From: arne@vajhoej.dk   
      
   On 11/20/2025 6:09 PM, Lawrence D’Oliveiro wrote:   
   > On Tue, 18 Nov 2025 13:52:44 -0500, Arne Vajhøj wrote:   
   >> But even for the MongoDB k8s case the mainframe contribute to the   
   >> expected uptime due to the low number of active physical boxes.   
   >   
   > No they don’t. They don’t make any contribution to the nines at all;   
   > all that is coming from the Linux stack.   
      
   You need to do the math.   
      
   If we say Pn means uptime for n systems, then:   
      
   Pn = 1 - (1 - P1)*n   
   =>   
   P1 = 1 - (1 - Pn)**(1/n)   
      
   and combined with:   
      
   P1 = MIN(P1hardware, P1os, P1app)   
      
   then we know that:   
      
   P1hardware >= 1 - (1 - Pn)**(1/n)   
      
   Or more specifically then for n=2 we know that   
   P2 = 8 nines means that P1hardware >= 4 nines.   
      
   Arne   
      
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