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Home > Virtual Data Centre News > Getting your virtual architecture right |
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The approach to managing physical data centres has had to continually change, as the IT industry has moved from mainframe through midi computers, tower computers and on to rack mounted servers and blades. However, even the old way of doing things -- one or two applications per server -- meant relatively low stress levels in data centres when it came to power requirements and heat production, even as overall equipment densities increased. Virtualisation, however, changed all of this. As virtualisation projects are used as a review point for rationalisation and consolidation of the software, as well as hardware assets, many organisations find themselves not just with higher hardware densities but also individual servers with far higher utilisation. This can end up really stressing power and cooling capabilities. Indeed, in many early implementations of virtualisation, Quocirca saw how simple approaches of trying to stack similar equipment in specific areas led to disastrous results. For example, placing all power converters in one stack, all CPU capability in another and all storage in yet another led to severe problems at the power converter level. Removing heat effectively from such a high dissipative source would require highly specialised cooling systems to ensure that overheating would not occur. The majority of vendors and data centre implementation partners will have built up their own "best practice" capability in how to architect a platform for a highly virtualised environment. Even these, however, may not take everything into account when it comes to building an overall best architecture that looks not only at the main assets, but also at the interconnects, the incoming power distribution and the need for a dynamic capability to grow and shrink resources to meet the workloads. Power distribution For a high density environment, cable management can be a major issue. Quocirca recommends that underfloor cabling in a virtual environment be avoided and that power and data cables be kept away from each other. By doing this, cabling can be more easily maintained, and crosstalk between power and data cables is minimised. Rack engineering also has to be taken in into account. A mix of power, CPU, storage and networking units in one rack is not a problem. It enables high-heat, dense units such as power converters to be mixed with less dense and less hot units, such as a router or switch. This, then, enables cooling systems to be better engineered: Approaches such as cold/hot aisles and forced, ducted cooling will minimise costs while maximising cooling efficiency. Indeed, a well engineered cooling system can provide outlet heat that can be used elsewhere in a building for space heating or, via a heat pump, hot water. Although a well-architected virtual environment can lead to increased system availability, a basic system can reduce it. A move from a one application per server to a "many virtual applications per physical server" model can mean that the failure of a single physical server can bring down many applications or services, essentially bringing an organisation's IT services to its knees. Using virtualisation to create "n+fractional" resilience, however, should be considered. In a physical setup, availability is generally provided through clustering and/or mirroring. At the simplest level, this will involve an "n+1" approach -- the servers (and other assets) that are in use will require at least one extra asset to be in place to ensure a degree of availability. In fact, many organisations use an "n+m" approach, with multiple backup systems in place to provide higher levels of availability. Bearing in mind that in pre-virtualisation environments the majority of Windows-based servers are running at less than 10% utilisation, "n+1" will make it 5% (if n=1), and "n+m" will drive it down even further. Active and passive images Either image will optimise utilisation and lower the costs of having a highly available single data centre. For ultimate availability, an "n+fractional" multi-data centre approach may also be required. This can be provisioned through using an on-demand IT infrastructure or a third party colocation provider's data centre facility as a primary failover site. Virtualisation gives us an opportunity to review a data centre's architecture and to ensure that a highly effective, dynamic and responsive infrastructure is put in place. As in architecting a building, it's function that has to win over form. For a data centre, bear in mind that the function is variable: The form must be enabled to allow for this. Clive Longbottom is a service director at UK analyst Quocirca Ltd and a contributor to SearchVirtualDataCentre.co.uk.
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