High Availability – what do these nines mean?
Posted by decipherinfosys on December 7, 2008
If you have ever worked in an environment where you have had to either interact with the Hosting Providers (Perot Systems, Sungard, DST etc.) at their data centers or have to host your servers yourself, one question that will be asked early on is – what nines of availability do you need? A nine is essentially the number of nines in a percentage that represents the uptime of a system or network or an application or a solution etc.. So, 99.999% of uptime is essentially 5 nines of availability which translates to that that application/solution is up 99.999% in a year. The remaining 0.001% is the downtime which could because of planned maintenance, upgrades or failure related downtime.
However, this is also one of those things which is not well understood by some shops and they un-necessarily go for 5 nines even when they do not have a business need to do so. We have seen clients pay a lot of money to get the 5 nines availability even when just having 99% would have sufficed for them. Sometimes, this happens because of lack of communication between the business side and the IT side – business really wants close to 0% downtime between the business hours of 8 A.M. – 6:00 P.M. but is ok with maintenance outages in the night shift. This should not be interpreted as 5 nines of availability.
In terms of minutes/hours, this is what the table looks like when you compute the availability in terms of nines: Considering a normal year (not taking leap years into account for this calculation): 365 days * 24 hours * 60 minutes = 525,600 minutes per year. So, now if we want to calculate 5 nines availability, it means:
525600 – ((99.999 * 525600)/100) = 5.256 minutes of downtime (includes both planned and unplanned downtime).
So, do you really need 5 nines availability? A majority of the applications don’t. It’s not that it cannot be achieved – it most certainly can but then the cost goes up exponentially so you should evaluate carefully when the business area or the CxO (CEO, COO, CTO, CIO) proposes a 5 nine solution for the application. A 99.9% (3 nines availability) translates to 525.60 minutes or roughly 8.7 hours of downtime in a year which for even the stringent requirements for uptime is acceptable. Even 99.0% translates to roughly 87.6 hours of downtime per year which was what our client finally agreed to when we showed them the calculations for the application upgrades, patches etc. vs the cost that they would incur with the data center as well in terms of planning and resources in order to get the 5 nines of availability.