# High Availability

*[Main Index](../SKILL.md)*

## Contents

| Section                                 | File                                                           |
|-----------------------------------------|----------------------------------------------------------------|
| 15.1-15.5 HA Overview and How It Works  | [overview-and-how-it-works.md](overview-and-how-it-works.md)   |
| 15.6 HA Configuration                   | [configuration.md](configuration.md)                           |
| 15.7-15.9 HA Fencing and Error Recovery | [configuration-fencing.md](configuration-fencing.md)           |
| 15.7-15.9 Fencing and Recovery          | [fencing-and-recovery.md](fencing-and-recovery.md)             |
| 15.10-15.12 Maintenance and Scheduling  | [maintenance-and-scheduling.md](maintenance-and-scheduling.md) |


## See also

- [Cluster Manager](../ch05-cluster-manager/_index.md)
- [ha-manager CLI Reference](../appendix-a-cli/ha-manager.md)

## Overview

Our modern society depends heavily on information provided by computers over the network. Mobile devices
amplified that dependency, because people can access the network any time from anywhere. If you provide
such services, it is very important that they are available most of the time.
We can mathematically define the availability as the ratio of (A), the total time a service is capable of being
used during a given interval to (B), the length of the interval. It is normally expressed as a percentage of
uptime in a given year.

Table 15.1: Availability - Downtime per Year
Availability %
99

## 99.9 99.99


## 99.999 99.9999


## 99.99999 Downtime per year


## 3.65 days


## 8.76 hours


## 52.56 minutes


## 5.26 minutes


