What is a datacenter U rack unit?
Colocation and datacenter cabinets and racks are measures in rack units. The height of a cabinet is a set number of rack units. These rack units are usually represented by the letter "U". A common datacenter cabinet height is 42U or 42 rack units. Servers are also measures in rack unit height. So you may have a 1U server, which is a server which fits within 1U of space inside of a colocation cabinet.
A rack unit is 1.75 inches tall (44.45 mm). This is generally the minimum amount of spaces a colocation provider will sell. It is also the smallest server size for common servers. The rack unit does not have anything to do with width or depth. Cabinet width and depth is measured in inches and centimeters. The depth usually ranges between 36 and 42 inches.
A data center cabinet post is filled with holes which servers and shelves are attached into. This is how equipment is mounted into a cabinet. A 1U space typically contain three holes. The spacing between these post holes is the same regardless of if the posts use square or round holes. A rack unit is always a rack unit, no matter the type of cabinet, rack or posts.
If you are moving into a cabinet, be sure to know the depth of the cabinet and the length of your servers. It is not uncommon to have a cabinet be shorter than a server. The solution to such a problem depends on the policies of the data center. Some colocation providers allow for servers to stick out the back of the cabinet. This is not an ideal solution, as the servers are no longer secured behind a cabinet door. However, it is a better solution than not being able to rack the servers are all.
Many colocation providers offer colocation in shared cabinets. What this means is that instead of renting out a whole 42U of space, you instead rent out part of the cabinet (7U for example) and other customers rent out the rest. The main benefit is saving money. The main drawback is that other customers will have physically access to the cabinet. This increases security risks and also increases the chance that a cable attached to your equipment will be bumped and become dislodged.
If you need a colocation service but are short on cash, see if the colocation provider you are looking at offers shared cabinet space.
Another consideration is square versus round rack posts. The rack unit measurement is the same for both, but some servers require either round or square posts. It is important to know the specifications of the server you will be mounting and make sure that your colocation provider offers an exact fit.
The rack unit measurement was standardized by the Electronic Industries Association who were a trade organization. They no longer exist, but the rack unit is here to stay.