1U is defined as the height of three consecutive holes built into the rack, to which the hardware can be secured. The holes could be round or square or threaded – the size of each hole, and the gap between them, is standardized across companies by the rack unit. For example, a typical full-size rack cage is 42U high, while equipment is typically 1U, 2U, 3U, or 4U high. The rack unit size is based on a standard rack specification as defined in EIA -310. 66 millimeters in height rather than the full 1. This article explains definition, planning, installation tips, and trends. Important: U describes height only, but a server's real "capabilities" are also determined by chassis depth, internal layout, airflow, rails, power, and expansion (PCIe/risers, NVMe. 1 Rack Unit (1U) = 1.
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