Having only one mirror is scary. If one goes down, its like murphy's law kicks in. So you want at least 3 things to go wrong in order to take down your system,

Having only one mirror is scary. If one goes down, its like murphy's law kicks in. So you want at least 3 things to go wrong in order to take down your system, 2 is not enough. Also have redundancy everywhere if your checker agent stops working for example. You want 2 of everything and at least 3 of those that should never fail.