There’s an older blog post (circa 2013) by WP Engine founder Jason Cohen titled Do You Have 100% Uptime? which discusses the challenges (technical and cost) of what 100% web services / web site uptime really means.
Cohen points to examples of 4 well known WordPress specific hosting companies as well as major services like Gmail, Facebook, and Twitter all experience downtime over the course of a 10 day period.
A quote from the post:
Every “9” you add to uptime (e.g. 99%, 99.9%, 99.99%) is not only an order of magnitude more uptime, it’s often a multiple more complex and expensive. At some point, trying to eliminate a few minutes of downtime now and then means doubling or tripling the cost of the service.
It’s a great read on the challenges and compromises an organization needs to make when chasing the elusive (and impossible) 100% uptime target. Well worth checking out.