> It’s 1/2 + 1 isn’t it? > That would mean in 3 servers you need 2.5 aka 3 machines to commit a change. Then 4/5, 5/7, 6/9, 7/11. No, the requirement isn't 1/

> It’s 1/2 + 1 isn’t it? > That would mean in 3 servers you need 2.5 aka 3 machines to commit a change. Then 4/5, 5/7, 6/9, 7/11. No, the requirement isn't 1/2 + 1. Any strict majority of the cluster is enough to elect a leader. So you need 2/3, or 3/4, or 3/5, and so on. > Not to mention the time during which 1/n machines are down due to an upgrade gets longer and longer the more machines you have, increasing the chances of double fault. Generally, this is not the case. If individual machine failures are random and equally probable, and if each machine is down on average less than 50% of the time, then adding more machines makes things better, not worse. (This is a basic property of the binomial distribution.) Of course, if you have a single point of failure somewhere -- e.g. a network switch -- this assumption can be violated, but that's true regardless of how many machines you have.