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2 Nines is sufficient at scale?

It’s Twitter. A nine is more than enough for their most critical workloads.

One nine allows 10% downtime. That's 36 days per year. I doubt that.

Wouldn’t 1 nine be 9%?

One nine is atleast 90.0%.

Two nines is atleast 99.0%.

Three nines is atleast 99.9%.

and so on.

For a consumer facing surface. Yes.