Every server in a datacenter lives in a rack: a standardised steel frame 19 inches wide, with mounting rails on which equipment is bolted in stacked slots. The slots are measured in rack units, and once you know what a U is, a datacenter floor stops looking like a wall of blinking boxes and starts looking like a filing system.
The unit
One rack unit — 1U — is 1.75 inches, 44.45 mm, of vertical space. A 1U server is a flat box that height; a 2U server is twice it, with room for more drives, bigger fans and full-height expansion cards. A standard full rack is 42U tall, just over 1.8 metres of mountable space. Larger 45U and 48U racks are common in new datacenters.
Width is fixed at 19 inches between the rails (the equipment's mounting ears make up the difference to the rack's 600 mm outer width). Depth varies; modern server racks are 1,000–1,200 mm deep to fit long chassis and cabling.
What goes in a rack
A typical hosting rack, top to bottom:
- Top-of-rack switches (1–2U). Every server in the rack connects here; the switches uplink to the datacenter core. Putting them at the top keeps server-to-switch cables short.
- Patch panel (1U). Where the rack's fibre and copper uplinks land.
- Servers — 30–36U of them. Twenty 1U servers, or a mix of 1U and 2U, or a few 4U storage servers with 24 drive bays.
- Blank panels in any empty slot, for airflow (below).
- PDUs — power distribution units — usually mounted vertically down the back rails, not consuming U-space. Two of them, on separate feeds, so each server's two power supplies go to different sources.
- Occasionally a KVM or console server, and a shelf for anything that is not rack-mountable.
Each server is on sliding rails, so a technician can pull it forward, open the lid and replace a drive or a DIMM without unplugging anything.
Airflow decides everything
Servers pull air in at the front and push it out at the back. A datacenter arranges racks in rows, alternating cold aisles (fronts facing each other, fed with chilled air from the floor or overhead) and hot aisles (backs facing each other, where the exhaust collects and is drawn back to the cooling units). Containment — doors and roofs over an aisle — stops the two mixing, which is worth 20–30% of the cooling bill.
Inside the rack, that is why every empty slot gets a blank panel: without one, hot exhaust from the back curls round to the front and gets pulled into the server above. Cable management matters for the same reason; a tangle behind the servers blocks the exhaust. More on the building side in power and cooling in a datacenter.
Power and weight
A rack of modern servers draws 5–15 kW, and dense compute racks 30 kW or more. Datacenters sell space by power as much as by U: a rack with a 5 kW feed holds however many servers fit in 5 kW. Weight adds up too — a full rack is 500–1,000 kg, which is why raised floors have load ratings and why racks are bolted down in seismic regions.
Density: 1U vs 2U vs blades
A 1U server is the densest general-purpose option: up to 42 in a rack. 2U gives more drives per server and quieter, more efficient cooling. Blade chassis pack 16 servers into 10U with shared power and networking, at the price of vendor lock-in. Most hosting fleets are 1U and 2U machines from a single vendor, chosen for parts commonality — one spare drive model, one spare PSU, one spare DIMM type for the whole floor.
Renting rack space
Colocation is renting U-space in someone else's rack — a quarter, half or full rack — with power and network, for your own hardware. It is how many hosting companies start. Prices in UK datacenters run roughly £60–120 per month for 1U with 100 W of power and a network port, and £700–1,500 for a full rack with a few kW. What you get for it is the building: power redundancy, cooling, physical security, and hands on site.
What this means for a VPS customer
Nothing you configure — but it is where your server physically is. A VPSPioneer managed VPS runs on 1U and 2U hosts in racks exactly like this, in a UK datacenter with dual power feeds per rack and hot-aisle containment. The differences between such buildings — and what a "Tier III" badge does and does not promise — are in datacenter tiers explained.