Peak load needs 6 servers. They are spread evenly over 3 availability zones, and the service must survive losing a whole zone. How many servers do you run?

Options

• ✗ 6
• ✗ 7
• ✓ 9 Correct
• ✗ 18

Why

After a zone fails, the 2 remaining zones must carry all 6 servers' worth of load: 3 per zone. 3 zones × 3 = 9. In general, with Z zones you need N × Z ÷ (Z − 1).

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