering why I have categorized ESX and Xen together, after all, aren’t they utilizing different architectures?
Yes and no. Enter the difference between full- virtualization and para-virtualization. Full-virtualization is where the VM’s guest OS has no idea it is being virtualized while para- virtualization requires the VM’s guest OS be modified in order to be virtualized. ESX has traditionally used full-virtualization while Xen pioneered para-virtualization. In truth both of these forms are still bare-metal virtualization and both forms are used by ESX and Xen today. So for the pur-poses of this article full and para- virtualization are both categorized under the auspice of bare-metal virtualization. There are some downsides to using bare-metal virtualization. Typically the vendor publishes a hardware compatibility list (HCL) that dictates what hardware can be used with their virtualization product. This is because in order to keep
Expert Tips on Virtualization Selection and Strategies Weighing hosted, bare-metal, OS and hybrid server virtualization
the hypervisor as slim as possible, the number of device drivers in the hypervisor kernel is kept to a minimum. Some hypervisors have workarounds to this, such as Xen and driver domains, but these are not for the faint of heart. The aspect of bare-metal virtualization that makes it so appealing for data center use is not its performance in my opinion, but the fact that products that implement it are distributed as appliances or server OSes. Take VMware ESX or XenServer for example: you simply boot the server with an installation CD-ROM and it installs on the hard drive without the fuss or muss of messing with an existing OS. Embedded hypervisors are great examples of virtualization appliances. Turn the server on and it configures itself for your virtualization infrastructure. However, none of these features of bare-metal virtualization are derived from the architecture itself, which is why bare-metal virtualization may face serious competition this coming year from the fourth architecture in this list.
Operating system virtualization
OS virtualization has been making waves lately because Microsoft is rumored to be in the market for an OS virtualiza-tion technology. The most well-known products that use OS virtualizavirtualiza-tion are Parallels Virtuozzo and Solaris Containers.
OS virtualization has a very low overhead despite an existing OS because it does not utilize a traditional hypervisor to manage VMs. Instead, the OS virtualization model divides a single OS into containers and uses a container manager to facilitate management. This virtualization architecture has many benefits, speedy performance being the foremost. Another benefit is reduced disk space requirements. Many containers can use the same files, resulting in lowered disk space requirements.
The big caveat with OS virtualization is the OS requirement. Container OSes must be the same OS as the host OS. This means that if you are utilizing Solaris containers then all containers must run Solaris, or if you are implement-ing Virtuozzo containers on Windows 2003 Standard Edition then all its containers must also be runnimplement-ing Windows 2003 Standard Edition.
For some people the container OS requirement is too much of a no-go, but many other IT administrators see OS virtualization as the perfect architecture for implementing virtual desktops and Web servers since those platforms share many common files. However, much like the preceding two architectures, OS virtualization may soon see its proponents jumping ship to a hybrid model.
Hybrid virtualization
I have been deferring to this architecture for the duration of this article, and now I will explain why.
The hybrid model uses a host OS like hosted virtualization, but instead of laying a hypervisor on top of the host OS, a Kernel-level driver is inserted into the host OS kernel. This driver acts as a virtual hardware manager (VHM), coordinating hardware access between the VMs and the host OS. As you can see, the hybrid model relies on the memory manager and CPU scheduler of the existing Kernel. As with a bare-metal and containerized architecture, the absence of redundant memory managers and CPU schedulers increases the performance capabilities of this model. Yet unlike OS virtualization, the hybrid model does not have the restriction of only being to create guests with the same OS type as the host.
Hybrid virtualization offers all of the benefits of the aforementioned architectures and hardly any drawbacks, yet some negative aspects do exist. The hybrid model requires the underlying processor have virtualization extensions (such as Intel-VT and AMD-V) to function. This means that older hardware that could otherwise be utilized by other virtualization architectural models is useless to hybrid products. And while some people see the reuse of the existing kernels’ memory managers and CPU schedulers as a good thing, some industry analysts assert that relying on an uncontrolled entity such as a third-party kernel is not a good thing. It puts the future of the VHM in the hands of the kernel it is loaded into, because remember, despite all assertions to the contrary, in a hybrid architecture the HVM is *not* a hypervisor. For example, many people think that KVM is a hypervisor, and this is simply not the case.
Virtualization in 2008
So what’s in store for 2008 and server virtualization? Microsoft will acquire an OS virtualization technology in order to expand their portfolio, but internally they will likely be working on creating a hybrid model with their NT kernel. Windows 7 will likely ship with a HVM that allows the easy creation of VMs using Microsoft’s yet-to-be- announced built-in VM manager. On the other hand, virtualization product vendors that do not have access to a Kernel’s source code will be forced to continue to release hosted products. This will give Microsoft an edge in terms of VM performance. OS virtualization will eventually disappear as the hybrid model replaces it. Disk space is incredibly inexpensive, so the remaining benefit of OS virtualization over the hybrid architecture will not be enough of a reason to not move toward hybrid architecture which has the performance of an OS virtualization model and removes the single OS
Expert Tips on Virtualization Selection and Strategies Weighing hosted, bare-metal, OS and hybrid server virtualization
drawback. Bare-metal virtualization products will continue to thrive due to the sheer cost investment that vendors have put into them. However, hybrid model-based appliances will begin to appear that become very inexpensive alternatives to the more expensive bare-metal competitors.
About the author:Andrew Kutz is an avid fan of .NET, Open Source, Terminal Services, coding and comics. He is a Microsoft Certified Solutions Developer (MCSD), a SANS/GIAC Certified Windows Security Administrator (GCWN) and a VMware Certified Professional (VCP) in VI3.
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Citrix, Microsoft and VMware: Comparing virtualization strategies
Steve Shah, contributor to SearchServerVirtualization.com
When people ask me which virtualization platform they should choose, I give them the same answer I offer to home users asking for PC or digital camera recommendations: “It depends.”
Many people look at product reviews and research reports when choosing IT platforms. They want a quick “Consumer Reports” answer. Yet, when choosing core products, a critical step in evaluating vendors is examining their strategies and approaches, not specific products. So, in this comparison of three virtualization heavyweights, VMware, Citrix (which I’m pairing with Xen) and Microsoft, I’m looking at their positions more than their products. At the end, we should understand the vendors’ take on the virtualization market, its direction and the priorities chosen to achieve their goals.
This approach may fly in the face of product evaluation traditions; but it can be just as important as figuring out whether their products meet technical requirements. After all, if vendor X says that server virtualization is dead, and desktop virtualization is the future, buying vendor X for server virtualization would be a bad idea even if their product works well in that approach.
Citrix and XenSource
“We want a strong ecosystem. We want to enable choice.”
Those are the words of Simon Crosby, CTO of Virtualization and Management Division at Citrix and former CTO of XenSource, when asked about his top focus. Depending on where your own business priorities lie, this approach can be either a good or bad thing. It depends on how you look at leveraging multiple vendors to solve business problems. Historically, Citrix has had a solid history of supporting multiple ecosystems and encouraging their channel to support them. This approach has resulted in an impressive array of products being developed for their suite that address many verticals. ISVs invest in the Citrix machine because they see a viable business and—more importantly—because they know that Citrix tries not to step on its friends. This is great for IT managers in these verticals.
From a product perspective, ecosystem building is nice but it doesn’t tell you where a company is going with their engineers. Crosby stated his belief that hypervisors will be commoditized as core components of servers. He thinks virtualization has to be part of the “iron.”
When virtualization is done right, said Crosby, it is part of the infrastructure itself. For example, in Crosby’s world, the SAN (Storage Area Network) isn’t virtualized by software; the SAN is instead aware of the virtual infrastructure and works optimally with virtual infrastructure. In some cases, this means helping infrastructure vendors by giving them the hypervisor. For others, it simply means running an open hypervisor platform.
every-thing that needs to be done that the infrastructure knows how to abstract itself instead of depending on others to do the abstraction for it. The concept is definitely a little heady at first, but it doesn’t take long to identify the potential of a fully-virtualized data center where everything is responsible for virtualizing itself.
As a result of this fully-commoditized vision, Citrix identifies their contribution at the application level. In short, how Citrix can deliver an application, regardless of what the application needs of its infrastructure, that makes both the operational and end user experience better.
This approach requires that everyone agree to play nice with virtual environments and standards to be established in this area. It’s a tall order. Judging from the show of support from infrastructure vendors at Citrix’s latest App Delivery Expo (iForum), however, Citrix’s aggressive push is paying off.
The bottom line is Citrix/XenSource will differentiate based on what they do with their core technology and how they build a community a products around it to create the fully virtualized datacenter as opposed to differentiating based on the core technology itself. It is an atypical approach, but one that makes sense given XenSource’s roots in open source software.
IT managers who agree with this philosophy, and it does have inherent value, have to determine if the ecosystem surrounding Citrix/XenSource is strong enough to stay the course. Those that do agree should look at the
Citrix/XenSource virtualization ecosystem and ask whether the products there help your applications run better. If the philosophy and the products fit, Citrix/XenSource should be on your final evaluation list.
Microsoft virtualization
“Virtualization is like TCP/IP. It’s a means to an end.”
David Greschler, Directory of Integrated Virtualization Strategy for Microsoft, makes no bones about Microsoft’s belief that hypervisor technology will become a commodity and uninteresting. Sounds like Microsoft and Citrix might be singing from an awfully similar hymnbook!
Historically, Microsoft reaches feature levels that satisfy most users such that many don’t feel compelled to seek better alternatives, even when better alternatives exist. Right now, Microsoft isn’t a strong alternative in virtualization. Greschler himself noted that “we aren’t there yet,” but he thinks Microsoft will be a strong, or even the strongest, alternative in the near future. In the meantime, someone looking at Microsoft’s technology has to ask themselves whether today’s implementation is indeed good enough to use until then, if it’s better to wait and virtualize when Microsoft does deliver, or if the immediate need is so great that an alternative, like VMware, should be used right now. In all its spheres, from the desktop to the server, tight integration is the focus and primary value of a Microsoft ecosystem. In virtualization, Microsoft promises depth of management, achieved with its System Center’s integrated view of all the virtualization technologies used in a given data center. In virtualization, according to Greschler, con-sider than an application running inside of a SoftGrid application streaming virtual environment inside of a virtual server in the data center—and piping its display using Terminal Services to a Virtual PC inside of someone’s desk-top—has so many layers of encapsulation that traditional management tools simply don’t have an accurate view of
Expert Tips on Virtualization Selection and Strategies Citrix, Microsoft and VMWare: Comparing virtualization strategies
what’s happening. Microsoft’s focus with their System Center management suite is to provide that visibility down to the iron itself.
What’s good about this? For one thing, the concept does address a challenge that happens with too many layers of encapsulation. Networking people have repeatedly experienced this phenomenon, most recently with encapsulating XML in HTTP, when the tools for monitoring no longer have sufficient depth to accurately see what’s happening. In Microsoft’s view of the virtualized data center, multiple layers of virtualization are inevitable. This, of course, means that tools to identify resource allocation and provisioning are going to be critical.
Is there anything that might not be so good? That all depends on whether you believe Microsoft’s commoditized virtualization technology will deliver what you need for virtualization in the data center in the first place. Microsoft does have a strong position with their Virtual PC, Terminal Services and SoftGrid (Softricity) technologies. Hyper-V, once called Viridian, has yet to be seen.
The bottom line for Microsoft’s position that the infrastructure doesn’t matter. Focus on the management story. Operational teams may agree with this view, but data center architects may want to see some more out of the infrastructure before they look away.
VMware
“Being able to virtualize everything is the number one thing that drives VMware.”
Raghu Raghuram, vice president of VMware Products and Solutions take on virtualization infrastructure, offers a significant contrast to Microsoft’s and Citrix’s views. Where Microsoft and Citrix consider core virtualization technolo-gy as commodity, VMware sees the opposite: Core virtualization technolotechnolo-gy is the centerpiece. By focusing on what Raghuram calls the “Virtual Infrastructure”, VMware looks to provide a fully virtualized data center where adminis-trators see a single consistent view of everything in the data center even as the infrastructure itself changes. VMware, in its relatively short history, has succeeded in virtualizing significant parts of the infrastructure—such as SANs with VMFS—and moving the ball forward with hypervisor technologies, such as live migration using VMotion. In other words, VMware’s goal to fully abstract the data center environment so that all virtual hosts only see the view that VMware gives them appears to be an achievable endgame.
The significance of this approach is that it becomes possible to present a uniform view of the world, regardless of whether the world knows what to do with virtual machines or not. For data centers that would otherwise need to upgrade infrastructure to support virtualization, this can be a very appealing proposition; virtualization can be achieved without further investment. For system administrators and application owners, this means having a single view of the world around them, even when the physical world changes.
The challenge to this approach is that it remains a tall order to virtualize every conceivable piece of infrastructure and keep up with the changes in technology. History may be on VMware’s side with respect to their ability to exe-cute on this vision, but the challenge doesn’t become any easier. If anything, success brings more work to virtualize more infrastructure.
For VMware, succeeding in this area would crack the holy grail of utility computing, where any unit of capacity is genuinely interchangeable with any other unit of capacity, regardless of the physical world that surrounds it. From VMware’s perspective, the bottom line for administrators is: Core technology is important to delivering what they see as a true potential for virtualizing the datacenter; that is, utility computing. It is here where you’ll find the greatest strides forward by VMware.
VMware, Microsoft Virtual Server, or Xen? It depends
Between starting this article and reaching this point, I received no less than three “what’s the best” questions about cameras, phones and laptops. I, in turn, frustrated no less than three buyers with my stock answer: “It depends.” Having heard the raw, unfiltered words of Citrix, Microsoft, and VMware and where they plan to put their research and development dollars, I can firmly say that the same answer applies to “What’s the best virtualization solution?” It depends.
It depends on what you value the most in a solution. It depends on what you need to address in your own data center. It depends on what you need out of a virtualization technology today. It depends on what you need out of a virtualization technology tomorrow. It depends on what you expect from a management perspective. It depends on what you need out of vertically-focused virtualization solutions. It all depends.
Once you can succinctly define your own needs, re-read the positions of each vendor. Chances are that you’ll notice yourself aligning with one over the rest. It’s an approach that lacks the energy and fun that a pundit-fueled debate has or the simplicity of a “Consumer Reports” ranking. Also, it’ll rob you of excuses to play Bejeweled during end-less committee meetings.
The good news is that it will spare you from hearing smart-aleck, overpriced consults telling you: “It depends.”
About the author:Steve Shah is principal at Rising Edge Consulting, based in the San Francisco area. A 15-year IT veteran, he is the author of the book,Linux Administration: A Beginners Guide, and a contributing author to books on content delivery networks, Unix and Red Hat Linux.
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