The odds of a particular server or service being up and running at any point in time, usually measured in percentages. A server that has 99% availability will be operational 99% of the time (this would be described as having twonines of availability).
Used to describe systems that have particulary high levels of availability, typically 5 nines or more; sometimes abbreviated HA.
Typically refers to percentages of uptime. For example, 5 nines of availability means an uptime of 99.999% of the time. Below are the downtimes expected per year depending on those 9s:
- 99% (two 9s): 87.7 hours
- 99.9% (three 9s): 8.8 hours
- 99.99%: 52.6 minutes
- 99.999%: 5.3 minutes
The process of replicating parts of a system in an effort to make it more reliable.
Short for "service-level agreement", an SLA is a collection of guarantees given to a customer by a service provider. SLAs typically make guarantees on a system's availability, amongst other things. SLAs are made up of one or multiple SLOs.
Short for "service-level objective", an SLO is a guarantee given to a customer by a service provider. SLOs typically make guarantees on a system's availability, amongst other things. SLOs constitute an SLA.