• A common way to talk about availability is in a "number of nines"
• It refers to how many 9s are in the percent uptime
• One 9 = 90% uptime = ~36.5 days per year
• Two 9s = 99% uptime = ~3.7 days per year
• Three 9s = 99.9% uptime = ~9 hours per year
• Four 9s = 99.99% uptime = ~52 minutes per year
• More 9s means more work

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