99.999% Uptime? Come on now, what does that even mean.
If any of you came by WHW earlier today (between 10:30-11:00) you may have been surprised to find that the site was down. Plagued by nasty spam attacks all weekend, the site just couldn’t take any more abuse and checked out for nearly 30 minutes (28 minutes and 11 seconds to be exact). Thanks to the monitoring services i’ve been using, i was immideately notified via email. But, the whole experience made me think about server availability and redundancy.
Many Hosting providers guarantee 99.9% or 99.5% uptime. How much is that? And how does WHW measure up against some of the other, larger web hosting firms?
Well, for one thing, many experts in this field would say that bigger does always mean better. The simple reason being:
The bigger you are the more points of failure.
So if i administer 12 websites, it is likely that 1 of them will go down - even for just a few minutes - at some point this month. Whereas if you only had 1 website, you might go months without experiencing any downtime at all.
On the otherhand, if i take a brief look at the various definitions of maximum server availability (ie: 99.999% uptime) - there seems to be a degree of uncertainty surrounding the calculation of this mythical number.
From the Mon.itor.us Blog:
| If it’s up… | It’s down… per year |
| 90% | 876 hours |
| 95% | 438 hours |
| 99% | 87 hours, 36 minutes |
| 99.9% | 8 hours, 45 minutes, 36 seconds |
| 99.99% | 52 minutes, 33.6 seconds |
| 99.999% | 5 minutes, 15.36 seconds |
| 99.9999% | 31.68 seconds |
The Wikipedia offers a whole formula for calculating this percentage but their numbers don’t exactly match up:
| availability % | downtime per year | downtime per month* | downtime per week |
|---|---|---|---|
| 98% | 7.30 days | 14.4 hours | 3.36 hours |
| 99% | 3.65 days | 7.20 hours | 1.68 hours |
| 99.5% | 1.83 days | 3.60 hours | 50.4 min |
| 99.9% | 8.76 hours | 43.2 min | 10.1 min |
| 99.99% | 52.6 min | 4.32 min | 1.01 min |
| 99.999% | 5.26 min | 25.9 s | 6.05 s |
| 99.9999% | 31.5 s | 2.59 s | 0.605 s |
Even within the Wikipedia site itself, there are currently some inconsistencies between these figures. This chart shows “high availability” as:
- 99.9% ≡ 43.8 minutes/month or 8.76 hours/year
- 99.99% ≡ 4.38 minutes/month or 52.6 minutes/year
- 99.999% ≡ 0.44 minutes/month or 5.26 minutes/year
Anyway, i just thought it was interesting to take a look at these numbers again. I put them here because i thought it might be useful for some of you out there - but, i personally haven’t experienced anything close to 5 nines - maybe that’s why it’s often referred to as a myth.
