Reliability? I guess you mean an uptime of 99.999

I was at a customer site a few days ago where the client said that they wanted a completely reliable system -- "five nines." Well, that was fine with me, except that we couldn't agree on what five nines meant.

So we had to calculate it out, and I thought I'd share that with you. Mail is one of those things where reliability is key, and this may come in handy when you're not discussing Y2K problems.

My rule of thumb to remember is that five nines means five minutes of downtime a year. That's the easiest way to remember it. But the chart below might be a help. I've expressed things in two ways. One is the standard way of thinking about unplanned downtime, as a 24-hour thing.

However, for many companies, they need super-high reliability, but not 24 hours a day. In that case, an 8-hour window is critical, but if the mail system is down for 30 minutes at 3 am local time, this may not be a big deal.

One of the keys is that traditional thinking about being up 99% of the time isn't a good way to think about the problem. You could be up 99% of the time, and be down for over two entire work weeks during the 9-5 shift. That doesn't sound like 99% of the time to me!

For normal year-round operation:

Availability	Downtime
90%	876 hours, or 36.5 days
99%	87.6 hours, or 3.65 days
99.9%	8.76 hours
99.99%	3153 seconds, or 52.55 minutes
99.999% "five nines"	315 seconds, or 5.25 minutes
99.9999%	31.5 seconds
If you only care about an 8-hour window during the week:
99.999%	75 seconds
99.9999%	7.5 seconds

Joel Snyder is a senior partner with Opus One, a consulting firm in Tucson, Arizona. He spends most of his time on the road helping people build larger, faster, better, and more reliable networks. His professional travels have taken him from San Francisco to St. Petersburg, where he always carries his trusty Macintosh and modem, neither of which have cute names. He is also a member of the Network World Test Alliance and writes extensively on networking topics. Reach him at joel.snyder@opus1.com.

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