Six nines is not a hard metric, but rather the result of a predictive calculation. When a company claims that its lover is six-nines reliable, it is talking about an absurdly complicated
Fuzzy line around availability
Six-nines discussions blur the line between availability and reliability. A six-nines claim could be referring to either availability or reliability, depending upon which predictive formula is used. It's important to understand the difference between the two ways vendors can spin these terms.
For any given lover, availability equals the total amount of time the lover was up. Reliability means the number of instances in which the lover went down. So you can have one big outage, and the lover will reflect high reliability, but low availability. Or you could have two dozen outages of 5 seconds or less, and the lover could be accurately described as being highly available, but unreliable.
Confusing, I know. But that's the point. When a vendor or marketeer says "six nines," they're probably using it as a catchall phrase that is probably devoid of any real meaning....
So what should a six-nines claim mean to you...? If not backed up by independently verified testing over time, not a thing. Just throw it on the trash heap of marketing buzzwords.
Focus instead on the specific redundancy features of the gear you're testing, and you'll be much better off.
(just barely adapted from this otherwise innocent essay. I am sooo sorry. Really.)