Infrastructure & Reliability · Chapter 5 of 8
The physical layer, and why AI broke the datacentre
Power, cooling and space. Unfashionable, non-optional, and currently the binding constraint on the entire industry.
Cloud abstracted the physical layer without removing it, and AI has made it visible again. A quarter of the IT infrastructure postings in this atlas are AI cluster buildout, and the reason is that training workloads break the assumptions datacentres were designed around.
The arithmetic that decides everything
| Conventional hall | AI training | |
|---|---|---|
| Power per rack | 5–15 kW | 40–130 kW |
| Cooling | Air, comfortably | Rear-door heat exchangers or direct-to-chip liquid |
| What binds first | Space, sometimes power | Power distribution, then heat rejection |
| Failure impact | One node degrades a service | One node stops a synchronous job across thousands |
| Lead time | Weeks for servers | Quarters to years for transformers and switchgear |
The practical consequence: a hall with plenty of floor space frequently cannot host a single row of training racks, because the busway to that position cannot deliver the current and the building cannot reject the heat. Space is almost never the constraint, and saying so is the fastest way to demonstrate you have worked at this layer.
Redundancy notation, decoded
- N — exactly what is needed. Any failure is an outage.
- N+1 — one spare of each component. The common enterprise target.
- 2N — two complete independent systems.
- Concurrently maintainable — every component can be serviced without dropping load. This is what Tier III actually means and it is the phrase that matters in practice, because most real outages happen during maintenance rather than during failure.
PUE, and why it is often quoted misleadingly
Power usage effectiveness is total facility power divided by IT power. 1.0 would mean every watt reaches the equipment; 1.5 means half again is spent on cooling and losses.
It is quoted selectively — at design load, in winter, excluding certain draws. Ask what load it was measured at and over what period, because a PUE quoted at full design load says nothing about a hall running at 40%.