Understanding “availability” is crucial to implementing IT solutions
In the current business world where an increasing number of solutions are purchased as SaaS or cloud based offerings from vendors, and IT support is getting outsourced to various third party companies, the importance of “availability” in the context of business risk can be easily missed. Usually, this is not well communicated to the business by IT and sometimes not understood clearly by IT staff purchasing and implementing systems. The business decision makers need to be made aware about what it means in terms of risk to the business to ensure an appropriate IT system is designed or selected to provide the reliability required.
Usually, an IT system or service is outlined as part of the service level agreement (SLA) by the vendor or part of the support contract with an external organisation. This is typically outlined as a percentage of uptime for a given year, but what does it really mean? The following table outlines the availability percentage and the uptime it guarantees:
| Availability % | Downtime per year | Downtime per month | Downtime per week |
| 90% (“one nine”) | 36.5 days | 72 hours | 16.8 hours |
| 95% | 18.25 days | 36 hours | 8.4 hours |
| 97% | 10.96 days | 21.6 hours | 5.04 hours |
| 98% | 7.30 days | 14.4 hours | 3.36 hours |
| 99% (“two nines”) | 3.65 days | 7.20 hours | 1.68 hours |
| 99.50% | 1.83 days | 3.60 hours | 50.4 minutes |
| 99.80% | 17.52 hours | 86.23 minutes | 20.16 minutes |
| 99.9% (“three nines”) | 8.76 hours | 43.8 minutes | 10.1 minutes |
| 99.95% | 4.38 hours | 21.56 minutes | 5.04 minutes |
| 99.99% (“four nines”) | 52.56 minutes | 4.32 minutes | 1.01 minutes |
| 100.00% | 26.28 minutes | 2.16 minutes | 30.24 seconds |
| 99.999% (“five nines”) | 5.26 minutes | 25.9 seconds | 6.05 seconds |
| 99.9999% (“six nines”) | 31.5 seconds | 2.59 seconds | 604.8 milliseconds |
| 99.99999% (“seven nines”) | 3.15 seconds | 262.97 milliseconds | 60.48 milliseconds |
| 99.999999% (“eight nines”) | 315.569 milliseconds | 26.297 milliseconds | 6.048 milliseconds |
| 99.9999999% (“nine nines”) | 31.5569 milliseconds | 2.6297 milliseconds | 0.6048 milliseconds |
(above table was sourced from http://en.wikipedia.org/wiki/High_availability)
For example, a system which has an availability guarantee of 99.9% can sustain a total system outage of 8.76 hours for the year. Typically, this can happen across many small outages over the year or it can happen as one single outage. Therefore, a system with 99.9% availability for the year can be unavailable for one entire business day and still be within the agreement with the service provider.
The key questions to ask here are:
- Can the business sustain an outage of this magnitude, and does it have a business continuity plan in the event of such an outage?
- Does the availability for each IT system in the organisation accurately reflect the business importance and risk?
Another key consideration is to ensure that any other system or infrastructure underpinning the solution also has the same or higher availability. For example, paying for a high availability cloud solution with 99.99% can be pointless if the corporate network or internet capability does not provide a similar level of availability to access the system.
Today, an organisation’s IT ecosystem is a very complex environment with many moving components relying on each other.Therefore, it is essential to ensure the IT landscape is architected with availability in mind to mitigate business risks appropriately.
