{"id":744,"date":"2014-05-23T13:46:18","date_gmt":"2014-05-23T20:46:18","guid":{"rendered":"http:\/\/itiltopia.com\/?p=744"},"modified":"2017-03-02T21:50:30","modified_gmt":"2017-03-03T05:50:30","slug":"availability-vs-reliability","status":"publish","type":"post","link":"http:\/\/itiltopia.com\/?p=744","title":{"rendered":"Availability vs Reliability"},"content":{"rendered":"

\"Timex\"<\/a>IT prides itself on ensuring that our services are highly available. We measure availability to ridiculously high levels of precision. Indeed, we have a magical number that represents availability nirvana. We call it \u201cFive Nines\u201d.<\/span><\/span><\/span><\/p>\n

The term Five Nines indicates that our systems are available for 99.999% of the agreed time. <\/span><\/span><\/span><\/p>\n

Notice that I said \u201cagreed time\u201d. If our users agree to a planned outage of 120 hours for system maintenance every week, meeting that Five Nines of availability becomes much easier to achieve. Of course, generally our users don\u2019t give us the privilege of taking systems down for extended periods on a frequent basis, so we have to eek out a few hours here and there to do our periodic house-keeping.<\/span><\/span><\/span><\/p>\n

To give you an example of how reliable a system that meets Five Nines of availability is, if you were to calculate the downtime for a service that is agreed to be up 24 hours a day, 7 days a week, the total unplanned\u00a0outage time over the course of a year would need to be less than 5 minutes and 15 seconds. That is seriously expensive to do.<\/span><\/span><\/span><\/p>\n

Some organizations don\u2019t allow for even a single moment of downtime because their systems are so heavily relied upon. They go to extreme measures to ensure the highest availability that money can buy. I know of one organization that claims it loses\u00a0a million dollars in revenue for every minute their most critical system goes down. That company\u00a0is American Express, and the system would be their credit card transaction processing service.<\/span><\/span><\/span><\/p>\n

Fortunately, most of us don\u2019t work for companies that have such stringent standards for high availability, but that doesn\u2019t mean we don\u2019t feel the heat when the services do fail.<\/span><\/span><\/span><\/p>\n

ITIL teaches us that there are two aspects we need to measure when addressing the business\u2019 requirements:<\/span><\/span><\/span><\/p>\n