An entire rack stops responding at 2 a.m. Walk me through the first thirty minutes.

A whole rack failing at once is a shared dependency, which narrows the causes to four before anyone touches anything. What the out-of-band network tells you in the first minute, why the order of restoration matters, and the decision about liquid cooling that has to be made before power returns.

Updated Sep 2026 · Grounded in real AI infrastructure interview loops and written to a senior-engineer editorial bar, with every number worked and every diagram hand-built.

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The concepts behind this question

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Foundational

🖧 Hardware & Cluster Build-Out

SXM, PCIe and Rack-Scale Form FactorsThe same silicon ships in three shapes and the shape decides the deployment. An SXM module is soldered to a baseboard with a full NVLink mesh and needs 700 to 1,400 W of direct power and usually liquid cooling. A PCIe card slots into a standard server, draws through the slot and a cable, and has no NVLink. A rack-scale system like GB300 NVL72 makes the whole rack one NVLink domain and stops being a server at all. Choosing between them fixes your power, cooling, cabling and scheduling story.

Foundational

🖧 Hardware & Cluster Build-Out

Direct-to-Chip Liquid Cooling and CDUsAbove roughly 40 kW a rack cannot be cooled by air in any practical hall, which is why every dense GPU deployment now runs liquid to the chip. A cold plate sits on each GPU, a coolant distribution unit isolates the clean rack loop from facility water, and the facility side runs warm, typically 30 to 40 degrees supply, because warm water is cheaper to make. The design numbers are flow rate and temperature rise, and both fall out of one equation that every operator should be able to do from memory.

Foundational

🖧 Hardware & Cluster Build-Out

Cables, Transceivers and the Optics Power BudgetCable choice is set by distance and it is the most common ordering mistake in a GPU cluster build. Passive copper reached 3 m at 400G and tops out near 2 m at 800G, so a bill of materials copied from the previous generation produces links that will not come up. Beyond copper come active copper, then active optical cables, then transceivers and fiber. Each step adds reach and adds power, and at cluster scale the transceivers alone draw tens of kilowatts that nobody budgeted.

Foundational

🖧 Hardware & Cluster Build-Out

Accelerator Selection: H100 to B300 and RTX PRO 6000Three published numbers decide which accelerator suits a workload, and they are independent: memory capacity gates what fits, memory bandwidth gates decode speed, and tensor FLOPS gate prefill and training. As of September 2026 the parts NVIDIA sells for datacenters span 80 GB to 288 GB and 1.6 TB/s to 8 TB/s, and the gap between the compute number and the bandwidth number has widened every generation, which is why a part that looks four times faster on a slide is often twice as fast on a decode workload.

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Closest to this topicYou have budget for roughly 1,000 GPUs and a greenfield site. Specify the cluster.Closest to this topicYou have exactly 1 MW of facility capacity. Maximize training throughput.Closest to this topicHow much power does a 512-GPU cluster actually draw, end to end?

FEDITOR'S NOTE

Scored on using out-of-band to distinguish a power loss from a network loss, on the four shared-dependency causes, and on not restoring power before the cooling state is known.

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