High Availability

Availability is traditionally measured according the percentage of time that the system is available to its end users.

Therefore, 100 percent availability means that the system is available all of the time and there is no downtime. However, achieving 100 percent availability is virtually impossible. Too many technical and human factors are involved for that to be a realistic possibility. Even so, by utilizing technology and creating a suitable operating environment, very high levels of availability are achievable. The highest practical measure of availability is typically expressed as “five nines” or 99.999 percent. The percentage of availability can be calculated using the following formula:

Percentage of availability = ((total elapsed time – sum of downtime)/total elapsed time)

The percentage of system availability equals the total elapsed time minus the sum of the system downtime. This result is then divided by the total elapsed time.

Let’s take a little deeper look at what this means in a practical sense. A year has a total of 8,760 hours (24 hours per day × 365 days per year = 8,760 hours). Therefore, an availability of 8,760 hours over a year would be 100 percent uptime as you can see in the following equation:

100 = ((8,760 – 0)/8,760) × 100

A much more common scenario is for a system to have a regular period of downtime every month. For many organizations this might be as little as eight hours of downtime in every month, or essentially two hours per week. This downtime might be the result of planned system maintenance such as system backup procedures. Two hours of downtime per week or eight hours per month results in 98.9 percent availability, as the following formula illustrates:

98.9 = ((8,760 – (8 × 12)/8,760)) × 100

Many organizations don’t achieve that level of availability. However, when expressed as a measure of nine, 98.9 percent isn’t even two nines, as the base level of availability is lower than 99 percent. While one nine may not seem like a high level of availability, for many organizations one day of downtime per month would be perfectly adequate.

However, many businesses are running critical lines of business and e-commerce applications where one nine of availability is not enough. These organizations require three nines or even five nines of availability. So how much actual downtime is associated with these levels of availability? Table 1-1 gives you an idea of the amount of downtime that is permitted with each increasing level of “nines.”
Number of Nines and Downtime

Number of Nines	Percentage Availability	Downtime per Year	Downtime per Month	Downtime per Week
Two nines	99.0%	3.65 days	7.30 hrs	1.68 hrs
Three nines	99.9%	8.76 hrs	43.8 mins	10.1 mins
Four nines	99.99%	52.6 mins	4.38 mins	1.01 mins
Five nines	99.999%	5.26 mins	26.28 secs	6.06 secs

The above table shows how each increasing nine level of availability requires significant decreases in downtime. While reaching two nines of availability can be accomplished with a total of 1.68 hours per week of downtime, five nines of availability is only slightly more than five minutes of downtime per year.

Five minutes of downtime per year is an impressive and difficult number to achieve. Another important factor to remember is that the costs and operational disciplines increase substantially with each successive level of availability. Achieving these higher levels of availability cannot be accomplished using technology only. Creating a highly available environment requires a combination of several factors.