The Myth of the Nines
The number of nines N
In general, the number of nines is not much used by engineers modelling and measuring availability, because it is hard to apply in formulae. More often, the unavailability expressed as a fraction (like 0.00001) or as a downtime per year is quoted. Availability specified as a number of nines is more often seen in marketing documents, presumably because it looks impressive.
The following table tries to elaborate the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously.
| availability % | downtime per year | downtime per week |
| 98% | 7.3 days | 3.3 hours |
| 99% | 3.65 days | 1.7 hours |
| 99.9% | 8.75 hours | 10.1 min |
| 99.99% | 52.5 min | 1.0 min |
| 99.999% | 5.25 min | 6.0 s |
| 99.9999% | 31.5 s | 0.6 s |
This model does not take into account the impact that an outage would have on business if it occurred at a critical moment.
The myth of the nines is the implicit assumption that if the computer is operating 0.99999 of the time, then the user's business is operating 0.99999 of the time. In fact, this is often far from the truth. After an outage, the humans using the computer have to scrabble to catch up, perhaps apologising to customers, calling them back, entering data written down with ink and paper during the outage, and other unfamiliar chores. A computer outage of a minute might cause a business outage of hours.
A further assumption in this model is that ten outages of one minute each have the same effect on the user as one outage of ten minutes. Again, this is not usually true. If a system is experiencing repeated outages, the user is justified in believing that the system cannot be trusted. There must be something wrong with it that nobody can fix. In this case, the user may regard the computer as a liability. The user may measure ten one-minute outages over a period of six months as a downtime of six months, while the computer's manufacturer measures it as a downtime of ten minutes.
Another factor is that computer systems do actually tend to experience outages at the most inconvenient time, when the system is most heavily used.
