In today's complex IT environments, server virtualisation simply makes sense. Redundant server hardware can rapidly fill enterprise datacenters to capacity; each new purchase drives up power and cooling costs even as it saps the bottom line. Dividing physical servers into virtual servers is one way to restore sanity and keep IT expenditures under control.
With virtualisation, you can dynamically fire up and take down virtual servers (also known as virtual machines), each of which basically fools an operating system (and any applications that run on top of it) into thinking the virtual machine is actual hardware. Running multiple virtual machines can fully exploit a physical server's compute potential — and provide a rapid response to shifting datacenter demands.
The concept of virtualisation is not new. As far back as the 1970s, mainframe computers have been running multiple instances of an operating system at the same time, each independent of the others. It's only recently, however, that software and hardware advances have made virtualisation possible on industry-standard, commodity servers.
In fact, today's data centre managers have a dizzying array of virtualisation solutions to choose from. Some are proprietary, others are open source. For the most part, each will be based on one of three fundamental technologies; which one will produce the best results depends on the specific workloads to be virtualised and their operational priorities.
What's behind the virtualisation buzz
Server virtualisation breaks up the marriage of hardware and software (in this case, between the physical system and operating system software), and thus allows a single physical server to host many virtual servers running different operating systems. The benefits of this basic capability border on computing nirvana, not the least of which is server consolidation. Indeed, the front lines are awash with server virtualisation success stories — and the drumbeat grows louder every day. However, there hasn’t been a widespread adoption of server virtualisation in the region for a variety of reasons. So, what’s holding the users back? “That's because a hypervisor is just “the first step” toward virtualisation, and users want a whole infrastructure – not just server virtualisation, but also applications and desktop virtualisation with a strong ecosystem built around it. The technology is mature now and is well supported by the industry, and we are now moving from test platforms to real production. Citrix and Microsoft will democratise the server virtualisation market by introducing real alternatives,” says Antoine Aguado, Regional Director- ME, Citrix Systems.
Mohammad Arif, Severs and Tools Manager, Microsoft Gulf, offers a similar perspective: “The buzz around virtualisation has been building over the past few years, with limited adoption. With all the benefits that server virtualisation brings, it adds some complexities too – support, licensing, management, security, availability and so on. Customers have also been waiting for the technology to become mainstream. That said, we do see interest and adoption finally picking up. With the release of Microsoft’s hypervisor Hyper-V and the upcoming Virtual Machine Manager, we expect server virtualisation adoption to grow massively over the coming year.”
Full virtualisation
The most popular method of virtualisation uses software called a hypervisor to create a layer of abstraction between virtual servers and the underlying hardware. VMware and Microsoft Virtual PC are two commercial examples of this approach, whereas KVM (kernel-based virtual machine) is an open source offering for Linux.
The hypervisor traps CPU instructions and mediates access to hardware controllers and peripherals. As a result, full virtualisation allows practically any OS to be installed on a virtual server without modification, and without being aware that it is running in a virtualised environment. The main drawback is the processor overhead imposed by the hypervisor, which is small but significant.
In a fully virtualised environment, the hypervisor runs on the bare hardware and serves as the host OS. Virtual servers that are managed by the hypervisor are said to be running guest OSes.
Para-virtualisation
Full virtualisation is processor-intensive because of the demands placed on the hypervisor to manage the various virtual servers and keep them independent of one another. One way to reduce this burden is to modify each guest OS so that it is aware it is running in a virtualised environment and can cooperate with the hypervisor. This approach is known as para-virtualisation.
The advantage of para-virtualisation is performance. Para-virtualised servers, working in conjunction with the hypervisor, are nearly as responsive as unvirtualised servers. The gains over full virtualisation are attractive enough that both Microsoft and VMware are working on para-virtualisation technologies to complement their offerings.
OS-level virtualisation
Still another way to achieve virtualisation is to build in the capability for virtual servers at the OS level. Solaris Containers are an example of this, and Virtuozzo/OpenVZ does something similar for Linux.
With OS-level virtualisation, there is no separate hypervisor layer. Instead, the host OS itself is responsible for dividing hardware resources among multiple virtual servers and keeping the servers independent of one another. The obvious distinction is that with OS-level virtualisation all the virtual servers must run the same OS (though each instance has its own applications and user accounts).
Easier but harder
Unlike mainframes, PC hardware wasn't designed with virtualisation in mind — software alone had to shoulder the burden, until recently. With the latest generation of x86 processors, AMD and Intel have added support for virtualisation at the CPU level for the first time.
Although x86 virtualisation is not as mature as the mainframe virtualisation tools, vendors says things are changing.
“Although virtualisation was pioneered on the mainframe, virtualisation on the x86 platform has made rapid strides over the past few years, enabling more customers to adopt the technology easily. Servers based on today’s 64-bit, multi-core processors and massive memory can support heavier workloads and more VMs per server, while conserving datacenter space, power, and cooling resources. It also simplifies implementation and reduces the total cost of ownership,” Says Arif.
Security is another major benefit being touted by vendors for going down the virtualisation route. “Virtualisation will make it easier to secure your data and services because generally the virtual machines are managed centrally which means that perimeter security is already in place. In addition, anti-virus, backups, Disaster Recovery and Business Continuity can be implemented in a controlled and managed way,” says Nick Black, Manager, Systems Engineering MEA, Citrix Systems.
A virtual toolbox
Each method of virtualisation has its advantages, depending on the situation. A group of servers all based on the same operating platform would be a good candidate for consolidation via OS-level virtualisation, but the other technologies have benefits as well.
Para-virtualisation represents the best of both worlds, especially when deployed in conjunction with virtualisation-aware processors. It offers good performance coupled with the capability of running a heterogeneous mix of guest operating systems.
Full virtualisation takes the greatest performance hit of the three methods, but it offers the advantage of completely isolating the guest OSes from each other and from the host OS. It is a good candidate for software quality assurance and testing, in addition to supporting the widest possible variety of guest OSes.
Full virtualisation solutions offer other unique capabilities. For example, they can take “snapshots” of virtual servers to preserve their state and aid disaster recovery. These virtual server images can be used to provision new server instances quickly, and a growing number of software companies have even begun to offer evaluation versions of their products as downloadable, prepackaged virtual server images.
It's important to remember that virtual servers require ongoing support and maintenance, just like physical ones. The increasing popularity of server virtualisation has fostered a burgeoning market of third-party tools ranging from physical-to-virtual migration utilities to virtualisation-oriented versions of major systems management consoles, all aimed at easing the transition from a traditional IT environment to an efficient, cost-effective virtualised one.
