This post is the first in a series of questions that need to be asked before embarking on a data center build project.
How big is big enough?
The first question asked is often the most difficult to answer, or the simplest. “It depends” might be valid for an analyst, but not when you’re potentially spending 10’s of millions of dollars on a new data center. And the difficult part of this question is not figuring out how much you need – it’s figuring out what you need in 15 years.
In the old days (you know, last year) the method of figuring out how large the new data center was going to be was a combination of guesswork and extrapolation (based on that guesswork). I remember doing future value calculations on floor space by substituting financials with square feet in order to come up with a defensible number for our plans. This sounded good at the time, and impressed the bean counters, but essentially it was a more formalized version of traditional back of the napkin analysis – resulting in a number we could believe in – not necessarily one with any level of accuracy. Sometimes called GIGO (garbage in, gospel out), planners tend to believe spreadsheets, regardless of the results. (This has always baffled me, but that’s the topic of a future post….)
In today’s world though I would recommend some reverse engineering to get to a reasonable size. Take a walk out on your floor (if it’s handy) and get a good feel for how many racks you have, and at what capacity those racks are populated. A standard 42U rack today takes up about 30 square feet of floor space, if you include aisle ways, door swing space and room for maintenance. If your computer room is mostly racks it’s then an easy task to multiply 30 square feet times the number of racks you have, and then apply a growth factor to it. You’ll be surprise how much space is actually needed.
The second factor to consider is the capacity of those racks you have today. In most IT shops racks are populated in the 50-60% range, primarily to reduce concentrations of heat on the floor. Racks and servers are spread out to share the heat, so to speak, but when considering building a new room you need to assume it will be build to handle proper heat loads, and therefore racks can be much more densely populated – upwards of 80-90%.
Modern data centers are being built smaller, not larger, and are designed to support high levels of vertical growth, resulting in the same relative compute density as older designs, but with less capital outlays and greater flexibility for future growth. We will address long term growth when question 5 is answered; How long should it last.