The fourth installment of this series on the basics of data center design focuses more on the computer room proper. In particular, it looks at one of the more controversial design aspects: a raised floor versus a slab floor. Dogmatists on both sides of this perennial debate tout their respective favorites, but companies seeking the optimal approach for their businesses may find the matter to be less than black and white.
Maintaining Perspective: Slab Versus Raised Floor
Like so many aspects of the data center, choosing whether to go with a slab or raised floor in the computer room is linked with numerous other design decisions, not the least of which is cooling. The numerous design permutations can quickly become overwhelming, but keeping at least two overarching considerations in mind can help narrow the field and guide the final selection.
First, pay attention to total cost. It’s easy to just assume that since adding a raised floor costs, say, $25 per square foot, choosing a slab floor translates into an instant $25,000 savings per 1,000 square feet of space. But if eliminating the raised floor necessitates installation of an anti-static layer on the concrete at $12 or so per square foot, that initial savings estimate is immediately cut almost in half. Other considerations may also balance the cost, potentially eliminating the monetary premium of one approach over the other.
Second, keep the future in mind. Once you build a data center, you’re largely stuck with it, so plan as much as possible for where your business (and server technology) is headed. For instance, if you’re designing for high (and increasing) density, load-bearing concerns may scuttle the raised-floor option. On the other hand, if regular rearrangement of the rack layout is in your future, a raised floor may ease the concomitant cooling reconfiguration. As with all aspects of data center design, knowing what you want and what you expect in coming years will help you make decisions that deliver the most value in the long term.
Overhead Cabling: Raised Floor Killer?
One espoused benefit of raised floors is that they provide a space for cabling that limits mishaps (such as accidental disconnections) and promotes a neat, professional appearance. Cable trays and other overhead cable-management infrastructure offer an alternative that proponents claim is less expensive than using a raised floor and more amenable to accessibility and efficient cooling. And indeed, assuming a raised floor is only for the purpose of making room for cabling, costs for the overhead approach are probably lower. But the raised floor is typically about more than just an out-of-sight location for cables; it is a convenient means of distributing cold air to rows because cold air naturally tends to reside below warm air.
Although a raised floor can serve as both a “cable stash” and a plenum for moving chilled air, this combined approach has dangers. Poorly managed cabling can lead to obstruction of airflow, which at a minimum decreases efficiency and at worst may cause hotspots, posing a risk to sensitive equipment. Using overhead cable management and a raised floor is one possibility that offers the best of both worlds, but it increases costs and can quickly run into vertical-clearance constraints. In addition to the fundamental limit of floor-to-ceiling distance for equipment, ceiling-mounted sprinklers must have sufficient clearance from the top of racks and cable trays to allow them to function properly in the event of a fire. A raised floor immediately consumes some of that precious space, so the decision to go that route should involve more than just incremental benefits and minor conveniences.
Your choice of a cooling system and your choice of whether to use a raised floor will go hand in hand. For instance, a high-density deployment using liquid-based cooling delivered to the rack level carries numerous reasons to avoid using a raised floor. Here are a few:
- Weight limits. A raised floor has less load-bearing capability than a concrete slab, so a data center with numerous fully loaded racks could run into structural problems. Furthermore, you need to think beyond just what raised-floor tiles can handle in terms of static weight; moving racks, dropping equipment and other factors must be considered.
- Lost vertical space. If you’re using this type of liquid cooling setup, a raised floor is probably an unnecessary waste of vertical space. A slab floor leaves room for taller racks, and overhead cable management can replace the under-floor approach.
- Easier maintenance. Cleaning and accessing the under-floor area can be a hassle, and tiles that are replaced improperly or not at all create a serious safety hazard. To be fair, however, overhead cabling typically requires ladders to access, thus carrying its own safety concerns.
Even if your design involves more-traditional air cooling, however, a raised floor is unnecessary. Cold air can still be routed through overhead infrastructure or using row-based cooling units in a hot-aisle/cold-aisle configuration. These approaches, however, tend to place limitations on how rows can be arranged, since major—or even minor—adjustments can necessitate changes to the duct or aisle-containment infrastructure (typically composed of sheet metal or plastic). A raised floor is more accommodating for such situations.
When choosing between a slab and raised floor, you should consider the following critical parameters. Naturally, some may be more important than others to your design.
- Weight limits
- Cooling system
- Preference: cabling hidden or in view
- Vertical space
- Seismic hazards (raised floors in earthquake-prone areas carry additional requirements)
- Data center growth
- New technologies (which can lead to changes in rack size or arrangement)
According to Uptime Institute survey results mentioned in a 2011 discussion regarding slab versus raised floors, some 90% of data centers at the time used a raised-floor architecture. Although raised floors are a present fixture of data center design, however, the trend is toward slab floors—the Uptime Institute also suggested that only 48% of future data centers would use a raised floor.
Raised floors were all but a necessary part of data center design in the previous decade (although certain segments, like telecommunications, relied heavily on slab floors), but the trend seems to favor slab floors in new designs. Despite some seemingly overwhelming advantages to a slab floor—such as the elimination of weight limits and the availability of overhead cable management—you shouldn’t immediately discount this approach. Your choice in this matter will depend on your budget and your other data center requirements; the industry as a whole may be leaning toward a slab floor, but your facility should use whichever option best fits your business needs.