Sunday, September 30, 2007

Making sense of 9s

A recent article on TechCrunch featured the uptime of Google for different countries around the World.

The table looked like this:

Hm... sometimes we take a glance at these digits, but we don't notice the difference between uptime and availability, which can lead to misleading impressions.

As usual, Wikipedia provides the answer:

Availability is usually expressed as a percentage of uptime in a given year. In a given year, the number of minutes of unplanned downtime are tallied for a system; the aggregate unplanned downtime is divided by the total number of minutes in a year (approximately 525,600), producing a percentage of downtime; the complement is the percentage of uptime, which is what is typically referred to as the availability of the system. Common values of availability, typically stated as a number of "nines", for highly available systems are:

  • 99.9% ≡ 43.8 minutes/month or 8.76 hours/year ("three nines")
  • 99.99% ≡ 4.38 minutes/month or 52.6 minutes/year ("four nines")
  • 99.999% ≡ 0.44 minutes/month or 5.26 minutes/year ("five nines")

It should be noted that uptime and availability are not synonymous. A system can be up, but not available, as in the case of a network outage.

Anyway, here are some tidbits for regular readers:
- Mac Leopard Wallpapers
- Mac Tiger Wallpapers
- The 5th Wave RSS Feed

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