The Five Nines: How High Availability Uptime Can Be Measured

You will need to be aware of uptime and availability, no matter if you are a veteran professional or a newbie to networking. These are the most important metrics that customers, vendors, as well as competitors, pay attention to. These statistics are often misunderstood and can lead to costly mistakes that could result in significant financial losses.
The “watermelon effect” is a dangerous way to look at metrics. This is where a service provider meets the specified thresholds (green outside), but provides a lower level of service than the customer desires (red inside).
You may have heard of the Five Nines. This is a measure of how much availability or uptime there is compared to periods of unavailability and downtime. Let’s look at the definitions of each of these metrics and how they are calculated. We’ll also discuss what type of availability and uptime your business might require.
What is the difference between availability and uptime?
Although availability and uptime are often used interchangeably they refer to two distinct concepts. The measure of system reliability, uptime is usually expressed as a percentage time that a computer, server or system has been available or working for use. However, availability refers to the likelihood that a system will function as needed during a mission period. Even more important is availability when your team works remotely.
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Start trainingUptime is a backward looking metric. It measures how reliable the system has been over a given period of time, usually a year. However, it can vary. Although it can be inferred that uptime is an indicator for availability, it is not a guarantee. This is crucial when evaluating service-level agreements (SLAs), with a service provider. While a guarantee of uptime is a confirmation of past performance, it does not guarantee what will happen in future.
The following algorithm is used to calculate operational availability:

These terms are used in the equation:
OT = Operating Time per Calendar Year
ST = Standby Time
TPM = Total preventive maintenance per calendar year
TCM = Total Corrective Maintenance Time per Calendar Year
ALDT = Administrative and logistic delay time while waiting for parts, maintenance personnel or transportation per calendar-year

Operating time (when the system is being used) and standby (when the system is not being used) are equal to the total time that the system is available. The sum of these two numbers is divided into OT and ST, plus preventive & corrective maintenance, administrative and logistic delays. The closer a system is to achieving 100% operational availability, the lower its TPM, TCM and ALDT.
What are the Five Nines?
There are many levels of availability. These are often called The Nines. The most popular is the Five Nines level (99.999%). Here are a few of the most frequently-encountered nine levels.
99.9%
99.99%
99.999%
99.9999%
99.99999%

“Five nines uptime” refers to a system that has been in operation or in standby for 99.999% of the year. This is a total of all but five minutes and 16 seconds over the entire 365 days. This chart shows the various measurements and how much downtime was experienced over the past year.

It is easy to see the importance of those numbers to the right decimal place. A server with a 99 percent uptime rate is still down for almost 88 hours every year. This can quickly add up, as the average downtime cost for a business is $163674 per hour.
Why are the Five Nines Important
This is a key metric when assessing the level of service provider SLA. The lower the number of nines, you are less likely to experience downtime.