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Availability in Plain Words

Availability is the share of time a system is up and able to answer requests. Availability is written as a percentage, and the missing share is downtime.

Downtime at 99 percent

A year holds 365 days. At 99% availability, one percent of the year is downtime.

days_in_year = 365
downtime_days = 0.01 * days_in_year
print("Days in a year:", days_in_year)
print("Downtime in days:", downtime_days)

Downtime at 99.9 percent

A year holds 8,760 hours. At 99.9% availability, the downtime is a small slice of those hours.

hours_in_year = 8760
downtime_hours = 0.001 * hours_in_year
print("Hours in a year:", hours_in_year)
print("Downtime in hours:", downtime_hours)
print("Rounded downtime in hours:", round(downtime_hours, 1))

Downtime at 99.99 percent

Four nines leave almost no downtime in a year. The code measures that downtime in minutes.

hours_in_year = 8760
downtime_minutes = 0.0001 * hours_in_year * 60
print("Downtime in minutes:", downtime_minutes)
print("Rounded downtime in minutes:", round(downtime_minutes))

Example

year_hours = 8760
year_days = 365
print("99% down in days:", 0.01 * year_days)
print("99.9% down in hours:", round(0.001 * year_hours, 1))
print("99.99% down in minutes:", round(0.0001 * year_hours * 60))

Key points

  • Availability: the share of time a system stays up and answers requests.
  • Downtime: the time a system is down and cannot answer requests.
  • Year: the measuring period for the figures in this lesson, with 365 days and 8,760 hours.
  • Nines: a short name for the count of 9 digits in an availability percentage.
  • 99% availability: down about 3.65 days in a year.
  • 99.9% availability: down about 8.8 hours in a year.
  • 99.99% availability: down about 53 minutes in a year.

In this example, each higher percentage shrinks the downtime from days to hours and then to minutes. A small change in the percentage makes a large change in the allowed downtime.

Note

a higher percentage always means less time down in the year.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

How much downtime does 99.9% availability allow in one year?