2.3 Availability Concepts Overview
The concept of availability extends across multiple fields, in addition to cloud computing. When dealing with computer systems and networking, as a general term,
availability can be described as “the degree to which a system or component is operational and accessible when required for use,” according to the IEEE 610 definition (see Section
6,
References).
The availability of a system or component is commonly calculated as a percentage (of the time the system or component is available when compared to the whole), and it is often measured in terms of number of 9s, for example, 99.99%.
A formula to capture the availability of a simple system or component is calculated as follows, by tracking the uptime and downtime of the system itself:
Availability = Uptime/(Uptime + Downtime)
The overall availability of a more complex system is calculated as follows, by multiplying the availability in percentage of each component present in the system:
Overall Availability = Element#1(availability%)*Element#2(%)*... Element#n(%)
A computer server running in a data center represents a complex system when approached as a set of separate devices, each with its own availability (calculated or most likely provided by the manufacturer). To the extent that the rack containing the server and additional devices represents a more evolved complex system to measure in terms of overall availability, the following items must be considered:
• Rack power supplies, cabling, and so on
• Top-of-the-rack network switches
• Server’s local storage / hard disks
• Server’s power supply (supplies, if redundant)
• Server’s internal components (CPUs, memory, controller, and so on)
A reference set of availability percentages, with their corresponding number of 9s and calculated downtime per year, is detailed in the following table.
Table 2. System Availability and Downtime per Year
Number of 9s | Availability % | Downtime/Year | System or Component Inaccessible |
1 | 90% | 36.5 days | Over 5 weeks per year |
2 | 99% | 3.65 days | Less than 4 days per year |
3 | 99.9% | 8.76 hours | Approximately 9 hours per year |
4 | 99.99% | 52.56 minutes | Approximately 1 hour per year |
5 | 99.999% | 5.26 minutes | Approximately 5 minutes per year |
6 | 99.9999% | 31.5 seconds | Approximately half a minute per year |