excuse me, can anyone explain to me what are conceptsof 4 9s, 5 9s, 6 9s, 7 9s

Hmm... let me try to help...

I assume you were reading from the slides.

Typically, when you go to a internet service provider(ISP), or webhost, they will normally give you a percentage reliability, say 99% quality of service. This means that it is 99% of the time up and running. Then you will notice that there are some that provides 99.9%, while some provides 99.999%, and even 99.9999% .

At the end of the day, they may sound and feel similar to any person, but ask a mathematician, he will do the maths as follow:

a year as 365 days, each day has 24hour, each hour has 60 minutes, each minute has 60 seconds. So in a year, you get 31536000. (Get your calculator now and start punching the numbers. You will appreciate it better later!!!)

So base on the fact that we have 31536000 seconds a year, if a service has a Quality of Service of only 99%, it means that we have a downtime (where the service is unavailable) of 315360s per year. Do the maths again, you realise that it translate to approximately 87.6 hours, or about 3.65 days.

Similarly, if the service has a Quality of service of 99.999999%, punch in the calculator, and you get a downtime of only 0.32s per year. That's less that 0.000088888 of a day....

So, in that sense, the more '9' there is, the better the quality of service.

I hope you understand this better. :-P

thank you