The term virtualisation is increasingly applied to all elements of an organisation's IT infrastructure. For an IT director wanting to consolidate IT services, virtualisation is certainly the route to take. However, the definition of virtualisation has become overly complex and an IT director can easily become submerged in confusion, lacking the resources to allocate the relevant virtualisation technology to their specific IT problem ? let alone identify the vendor that can deliver the best solution.  Broadly speaking, there are five categories of virtualisation and in this article I aim to provide a technical synopsis of how each category can be broken down by capability and application within the IT infrastructure, thus giving the IT director an easy reference tool and better understanding of what has become an all encompassing technology.

1. Server Virtualisation

Server virtualisation is the most common form of virtualisation and the Butler Group has categorised it into three distinct areas, emulation; partitioning; and clustering, and within each the technology can be subdivided even further. 

 Emulation is the virtualising technology that emulates the appearance of other IT resources appearing to requesting processes or applications to be like them. Emulation can incorporate two forms of virtualisation ? hypervisor and hosted virtualisation.

o Hypervisor virtualisation presents actual physical resource, such as processor, memory, disk and network, to guest virtual machines (VMs) that operate as if they have direct access to the underlying physical resource.

o Hosted virtualisation installs the virtualisation layer on top of a host operating system, and is typically considered to achieve higher server utilisation levels. The range of guest VMs that can be installed is likely to be limited by the host operating system.

 Partitioning enables a single resource to be presented as multiple resources to requesting processes or applications and can be split into two forms ? hardware and logical partitioning. 

o Hardware partitioning splits a resource into two or more electrically-isolated environments. This degree of isolation ensures that hardware or software problems experienced by one environment do not affect another.

o Logical partitioning provides for isolation of environments at a software level, allowing hardware resource to be dynamically allocated across environments in response to workload.

 Clustering combines and presents multiple resources to requesting processes or applications through a layer of abstraction so that they appear as one resource.

o NLB clusters are used to scale solutions to cope with increased workload and seen at the presentation layer of an architecture

o HA clusters are typically used to improve service availability and two or more nodes will exist in the cluster. If a node fails its workload will transfer to a sister node

o HPC clusters increase performance by sharing workload across multiple resources

Although much of the buzz regarding server virtualisation has concerned the x86 market over the last couple of years it is important to note that the above concepts have been available as mature products for some years in the wider server space.

2. Desktop Virtualisation

The desktop virtualisation market is principally concerned with the delivery of the Windows operating system and associated applications to end user devices.  Desktop virtualisation has three key applications within an IT infrastructure:

 Workstation hosted virtualisation is typically used to improve localised development and test functions.

 Server based desktop delivery allows the server to run one copy of the Windows operating system to where one or more applications are installed. Users then connect to the server from thin clients or PCs, and all processing is performed on the server with screen updates presented on the client.

 Virtual Desktop Infrastructure enables a central server to host logically isolated VMs each running their own guest operating system and applications. Users connect to the VMs though a connection broker that matches their requirements against available VMs.

3. Application Virtualisation

Application virtualisation allows for the centralised management and control of applications written to operate in a distributed environment.

Application virtualisation packages standard applications into a format which allows them to be delivered or streamed to clients as they are required, on demand. The streaming process does not install the application to the client in the traditional sense. Rather, it places the application into an isolated ?sandbox' to prevent compatibility issues with other client applications, or disruption to the client device.

Depending on end user requirements and IT department policy the application may only be available whilst the user maintains a connection to the corporate network, or it may be cached locally for use in off-line mode.  Server based desktop virtualisation may be combined with application virtualisation to minimise the risk of conflicts when multiple applications are installed on the same operating system. 

4. Storage Virtualisation

This is most confusing of all the five categories under the umbrella of virtualisation as there are more vendors competing in this space each bringing their unique definitions to the fore.  Essentially, all approaches incorporate a layer of abstraction between the requesting client or application and the underlying data.  Storage virtualisation technologies can be classified as ?in band' or ?out of band'. In band solutions place a device in the path of host requests to the physical data storage and present an abstracted view of that storage to the requesting hosts. Out of band solutions do not interrupt the data flow between host and data storage, and provide abstracted administrative and management functions.

 File level Virtualisation provides a layer of abstraction between underlying file systems presented using NFS (Unix) or CIFS (Windows) and requesting clients. Clients connect to a global namespace that then maps the client's request to retrieve a file to the underlying file system on which the data is stored. Management policies allow unused data to be moved to less costly storage after a period of time.

 Block Level Virtualisation provides a similar abstraction mechanism to file level virtualisation but for data accessed over Fibre Channel or iSCSI. Virtual disks are presented to the host which are mapped transparently by the virtualisation device to storage of the ?right' category or tier. The virtualisation engine controls the physical disk on which the data is stored  and can be changed non-disruptively and automatically.

 Disk Capacity Virtualisation allows individual disks, within or across systems, to be aggregated and presented as one virtual pool of storage for allocation to hosts. Hosts are connected to the storage pool or tier offering the required level of performance.

 Thin provisioning allows the allocation of physical storage resources by allowing administrators to present host systems with a logical amount of disk capacity, whilst the actual physical capacity is automatically allocated as required.

5. Network Virtualisation

Data and voice network virtualisation is well established within many organisations, but less established is the concept of data centre I/O virtualisation..

 I/O virtualisation places an abstraction layer between host systems and the existing multiple I/O interconnect types such as Fibre Channel, Ethernet, Fibre Channel over Ethernet, iSCSI and Infiniband.  It seeks to virtualise the different interconnect types by consolidating them into a resilient high throughput hub device, which then offers host systems access to necessary resources  through one interface.

Adopting a coordinated approach across these five categories offers the IT director potential for true infrastructure virtualisation, delivering significant improvements in service delivery and operational management; improved efficiency and cost reductions; tangible ?green' gains in terms of reduced power, cooling and space requirements and greater flexibility to changing business need. 

www.b2net.co.uk

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