Business impact and transparency: expressing system availability

“System availability was 99.83% last month!  That’s up from 99.75% the previous month!”

Sounds kind of good, no? I mean, that’s a high number, right? Right?

Actually, no.  It’s not a very useful number, in and of itself. In fact, I regard the publication of uptime metrics like that as a regrettable symptom of IT focusing on technical aspects, rather than business impacts.  Here’s a discussion of why I see it that way, followed by a presentation of an alternative focus providing much more business value.

So, what’s wrong with a time-honored metric like “the system was 99.83% available”?

  • The number is deceptive. Few people can mentally translate “99.83% availability” into a more meaningful real number, such as “system was down for 1.3 hours last month.”  Even fewer can tell you the real difference between 99.3% uptime (also sounds pretty good, right?) and 99.8% uptime.  Both 99.3% and 99.8% look (to the vast majority of business people) at first glance like pretty good numbers for uptime, but the first represents more than three times the number of “down hours” of the second.

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