The system was down for two hours. Your rate of availability just plummeted, and the customer is none too pleased. Unfortunately, it’s impossible to prevent outages 100% of the time, but part of availability is first figuring out how to predict it. If you need some help in this area, Stephen Mann has a few tips about how to agree on a rate of availability with your customer.
First, know the number and duration of outages. A system outage for two hours at one time is much different sixty outages of two minutes each scattered over several months. Also, look at the number of users affected. How many users have to be down before it quantifies as a system outage?
The criticality of the outage might depend on the business function as well. Sending email might be down 100%, but at the same time, reading public folders is 50% available. Make sure you discuss the different types of outages and how to record them with your customer. Then, put your plan into the SLAs. You and your customer will be much happier when you agree on the variables involved in availability.