Context, you are looking at hosting providers for you’re enterprise system and the hosting provider says their SLA guarantee is five nines but what does five nines actually mean?
Five nines is often the term used to describe 99.999% uptime of a service from unplanned downtime, now what time period is measured for this uptime guarantee varies on the provider and the type of service purchased, this may be between the hours of 0600 – 1800 each weekday or it could be the full 24hours x 7 days.
As an example 99.999% uptime over a full year (365 days x 24 hours per day) is equal to only having 5 minutes and 15 seconds of unplanned downtime anytime over this period.
Take a look at the below chart which has the most common service uptime levels
|Percentage||Uptime||Downtime||Also known as|
|99.99999||364d 23h 59m 56s||000d 00h 00m 03s||Seven nines|
|99.9999||364d 23h 59m 29s||000d 00h 00m 31s||Six nines|
|99.999||364d 23h 54m 45s||000d 00h 05m 15s||Five nines|
|99.99||364d 23h 07m 27s||000d 00h 52m 33s||Four nines|
|99.95||364d 19h 37m 12s||000d 04h 22m 48s|
|99.9||364d 15h 14m 24s||000d 08h 45m 36s||Three nines|
|99.72603||364d 00h 00m 00s||001d 00h 00m 00s||1 day|
|99.5||363d 04h 12m 00s||001d 19h 48m 00s|
|99||361d 08h 24m 00s||003d 15h 36m 00s||Two nines|
|98||357d 16h 48m 00s||007d 22h 48m 00s|
If you are buying a service that offers you 99.9% uptime or greater then you will need to look into this figure and ask yourself if its achievable from your service provider.
Ask the provider how they measure and report on the uptime figure, the answer you receive will give you a feel if what they state is correct and or feasible. Also ask about penalty clauses if they don’t achieve the figure that they commit to in an SLA, usually the providers that are confident of their infrastructure will give you some sort of recompense should they fail to achieve the service level.
And one last but by no means final thing is that common sense will usually tell you if a service uptime percentage is achievable, for example if a provider offers a 99.999% uptime so they are stating that there will only be 5 minutes of downtime over a year then they must have durability in their solution, ie multiple power sources, multiple muti-path external connectivity, clustered /load balanced firewalls; switches and servers, etc.…
And then there is the physical location itself do they have multiple data centres if so how does this affect uptime if not how do they recover from a localised incident such as flood or fire, this will also give you an indication on how serious they are with the uptime guarantee.
Just my two pence after asking these sorts of questions myself over the years ….