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Racks are meant to be standard, but IT equipment dimensions vary and the volume of cabling needed to power and network modern servers, switches and storage cause unexpected deployment problems.
Most data center equipment is installed in racks, which come in an array of standard size increments. Rack units -- U -- accommodate a range of air handling, cable management and accessibility features, such as removable doors or sliding rails.
When racking servers, check the dimensions and physical interoperability between the rack, rail assembly and servers or other IT equipment that you plan to deploy. Perform due diligence to check the fit in advance when piecing things together yourself.
Hidden problems in server racking
A rack typically accommodates servers that are 19 inches (482.6 mm) wide. Height is some multiple of 1.752 inches (44.50 mm), which is a standard rack unit. For example, a 42U rack provides an opening 19 inches wide and 73.6 inches high (1.8669 m).
A 19 inch rack will fit any 19 inch server, rack switch, power distribution unit (PDU), uninterruptable power supply (UPS) or other standard-width gear. Add up the vertical height and make sure that the rack opening is high enough to accommodate all the planned gear. For example, if you need to deploy 14 new 2U servers, a 4U PDU and an 8U UPS, you need a 19 inch rack at least 40U high. Taller racks are fine; especially if there's a chance you'll reconfigure later.
Two plus two
A two-post rack is a pair of vertical rails. Each rail attaches to the middle of each server or piece of equipment. Generally, two-post racks suit small deployments, for easy access without airflow containment issues.
A four-post rack is a four-corner box or cabinet. Each server or device installs along horizontal rails, secured with screws or quick-disconnect latches through holes in the front panel. Four-post racks enclose IT gear to add equipment security, protect power and network cabling, and shroud airflow for hot/cold containment.
The biggest problem with server racking is depth. Dell, HP, IBM and other server vendors put out models that range from 28.5 inches to 29.125 inches deep. Newer servers may not slide all the way into older racks. You'll need to replace them with deeper models to safely accommodate newer gear.
Some servers fit, but leave no room for power and network cabling. The problem multiplies when you consider that a 42U rack has room for 42 1U servers or other devices, each one with power and network cables. Without sufficient clearance, the mass of wiring may obstruct airflow or the rack's rear door.
Most server vendors offer a variety of power cord options for rack server models, or you can buy low-profile power cords from third-party vendors, like the YP-12L-9 from Signal & Power Delivery Systems, to free up space.
Cable management arms keep power and network cables in mechanical trays behind each rack server. Arms neaten up the rack, but they make locating, troubleshooting or replacing specific cables more difficult. Some arms are relatively deep and more likely to obstruct deep servers. Users can remove arms, though this may lead to cable "bird's nests." Look for smaller or low-profile arms from the rack vendor, reduce the server count in the rack to free up space, or replace the entire rack with a deeper model for adequate rear clearance.
Rack width, height and depth (internal and external) actually vary between makers. How will those dimensions fit within your existing floor plan? Oversized racks can disrupt aisle containment enclosures or shift other racks and airflow ductwork. Subtle size differences may not seem significant, but any increase in rack size can wreak havoc in a tightly configured data center layout.
A whole lot of holes
Racks have a series of holes to mount servers, switches and other equipment. However, rack manufacturers do not deploy any standard mounting hole type; threaded, unthreaded or square mounting holes can cause problems.
Threaded mounting holes use relatively thick metal posts with mounting holes threaded for common screw sizes, including 10-32, 12-24 or metric M6 threads. The server slides into place and the technician aligns the holes in its front panel with the corresponding threaded holes. Deeper, longer boxes without rails are difficult to mount on threaded holes, but it's easier for relatively small boxes.
A server latched onto sliding rails can be unplugged and removed within seconds. This is great for service, but puts expensive IT equipment -- and its data -- at risk. If the rack doesn't have locking doors, use a locking mechanism such as the Secure Server Unit. These devices require 3U of rack space, which is an issue for tightly packed racks.
Since threaded holes can strip and require thicker metal for threading, many rack manufacturers use the unthreaded hole approach. This works for equipment that relies primarily on rails or other mechanical elements for support -- the box slides in on a rail that holds it up. The server can easily slide out again for inspection or service. Use clip nuts to "thread" unthreaded holes when needed.
Screwing the server to the rack is cumbersome and adds time to service tasks. Most modern racks use an array of square holes with locking slides so technicians can add or move rails quickly. If the system owner wants to fix the servers into place with screws, they can snap square cage nuts into the square holes.
Use caution when selecting or installing server rail kits. Some accommodate multiple hole types in the same assembly and require rotating portions of the rail. Unfortunately, rails and racks are not universally interchangeable and such multi-purpose rail assemblies sometimes encounter obstructions. Check the rail length; a rail assembly can be too long for the rack -- especially older, shallow racks.
Never assume that racks and IT gear are automatically compatible. Check dimensions, review the data center layout and test-fit new gear before you buy.
About the author:
Stephen J. Bigelow is a senior technology editor at TechTarget, covering data center and virtualization technologies. He's acquired many CompTIA certifications in his more than two decades writing about the IT industry.
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