How not to build a data centre

Many businesses quickly fill their data centres to capacity due to common pitfalls in measuring the amount of space and power needed.

The need for data centre space is booming. Due to the convergence of a diversity of factors including synchronous applications growth, rising regulatory requirements and enhanced storage demands, a high percentage of existing data centres can expand no further. In fact, the Gartner Group estimates that over 50 per cent of existing data centres will be obsolete within the next five years.

For enterprises, it is important to ensure that the search for data centre space ends with a facility that can support both current and future requirements. Of particular importance is the need to accurately determine current and future space and power requirements. While this seems like a simple endeavor, if requirements are not expressed correctly, an organisation can easily wind up with a data centre whose obsolescence begins very shortly after the move-in date.

Square footage: Simple, yet so misunderstood

If your business has built data centres in the past, and it has specific requirements, wouldn’t you naturally just want to continue to build your own? 

Many organisations face this question, and frequently the right solution turns out to be a little different than they originally thought.

Typically, many data centre professionals use square footage as the standard unit to describe a site at its most basic level. This unit of measure is commonly used in all manner of commercial real estate transactions, and if you were looking for new office space to house the accounting department it would work just fine.

Unfortunately, in the realm of the data centre industry, the definition of square feet is not universal across all involved parties. For example, a real estate professional may use the phrase square footage to describe the entire rentable area within a building. Their IT counterpart may view this same terminology as reflecting the space’s cabinet footprint, and an engineer may visualise his conception of the term as the data centre’s gross raised floor area.

This diversity of interpretation can lead to a variety of issues including the dramatic overestimating of required power capacity, leading to the development of a facility that is unable to accommodate future growth because it is literally out of floor space.

Perhaps the most insidious drawback in defining your requirements solely in terms of square footage is that the real estate or ‘shell’ of a data centre represents only 5 to 20 per cent of its total cost of ownership. This figure pales in comparison to the site’s power delivery infrastructure, which contributes over 50 per cent of ownership cost.

The key to effectively addressing this inequity is to view your data centre in terms of small increments rather than as a single contiguous space.

Watts per square foot: It’s all in the maths

Although attempting to quantify your power and space requirements by expressing them in terms of watts per square foot might seem like the best method to use (especially compared to square feet), this alternative can be dramatically inaccurate. In fact, many data centre providers will use this measurement unit to entice customers by presenting them with wildly inflated measures of power capacity that only become apparent when they need to expand.

The principle problem with using W/ft2 as a measure for data centre space can be found in how this unit is actually determined.

Calculating W/ft2 is done by using a simple equation in which the total power capacity available for an entire space is divided by a prescribed amount of square footage.

For example, if we accept a power capacity of 2MW is going to service a 15,000 square foot data centre, the 2MW figure assumes that we have answered a number of underlying questions regarding our power requirements, including: Does this figure incorporate redundant components, and does it address utility requirements or only the estimated IT load? If we divide 2MW by 15,000 we get an average of 133 W/ft2 .

While this may seem to be a sound method of identifying capacity it is important to note that the operative term in this equation is the word ‘average’. The problem with this methodology is that the average capacity can be boosted by decreasing the amount of the denominator --the available square footage. Thus, if we maintain our same level of power capacity (2MW) but reduce square footage to 10,000ft2, my average rises to 200W/ft2.

Based on the previous example, the major flaw in using W/ft2 as the measurement unit for defining data centre capacity quickly becomes apparent -- the needs of your data centre provider and your potential for expansion may not be in synch. While you may be seeking an amount of space that will support your current applications with room to grow to accommodate future requirements, your prospective provider may have unwittingly curtailed your options for future expansion.

As indicated earlier, many data centre providers will base their proposals on quotations for W/ft2 that bring to mind the phrase “too good to be true”. By using our previous example, it is easy to see how the siren song of a seemingly high level of capacity can ultimately lead to a short lifespan for a customer’s ‘data centre of the future’.

KW: The true measure of a data centre

Viewing your power requirements in terms of kW of IT load is a fundamental principle. In my view of value engineering, the requirements of the data centre reflect the unique attributes of the space itself. Adopting this method of determining and expressing your data centre needs requires you to view your power requirements based on the footprint of all the components that will reside above your raised floor.

More succinctly, it provides a counter-balance to the tendency to overestimate your power needs. In this age of expanding high-density applications we regularly encounter prospective customers who come to us with needs of 250W/ft2 or more. What we have found to be most common in these situations is that rarely does a data centre have a homogeneous load profile.

While your racks of blade servers may indeed possess substantial power requirements, the other components that will reside in the data centre with them do not. Cabling and patch panels, for example, require no power at all, and other components such as your network/telco (10-50W/ft2) and spinning disk storage (100W/ft2) have rates of power consumption considerably below those of your high-density servers. Thus, your original 250W/ft2 requirement typically turns out to be considerably lower.

By using kW of IT load you have established a level of design flexibility for your data centre that square footage and W/ft2 cannot provide. Through using this unit of measure to determine your requirements, you are able to ensure that you have room for future growth through the realisation that the demands of high power consuming hardware are offset by other less voracious components. By achieving this balance within your data centre you have built in expansion capability from the standpoints of both footprint and power.

Getting the first step right

One thing is clear: the first and perhaps most important step in developing a new data centre is quantifying your space and power requirements correctly. Only by viewing needs in terms of kW of IT load can you be assured of accurately determining requirements. By using this measure you can factor in the entirety of your computing and support environment to ensure that your data centre will support your current, as well as, future applications needs.

Kris Kumar is senior vice president and regional head Asia Pacific of Digital Realty Trust.

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