Here are the differences between the two uptime SLAs at a monthly level, courtesy Wikipedia:
99.9% = 43.2 minutes downtime
99.99% = 4.32 minutes downtime
The difference of nearly 39 minutes a month might not seem like a big deal. However, if you are an online merchant that is going through a huge Christmas surge, that hypothetical downtime would create a significant business loss. For example, if one of our clients would have experienced the additional downtime during their heaviest signup period in 2011, they would have lost over 50,000 new customers in that 39 minute span. This is especially relevant because it is more likely that there would be a downtime during an extended peak surge.
If each customer is worth $20 that first year, that’s a million dollar mistake.
Of course this matters for your business. Let me reiterate: million dollar mistake. We do pretty well against our SLA’s. Wherever you go, ask them to prove that they can back their uptime guarantee or you’re stuck with empty promises and service credits. If you can get their track record over the course of a year or two, that’s good information to have.Update 3/20: looks like Aria is claiming they have 99.99% uptime for the year… (it’s only March) But on closer inspection that last year, they had three 9’s. (99.9%) Seems a little disingenuous to me to tout 99.99% when they aren’t fulfilling it… and I highly doubt they are guaranteeing it. Vindicia actually offers an SLA with four 9’s. Apples to oranges.