Availability vs. Capacity

Continuing the previous posts about  "Battle between "Gut-feeling" and Engineering." ....

Engineer 2: Igor - if you would like to find further extensions of your equation, you might check out Volume 1 of "Breaking the Availability Barrier," which I co-authored and which is available on Amazon. Also, check out several papers I published in the Availability Digest in the Geek Corner (http://www.availabilitydigest.com/articles.htm). A subscription is free.

Igor Trubin: Yes, I have already looked briefly at your book and referenced it in my blog, Very good book and I plan to read that all.

UPDATE: After reading one of suggested above papers ( "Calculating Availability - Redundant Systems" Some useful rules come out of the derivation of the availability equation.) I was able to show to my client that the following more general cluster (system) availability formula proves that for three-node cluster (n=3) the one spare node (s=1) configuration could provide approximately the same cluster availability as in the case of two spare nodes (s=2) but Capacity usage could be a critical factor as seen below. 
 

 

That could be a way to save money by allocating less capacity with the same number but more reliable nodes.