Virtualisation is a rapidly-expanding technology that is becoming more prevalent in computing circles as more and more IT managers see the benefits of being able to operate multiple operating systems, and by extension multiple servers, on a single physical server or set of servers. Virtualisation has become a mainstay in the data centre and storage arena as it allows for a more robust environment, by offering low-cost redundancy for servers and reducing electricity costs. It also allows companies more flexibility on how data centres are managed and simplifies the implementation of new software applications.

The virtualisation market is building momentum. Microsoft, HP, Intel and IBM are all building their solutions to embrace virtualisation in one respect or another. Virtualisation software, which allocates server functionality to partitions inside servers, has until recently been the domain of market leader VMware. VMware, an affiliate of data storage firm EMC Corp, holds more than 70 percent of the virtualisation market and its sales have doubled year-on-year for the past several quarters. VMware, with its server-based ‘Virtual Infrastructure’ has been the market leader since starting up in 1998. VMware is sold through Ingram Micro via HP and IBM.

Virtualisation is well-suited for open source operating systems and the rise in open source has spurred the adoption of virtualisation. Red Hat Enterprise Linux 5 is the first product to deliver commercial quality open source virtualisation with part of its operating system. Red Hat Enterprise Linux provides virtualised computing on your client’s server and client systems in one of three ways:

• Red Hat Enterprise Linux Advanced Platform includes fully integrated server and storage virtualisation, coupled with high availability clustering capabilities based on Red Hat Global File System and Red Hat Cluster Suite products. This robust environment supports servers of any size and an unlimited number of virtualised guest environments.
• Red Hat Enterprise Linux server provides integrated support for up to four virtualised guest environments.
• And the same goes for Red Hat Enterprise Linux 5 Desktop client system which can also provide support for up to four virtualised guest environments. RHEL5 with integrated vitualisation has been available since March of this year.

VMware initially sold core virtualisation technology for desktops, then servers, but later added its Virtual Infrastructure software to manage virtual machines and other higher-level software. That software lets administrators handle tasks such as starting and stopping virtual machines, moving them from one physical machine to another through a feature called VMotion, backing them up and restarting them elsewhere in the event of a data centre disaster and monitoring resource use to make sure servers aren't either overtaxed or idle.

Microsoft’s new Systems Center Virtual Machine Manager 2007, just released, allows companies to deploy virtualisation technology, centralise management and convert machines running VMware to a Microsoft format. Systems Center Virtual Machine Manager 2007 integrates with Microsoft's existing software tools into a single suite of applications to manage both virtualised and non-virtualised servers. The move is considered one of Microsoft's first major efforts to cut into the lead held by VMware - which went public this year in the hottest technology IPO in years - and thus undercut a potential threat to Windows server software.

Microsoft's main weapon against VMware is expected to be a piece of software, code-named Viridian, that will be included as a feature of its next generation of Windows Server software. Viridian will act as a ‘hypervisor’ (see below) the extra layer that sits between the hardware and the operating system. Microsoft expects Viridian to be available six months after the release of Windows Server 2008, which is due for completion before the end of 2007.
Microsoft also said that once Viridian becomes available, the software tools will work with other virtualisation software including ones from VMware and XenSource, a company recently acquired by Citrix Systems.

Benefits of Virtualisation
A benefit of virtualisation is that it allows clients to consolidate servers and optimise the workload amongst their server resources. Virtualisation is especially useful in disaster recovery as, when done correctly, it allows the quick restore of the server environment without any major significant losses. Another area where virtualisation can be useful is development. If people are developing applications which can cause crashes during the process, virtualisation allows them to isolate the development space so that even if the particular application causes the operating system to fail, no harm is done.

Another benefit of virtualisation is that it can reduce reduce energy consumption and cut operating costs for companies adopting the technology, according to a recent study on infrastructure virtualisation. The report estimates a company currently operating 250 dual-core servers can save US$4 million over the next three years by adopting virtualisation technology.

The study also estimates that a power savings on the order of US$157,500 for every 1,000 PCs per year can be achieved by businesses moving from a full desktop PC infrastructure to a server-hosted desktop virtualisation setup.

Many companies initially adopt virtualisation to save money through server consolidation and then start to notice other benefits so that virtualisation becomes part of the overall IT plan. The virtualisation revolution seems to be slowly marching on, with analysts and industry experts naming the technology as one of the most versatile tools in IT. Gartner has stated that virtualisation will be a key technology to help companies beef up security on corporate mobile devices.

Hypervisors: The Brains Behind Virtualisation
Most virtualisation solutions are based upon a mechanism called the ‘hypervisor’ which allows multiple operating systems to run, unmodified, on a host computer at the same time. The term is an extension of the earlier term, supervisor, which was commonly applied to the management of operating system kernels in the 1960s. Supervisors managed multiple users – therefore a hypervisor managed multiple kernels, ie. operating systems.

Virtualisation is not new, with the concept having roots in mainframe operations. Remember in the old time-share days how one big mainframe could run multiple applications for batch processing? Virtualisation was essential then as it extended the multi-user paradigm so that multiple operating systems could be run simultaneously. The IBM S/360, a revolutionary machine launched in the 1960s that brought computing to mainstream businesses, was designed specifically to support virtualisation of all kernel tasks. This allowed a single S/360 to run multiple operating systems for a variety of applications.

The kernel constitutes the central core of the operating system. It has complete control over everything that occurs in the system. A kernel can be contrasted with a shell which is the outermost part of an operating system that interacts with user commands. The kernel itself does not interact directly with the user, but rather interacts with the shell as well as with the hardware devices on the system including the processor memory and disk drives.

The early IBM mainframe included page translation table hardware for virtual memory and other techniques that allowed a full virtualisation of all kernel tasks, including I/O and interrupt handling. Prior to this point, computer hardware had only been virtualised enough to allow multiple user applications to be run. With the advent of the IBM S/360 however, the supervisor state was virtualised, allowing multiple operating systems to run simultaneously.

Hypervisors were developed to add stability to mainframe operations as different users had different applications to run. The hypervisor brought a measure of robustness and stability to the system; even if one kernel crashed, the others would continue working without interruption. Indeed, this even allowed beta or experimental versions of the operating kernels to be deployed and debugged without jeopardising the stable main production system and without requiring costly second and third systems for developers to work on.

Hypervisor Interoperability
The ability to manage clusters of virtual servers could be enhanced if the Linux kernel could deal with hypervisors from different vendors. VMware, Viridian and Red Hat all have different hypervisor designs but are now moving towards a common hypervisor extension called Open Virtualisation Machine Format (OVF) that would provide interoperability. Under such a scenario, it would be possible for a Red Hat virtual machine, running on failing hardware, to be automatically moved over to a VMware hypervisor on another piece of hardware.

Open Virtual Machine Format doesn't replace the three existing standards, but instead wraps them in a standard package of XML that includes necessary information to install and configure the virtual machines. OVF requires that the guest operating system be modified to make system calls to the hypervisor, rather than executing machine I/O instructions which are then simulated by the hypervisor. This is called paravirtualisation. The open source hypervisor, when running on a normal host operating system such as Linux, is able to run both paravirtualised and, with the help of the hardware virtualisation extensions such as Intel VTx, fully virtualised (i.e. an unmodified operating system) as guest operating systems.

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