99 Red flags

One of the most widely cited metrics in the IT industry is for availability, expressed in the form of a number of nines: three nines for 99.9% availability (minutes of downtime per year), extending to six nines — 99.9999% — or even, very rarely, seven nines. What this should mean in practice is show in Table 1.

Table 1. The availability metric

The metric is very widely used (more so in the context of IT equipment than facilities) and is commonly, if not routinely, cited in cloud services and colocation contracts. Its use is rarely questioned. Contrast this with the power usage effectiveness (PUE) metric, which has been the subject of a decade of industry-wide debate and is treated with such suspicion that many dismiss it out of hand.