ONID Service Level Agreement (SLA)
- Different goals for different services
- What can you actually do?
- What do customers want?
- Response time?
The following text was shamelessly copied from this page.
Uptime is the amount of time the entire system is available. By entire system we are saying that an entire transaction can be completed. Just having your web servers running when the needed application server isn't running cannot be defined as uptime.
Downtime is everything else.
Scheduled maintenance downtimes or windows are the periods of time (for example, from 1:00am to 3:00am Monday morning) when an IT team has the option, if they need, to bring down various components in a fashion that causes the system to be incapable of complete functionality.
Reliability is defined as uptime but where scheduled maintenance downtime is not counted against it. For example, if in a 24 hour period there was an hour of scheduled downtime, but otherwise full operational for the remaing 23 hours, then the system was 100% reliable.
So how do you translate the 'nines' into acceptable downtime? This chart provides the answer:
'Nines' Uptime % Minutes Per Year Minutes Per Month Two 99% 5256 438.0 Three 99.9% 526 43.8 Four 99.99% 53 4.4 Five 99.999% 5 0.4