Volume 26 • No 06 • 2003
ISSN 0742-468X • Since 1978
On-line Since 2000
Up and Down
Time
Every so often you run into the concept of uptime and downtime. This is particularly true in the computer industry. You may already have a service level agreement (SLA) with your IT department, but do you really understand what you are promised?
I realize that my readers are smarter than the average person, but just in case the second smartest person in the world is reading this, I thought you may want to explore the meaning of uptime in your SLA for your server
Most servers run 24/7, or 24 hours per day, seven days a week. When it comes to web servers this is not only the norm, but it is mandatory. Imagine surfing the web and finding that the web server is only available during normal business hours
The numbers please
The other day, someone told me about the good deal that got from their ASP (application service provider). This person was proud that they were guaranteed 95% up time.
"We can live with that," he boasted. Oh really? Take a look at what he really got.
- Uptime% = Downtime min/yr
- 90% = 52,600 minutes
- 95% = 6,280 minutes
- 99% = 5,256 minutes
- 99.999% = 5.25 minutes
While 95% uptime sounds good, ask yourself this: Can you really live without your server for 6,280 minutes per year? That’s 104.667 hours per year, or 4.63 days per year.
Even with 99% performance you can expect to be down for 3.65 days per year. That’s probably tolerable, unless it all happens at once. But, if you and your customers rely on accessing your server, then the standard is 5 9’s or an uptime of 99.999% — that’s just five minutes and 15 seconds in the entire year. However, a more realistic target might be what the Uptime Institute® refers to it as a Tier IV computer environment that delivers 99.991% availability. Accourding to the institute: “Tier IV’s 99.991% uptime is an average over five years. An alternative calculation using the same underlying data is 100% uptime for four years and 99.954% for the year in which the downtime event occurs.”
Conclusion
Availabilty comes at a cost. As you decrease the expected downtime, you add redunancy to your computer room in terms of servers, power, air conditioning, and security. Keep in mind that the performance numbers are averages. Thus, if the downtimes occur a few seconds each time, it may be annoying, but it may not be noticed. However, you must assume that the outages come in blocks of time, and you are kidding yourself if you think 95% uptime is acceptable service to you, or acceptable service from you to your customer. Now go checked out your SLA.
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