I'm interested in find a hosting that can offer really 100% of uptime.
Is it possible in hosting servers inside a cloud computing structure?
Real 100%? No. Five nines (99.999%)? Yep. Five nines is about five minutes unscheduled outage/year. You can get more reliability if you want, but five nines is where the cost for increased reliability is really taking off. You can approximate four nines as less than an hour outage/year, three nines as less than nine hours and six nines as about half a minute.
Why, you might ask, does "hosting servers in a cloud computing structure" not provide this? What are the chances of the cloud data center going down (tornado, fire, flood)? If at multiple data centers, how long does it take to detect and accomplish the failover? How much does that extra redundancy cost? You can see how the expense mounts rapidly.
Figure out how much application downtime will cost you and from that, determine how much you're willing to pay for increased reliability. Then go looking for appropriate Service Level Agreements.
There is no such thing as 100% uptime. Software breaks, hardware fails, and you can only mitigate these to a certain extent. Cloud computing is a misnomer, because it still relies on a hardware/software backend. That being said, I'm sure there are companies out there who have taken the necessary precautions to minimize the chance and duration of an outage. YMMV.
Mathematically spoken, this is also a question of probability. Each single server in the setup has only reliabilty of less than 100%, respectively a probability of failure of more than 0.
To get the reliability of your whole system, that is the opposite of failure, you have to multiply the single probabilities of failure. As each of them is greater than 0, the product will always stay greater than zero, be it very small, but it will never be zero.
And the reliability wich equals to 100%-"probaility of failure" will always be less than 100%.