> Each "fault-tolerance" criteria added adds some cost. At some point the cost of being resistant to errors exceeds the cost of downtime. Not to forget: those

> Each "fault-tolerance" criteria added adds some cost. At some point the cost of being resistant to errors exceeds the cost of downtime. Not to forget: those costs are not just in money and time, but also in complexity. And added complexity comes with its own downtime risks. It's not that uncommon for systems to go down due to problems with mechanisms or components that would not exist in a simpler, "not fault tolerant" system.