Power¶
10 cards — 🟢 3 easy | 🟡 4 medium | 🔴 3 hard
🟢 Easy (3)¶
1. What are the three types of UPS and which is standard for datacenters?
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Offline/Standby (5-12ms transfer time), Line-Interactive (2-4ms), and Online/Double-Conversion (0ms, always on inverter). Double-conversion is the datacenter standard because it provides zero transfer time and complete isolation from utility power quality issues.2. What is 1+1 PSU redundancy?
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1+1 means two power supplies in a server — one active, one standby. If the active PSU fails, the standby takes over immediately. Each PSU should be connected to a different PDU/power feed for full redundancy.3. What is a PDU and what types exist?
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A PDU (Power Distribution Unit) distributes power from UPS to server racks. Types: Basic (power strip), Metered (shows power draw), Monitored (network-connected, per-outlet monitoring), Switched (remote power cycling per outlet), Intelligent (monitoring + switching + environmental sensors).🟡 Medium (4)¶
1. How is dual-feed power redundancy implemented at the rack level?
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Each rack has two PDUs: PDU A from Power Feed A (UPS A) and PDU B from Power Feed B (UPS B). Each server connects one PSU to PDU A and one to PDU B. Either PDU can handle the full rack load if the other fails. This eliminates single points of failure from UPS to server.2. How do you monitor UPS status on Linux?
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Using NUT (Network UPS Tools): upsc myups (full status), upsc myups ups.status (OL=online, OB=on battery, LB=low battery), upsc myups battery.charge (percentage), upsc myups battery.runtime (seconds). For APC: apcaccess status. Server power draw: ipmitool dcmi power reading.3. What is PUE and what values are considered good?
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PUE (Power Usage Effectiveness) = Total Facility Power / IT Equipment Power. PUE 1.0 is perfect (impossible), 1.2 is excellent, 1.5 is average, 2.0 is poor (half the power goes to cooling/overhead). It measures datacenter power efficiency.4. How do you calculate power requirements for a rack?
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Sum the average power draw of all equipment. Example: 20 servers x 500W = 10kW, plus 2 switches x 150W = 300W. Total ~10.3kW. With 1+1 PSU redundancy, each PDU must handle the full 10.3kW. Typical rack capacity is 5-10kW (standard) up to 30+kW (high density).🔴 Hard (3)¶
1. What is the correct shutdown order during a power emergency?
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1. Applications (drain connections, flush buffers). 2. Virtual machines. 3. Hypervisors/bare-metal OS. 4. Storage arrays (after all servers are down). 5. Network switches (last — needed for management). Configure NUT/apcupsd for automatic shutdown when battery is low.2. What power-related alert thresholds should be configured for datacenter monitoring?
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UPS on battery: immediate page. Battery < 50%: warning. Battery < 20%: critical, initiate shutdown. PDU circuit > 80% capacity: warning (prevent overload trips). Inlet temperature > 35C: warning. Also monitor UPS input/output voltage, frequency, and server power consumption (watts).3. How does the power failover chain work from utility loss to generator?