"Gut-feeler 1" • A comment on the math for availability calculations: Certainly not being a math hero, I can still understand that playing with formulas can be much fun. But we ought to take care whether the results generated by these formulas are actually producing value in real world situations.
Misleadig fomulas can be dangerous and ought to be kept in a pen and paper / chalk and blackboard environment. When those formulas can be "googled", there is the risk that some younger and less experienced person working in the role of an IT architect takes them at face value and bases real world decisons on those - or even worse, someone might write a related Wikipedia article and multiplies the damage ...
Igor's initial formula suggests that any four node cluster is hundred times more reliable than a two node cluster, and that any cluster can easily achieve 99,999% availability if you only add enough nodes. Bill's revised formula would suggest an even stronger growth in availability when adding cluster nodes ...
"Engineer 2" has already pointed out that these are moot calculations, as the formulas are just not applicable in those real world environments we are talking about. You would not construct a large building or a bridge using elegant formulas that produce over-optimistic results which are in stark contrast with real world experience. Everybody knows that after the collapse, the architect would go to jail ...
IT architects do also carry responsibility - it might be somewhat limited when desining a webshop for selling cosmetics or toys, when only the shopowner would be disappointed after investing in additional nodes and not getting the expected reliability in return. But for instance, when building 911 emergency communication systems controlling police, ambulance and fire brigade services, lives are at stake and could be lost due to system outages. Here, only the most reliable IT infrastructure is good enough - and creating false expectations by misleading formulas would be fatal.
Misleadig fomulas can be dangerous and ought to be kept in a pen and paper / chalk and blackboard environment. When those formulas can be "googled", there is the risk that some younger and less experienced person working in the role of an IT architect takes them at face value and bases real world decisons on those - or even worse, someone might write a related Wikipedia article and multiplies the damage ...
Igor's initial formula suggests that any four node cluster is hundred times more reliable than a two node cluster, and that any cluster can easily achieve 99,999% availability if you only add enough nodes. Bill's revised formula would suggest an even stronger growth in availability when adding cluster nodes ...
"Engineer 2" has already pointed out that these are moot calculations, as the formulas are just not applicable in those real world environments we are talking about. You would not construct a large building or a bridge using elegant formulas that produce over-optimistic results which are in stark contrast with real world experience. Everybody knows that after the collapse, the architect would go to jail ...
IT architects do also carry responsibility - it might be somewhat limited when desining a webshop for selling cosmetics or toys, when only the shopowner would be disappointed after investing in additional nodes and not getting the expected reliability in return. But for instance, when building 911 emergency communication systems controlling police, ambulance and fire brigade services, lives are at stake and could be lost due to system outages. Here, only the most reliable IT infrastructure is good enough - and creating false expectations by misleading formulas would be fatal.
"Engineer 3": "In Theory, there is no difference between Theory and Practice. In Practice, there is" I don't know where I found the quote above but I like it. Nevertheless, I think one should in fact do both for any system of significant importance:
(1) Use Math, a.k.a."Theory" to calculate the expected availability and adjust as needed to match the required availability. Ignoring Math/Theory and replacing it with only gut feeling and/or trust in vendor statements does not sound right to me. (2) Apply gut feeling; maybe better called experience or " Practice"; Combine the two and you are off to a good start IMHO...
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