Shorthand for "
Rack Unit" when used in discussions of
rackmount equipment. 1 Rack Unit is (1.75
in. or 4.45
cm).
A 19" equipment rack (or telco rack, depending on the design) is about 19" wide, and about 2 meters high. The frame of the rack has a number of holes to which you can use screws or bolts to mount equipment. The holes are laid out account to an EIA standard in a alternating 5/8" - 5/8" - 1/2" hole pattern.
The layout of these holes, and the need for four screws or bolts to hold the equipment to the rack, means that the equipment must be at least tall enough so that its mounting 'ears' cross two bolt holes on each side. The minimum size is 1.75 inch. If you grow your equipment in the 1-unit increments, you are guaranteed to have a rack-friendly design.
The U shorthand is used in marketing servers. For example, the HP LPr series of servers are 2U high, so a stack of three of them would take 10.5" of rack space, or 6U. The Cobalt RaQ server is only 1U high, so you could pack six of them in the same space that the LPr servers would take.
Rack Units are also used in renting space for your servers.Co-location facilities charge by rack space used, so using 1U and 2U servers can save you a lot of money.