Datacenter Power Cooling¶
10 cards — 🟢 3 easy | 🟡 4 medium | 🔴 3 hard
🟢 Easy (3)¶
1. What are the four main types of PDUs and what does each add over a basic PDU?
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Basic PDU: simple power distribution, no monitoring. Metered PDU: shows per-outlet or per-phase power draw. Switched PDU: adds remote on/off control per outlet. ATS (Automatic Transfer Switch): automatically fails over between two input power feeds if one goes down.2. What is hot aisle / cold aisle containment and why does it matter?
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Servers intake cool air from the front (cold aisle) and exhaust hot air from the rear (hot aisle). Containment uses physical barriers (doors, curtains, panels) to prevent hot exhaust from recirculating to server intakes. Without containment, mixing reduces cooling efficiency, raises inlet temperatures, and can cause thermal throttling or shutdowns.3. What are blanking panels and why should empty rack U-spaces never be left open?
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Blanking panels are covers installed in empty rack unit spaces. Without them, hot exhaust air from the rear recirculates through the open gaps to the cold aisle, bypassing servers and creating hot spots. This reduces cooling efficiency and can cause adjacent servers to overheat even when overall room temperature is within spec.🟡 Medium (4)¶
1. What is A+B feed redundancy and why is it critical in datacenter racks?
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A+B feed means each rack receives power from two independent circuits connected to separate UPS systems and ideally separate utility feeds. Servers with dual PSUs connect one to each feed. If one feed fails (UPS failure, breaker trip, maintenance), the other feed keeps all equipment running with zero downtime.2. What is UPS and what happens when battery replacement is missed?
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A UPS (Uninterruptible Power Supply) provides battery backup during utility power outages, bridging the gap until generators start (typically 10-30 seconds). UPS batteries degrade over time and must be replaced on schedule (usually every 3-5 years). Missing replacement means the UPS may not hold load during an outage, causing an unplanned shutdown.3. What is the difference between a CRAC and a CRAH unit?
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A CRAC (Computer Room Air Conditioner) uses a compressor-based refrigeration cycle to cool air directly. A CRAH (Computer Room Air Handler) uses chilled water from a central plant and fan coils to cool air. CRAHs are more energy-efficient at scale and allow centralized chiller management, making them preferred in large datacenters.4. What is the ASHRAE A1 recommended temperature range for datacenter inlet air?
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ASHRAE A1 recommends 18-27 degrees Celsius (64-80 degrees Fahrenheit) for server inlet temperature. Operating outside this range increases hardware failure rates and can void warranties. Monitor inlet temps with BMC sensors (ipmitool sdr type Temperature) and environmental monitoring systems. Humidity should be 40-60% relative humidity.🔴 Hard (3)¶
1. How do you calculate a rack power budget and what is a typical limit?
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Sum the rated wattage of all equipment in the rack (servers, switches, storage) and compare against the PDU capacity (commonly 5-10 kW per feed). Account for peak vs average draw -- servers under full CPU load draw significantly more than idle. Maintain headroom (typically 80% of circuit capacity) to avoid breaker trips. Monitor actual draw with metered PDUs.2. What does N+1 redundancy mean for power and cooling, and how does it differ from 2N?
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N+1 means you have one additional unit beyond the minimum needed (e.g., 3 cooling units when 2 would suffice). 2N means fully duplicated capacity (e.g., two independent UPS systems each able to handle the full load). 2N is more resilient but costs twice as much. N+1 is the minimum for production datacenters; 2N is standard for Tier III/IV facilities.3. What is the difference between a power whip and a standard outlet in datacenter power distribution?