[simonebrunozzi]

Why too optimistic?

Probably closer to 4 9s.

With this outage of ~2 hours, we are at ~99.97% for this year. (I am not aware of any other downtime during 2021)

Rule of thumb (I strongly prefer minutes/year instead of 9s, to get an immediate sense of how good the availability is):

99.9% : down for 525 minutes / year, or roughly ~10 hours

99.99% : down for 52 minutes / year, or roughly ~1 hour

99.999% : down for 5 minutes / year

[protomikron]

Yes, but it would be nice to read some "official" numbers backed by HN's monitoring (although I'm on HN quite/too often, I would not notice every downtime).