U’s, Cabinets, Racks and Cages
The smallest measurement of data center space is the rack/cabinet Unit, typically referred to as a “U”. Data center cabinets, which resemble large lockers, as well as their open rack cousins, are designated as having a fixed 19″ usable internal width (which is actually less, as we’ll explain below). The depth of the cabinets vary a bit by manufacturer, with 36 inches being quite common, though some are as deep as 42 inches. The cabinet/rack “U” defines the third dimension: height. One U is a slice of cabinet or rack space that is the full depth and width, and exactly 1.75 inches high. Data racks and cabinets use the same “U” designation – the chief difference between data cabinets and data racks is that racks are open (no sidewalls), while cabinets are closed on all sides. Secondarily, some racks have only one mounting point on each side, located in the center of the rack – this is what’s commonly known as a “2 post” rack. Data cabinets are almost outfitted with two pairs of posts, one front and one rear, known as a “4 post” configuration (some open racks are also supplied in the 4 post version). Racks are typically only supplied for use inside private cages (see below), so unless you are shopping for a cage, you’ll be getting cabinet space.
Wikipedia has a good article on 19″ data racks which is worth reading if you have the time. Colocation providers who allow shared cabinet usage (purchasing fractional space in a cabinet that also contains other clients’ equipment) will sell you space in one or more U’s. Rackmount-ready equipment is designed to work with these divisions – in shopping for equipment you will find that the form factor of the equipment chassis is designated as “1U”, “2U”, “4U” etc. This tells you exactly how much data cabinet space is required to accommodate that unit, so determining your total space requirements is a simple matter of adding up the form factors of all of your equipment. Private cabinet spaces, which are most commonly available in 1/2-cabinet and full cabinet configurations, will vary in size slightly by manufacturer, but in general will be no less than 20U for a 1/2 cabinet, and 42U for a full cabinet.
The vertical mounting posts mentioned earlier are located inside of the 19″ horizontal width of the cabinet or rack. Rackmount equipment is designed to fit between these vertical posts, so consequently the width of any rackmount chassis is no more than approximately 17.5 inches. Rackmount equipment is designed to use the mounting hardware typically supplied with the equipment: wing brackets, fixed or sliding rails, or both. If the equipment that needs to be co-located does not have rack/cabinet mounting hardware, the usual solution is to mount a data cabinet shelf for the equipment to rest on. This is less desirable than having the proper mounting hardware, but is a workable solution.
One more thing about vertical mounting posts – the most common configurations are square-hole, or threaded round-hole, each with four holes per U. Most modern rackmount hardware is designed as a ‘snap in’ fit for the square-hole configuration, but there are still the odd situations where the need to use a fastening bolt requires a threaded round hole. Not so long ago the opposite was true, and therefor the threaded round hole posts were considered desirable, as they were the most flexible. These days, the square-hole configuration is almost certainly the first choice. In the rare mounting situation that requires a threaded hole, rack nuts that snap into square holes are easily obtained – many data centers keep them on hand for just such an occasion.
For very large colocation deployments, one may look beyond one or even several full cabinets, and instead consider a private cage. This kind of space is just what it sounds like – a fully enclosed wire cage with a locking metal door, to which only the colocation customer and designated facility staff have access. Where cabinet space is rented by the U or by unit of private space (1/2 cab or full cab), cage space is typically rented by the square foot. Since cage spaces are private, you can use open racks inside cages instead of enclosed cabinets to hold your equipment – doing so can make access to equipment a bit easier and promote more even cooling. Cage space is not for everyone – the entry price point for a cage, which will typically be 100 square feet at minimum, is generally much higher than for other types of colo. Cages provide unmatched flexibility when it comes to how you stack your equipment and allocate power, and if available space allows it you can even set up a small workstation inside your cage.