Power Management
Servers that draw up to 500 W each are installed in a rack with a power limit of 8 kW. If power capping sets a 400 W limit on each server, how does the number of servers the rack can hold change? (Ignore the power of other equipment.)
- AIt increases from 16 to 20
- BIt stays at 16
- CIt increases from 16 to 80
Show answer and explanation
Answer: A. It increases from 16 to 20
The rack's power limit must hold even if every server draws its maximum power at the same time. Without a cap, 8000 W ÷ 500 W = 16 servers fit, but if power capping guarantees that no server exceeds 400 W, up to 8000 W ÷ 400 W = 20 servers fit. In general, it is rare for all servers to draw their maximum power at once, so the performance impact can often be kept small. However, a server that reaches its cap has its CPU frequency and voltage lowered and runs more slowly, so the cap is set by looking at the workload.
- A. Correct. It goes from 8000 ÷ 500 = 16 to 8000 ÷ 400 = 20.
- B. Because the cap is guaranteed, less power can be budgeted per server, so more servers fit.
- C. This confuses the answer with 8 kW divided by the 100 W difference.