Built to the 9's - The 9's of Availability Explained
Have you ever heard the term "five nines" used in reference to availability and wondered just what it meant?
"five nines" refers to the percentage of uptime, or the amount of time the system is available for use. Expressed as a percentage 'five nines" would be 99.999%. This equates to 5 minutes and 15 seconds of downtime in a year. Other "nines" and the respective amount of downtime are listed in the table below.
| Nines | Percentage of Availability | Maximum Yearly Downtime |
| 1 | 90% | 36 days, 12 hours |
| 2 | 99% | 3 days, 15 hours, 36 minutes |
| 3 | 99.9% | 8 hours, 45 minutes, 36 seconds |
| 4 | 99.99% | 52 minutes, 34 seconds |
| 5 | 99.999% | 5 minutes, 15 seconds |
| 6 | 99.9999% | 32 seconds |
| 7 | 99.99999% | 3 seconds |
While the math is made clear above, what often is at issue is the definition of "downtime". Often, one will see downtime qualified as "unplanned downtime". This allows some to claim far higher availability than would otherwise be possible. In many industries there is simply no room for downtime - planned or not.
It is also important to determine exactly what equipment, process, or system is being referenced. Given the highly interdependent nature of modern computing systems, the availability of the entire system is limited to that of the least available sub-system. To insure availability, many components in the system will be duplicated - also know as being made redundant. It is for this reason we see modern servers equipped with dual power supplies. Not only does this mitigate against the failure of the supply itself, it also facilitates the connection of the server to separate power feeds. These feeds could then be connected to separate UPS's, standby generators, and ultimately separate, redundant power suppliers.
Due to the duplication of equipment, higher availability often times means higher costs. However considering the cost of downtime this is justified by the potential loss attributed to downtime.
What is the realistic availability of your computing system? What does an hour of downtime cost you?
Contact DTS today and let us help improve the availability of your infrastructure.