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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