Servers are mounted in standard-width racks, measured in rack units of 4.4 centimetres. A typical rack has 42 of them. Racks stand in rows, with aisles between, and each row has power and network cables running above or below.
Customers who rent space choose between a few units in a shared rack, a full rack, or a locked cage containing many. For hosting companies, the design goal is density without heat problems: more servers per square metre, as long as power and cooling keep up.
The unit and the frame
The rack is a steel frame with two pairs of vertical rails, drilled with a regular pattern of holes. Equipment is built to fit between the rails, which are 19 inches apart, so a server from any maker slots into a rack from any other. Height is counted in units, written U. One U is the 4.4 centimetres from the opening paragraph (strictly 44.45 mm), and a 42U rack therefore offers about 1.85 metres of usable mounting space.
| Equipment | Typical height | Notes |
|---|---|---|
| Switch or patch panel | 1U | Often at the top of the rack |
| Web or mail server | 1U or 2U | Most common shape |
| Storage server | 2U to 4U | Room for many drives |
| Blade chassis | 6U to 10U | Shares power and networking |
Rails are the part people forget. Sliding rails let a technician pull a server out like a drawer for service; cheaper static rails mean unplugging and lifting it out.
Rows, aisles and airflow
Almost every server draws cool air in at the front and blows hot air out of the back. Rows are therefore arranged back to back, so that fronts face a cold aisle and backs face a hot aisle. Cold air is delivered to the first, hot air is collected from the second, and the two are kept apart with doors, curtains or roofs over the aisle. Blanking panels fill empty units, because a gap in the rack lets hot air loop round to the front and back into the intake.
Power and cabling
Each rack gets power strips (rack PDUs) fed from the room's distribution. Servers with two power supplies plug one lead into each of two strips on separate feeds. Cabling for networking follows the same discipline: a top-of-rack switch lets every server in the rack connect with a short patch lead, and a few thicker uplinks run from the switch to the room's network core, usually along trays overhead or beneath a raised floor. Neat cabling is not vanity; a bundle of tangled leads blocks airflow and turns a ten minute swap into an hour.
What you can rent
| Option | You get | Suits |
|---|---|---|
| Shared rack, a few U | Space for one or two servers beside other tenants | Small colocation |
| Full rack | A lockable cabinet, usually with a power allowance | One organisation's whole estate |
| Cage | A fenced area holding several racks | Large or regulated deployments |
The price is driven by power more than by space. A rack with room for 42 servers is useless if the allowance is 3 kW and each server draws 400 W. Check the allowance, the plug type and what happens when you exceed it.
Density and what limits it
Every watt a server draws leaves the rack as heat, so a rack's power allowance is also its cooling budget. Older halls were designed around a few kilowatts per rack; modern ones may offer ten or more, and dense storage or GPU machines can ask for far beyond that. When a customer wants more than the room was built for, the operator has three choices: leave neighbouring units empty, move to a part of the hall with stronger cooling, or add rear-door coolers. Weight matters too. A full rack of storage servers can weigh a good fraction of a tonne, and the floor has a rated load per square metre.
This is why a rack that looks half empty may be fully sold. It is also why colocation quotes list kilowatts first and units second.
Checking it yourself
Before signing for rack space, ask for the power allowance per rack in kW, the circuit voltage, the number of independent feeds, and whether the aisles are contained. On a visit, look at the front of the racks for blanking panels and at the cable trays for tidiness. Then ask who can open your cabinet, and whether neighbours share its keys.
Things people ask
How many servers fit in a rack?
Up to 42 if every server is 1U, but power and cooling usually limit it to well below that.
Why are some racks half empty?
Because the power or cooling budget ran out before the space did. Empty units are cheap; kilowatts are not.
Can I bring my own rack?
Often only if it matches the operator's airflow and floor loading rules. Ask before buying one.