Comments on: In Search of Five 9s – Calculating Availability of Complex Systems http://www.edgeblog.net/2007/in-search-of-five-9s/ Notes from the edge Tue, 09 Mar 2010 19:19:17 -0500 http://wordpress.org/?v=abc hourly 1 By: Bill Highleyman http://www.edgeblog.net/2007/in-search-of-five-9s/comment-page-1/#comment-93410 Bill Highleyman Tue, 24 Feb 2009 00:12:57 +0000 http://www.edgeblog.net/2007/in-search-of-five-9s/#comment-93410 Hisham - I'm sorry that I am more than a half-year late in responding to your question. I wasn't aware of this great blog until my researcher found it. Your confusion is in the definition of spares. if we have n nodes and s spares, it takes the failure of (s+1) spares to take the system down. The Equation in question is the number of ways that you can select (s+1) systems out of n systems. If s = n, selecting (s+1) = (n+1) systems is meaningless, so the equation does not apply. In all cases, s < n or else the system will fail if all spares fail, which is not the definition of spares. If you want to continue this discussion, please feel free to contact me at editor@availabilitydigest.com. Thanks for your interest in the Digest. And thank you, Bill, for the plug. Bill Highleyman Managing Editor Availability Digest Hisham -

I’m sorry that I am more than a half-year late in responding to your question. I wasn’t aware of this great blog until my researcher found it.

Your confusion is in the definition of spares. if we have n nodes and s spares, it takes the failure of (s+1) spares to take the system down. The Equation in question is the number of ways that you can select (s+1) systems out of n systems. If s = n, selecting (s+1) = (n+1) systems is meaningless, so the equation does not apply. In all cases, s < n or else the system will fail if all spares fail, which is not the definition of spares.

If you want to continue this discussion, please feel free to contact me at editor@availabilitydigest.com. Thanks for your interest in the Digest. And thank you, Bill, for the plug.

Bill Highleyman
Managing Editor
Availability Digest

]]> By: Hisham Ghanem http://www.edgeblog.net/2007/in-search-of-five-9s/comment-page-1/#comment-67268 Hisham Ghanem Sun, 15 Jun 2008 14:13:02 +0000 http://www.edgeblog.net/2007/in-search-of-five-9s/#comment-67268 Hi, Great article and formulas posed through the site: http://www.availabilitydigest.com./public_articles/ article "Calculating Availability - Redundant Systems". I notived though that the "Multiple Nodes, Multiple Spares" formula has an issue with the (-1)!, when #of nodes = # of spares. Did I miss a point when reading the article? Thanks. Hi,
Great article and formulas posed through the site:
http://www.availabilitydigest.com./public_articles/
article “Calculating Availability – Redundant Systems”.
I notived though that the “Multiple Nodes, Multiple Spares” formula has an issue with the (-1)!, when #of nodes = # of spares.

Did I miss a point when reading the article?

Thanks.

]]> By: Chris Loosley http://www.edgeblog.net/2007/in-search-of-five-9s/comment-page-1/#comment-27583 Chris Loosley Thu, 01 Nov 2007 00:42:57 +0000 http://www.edgeblog.net/2007/in-search-of-five-9s/#comment-27583 <p>Bill,<br /> Nice analysis! </p> <p>I responded to your comment on my blog http://www.webperformancematters.com/performance-forum/post/296647 listing some of my own posts on this topic. My analysis was at a much higher level than yours, more like an introduction to the concepts you are working with in your formulas.</p> <p>--Chris</p> Bill,
Nice analysis!

I responded to your comment on my blog http://www.webperformancematters.com/performance-forum/post/296647 listing some of my own posts on this topic. My analysis was at a much higher level than yours, more like an introduction to the concepts you are working with in your formulas.

–Chris

]]> By: bill http://www.edgeblog.net/2007/in-search-of-five-9s/comment-page-1/#comment-27568 bill Wed, 31 Oct 2007 22:33:16 +0000 http://www.edgeblog.net/2007/in-search-of-five-9s/#comment-27568 Mr. Highleyman, Great tip! For anyone who liked my article, but wants to see the math expressed more eloquently, go immediately to this site and read: http://www.availabilitydigest.com./public_articles/0101/calculating_availability.pdf The basic calculations are the same, but his equations are much better than mine. -Bill Mr. Highleyman,

Great tip! For anyone who liked my article, but wants to see the math expressed more eloquently, go immediately to this site and read: http://www.availabilitydigest.com./public_articles/0101/calculating_availability.pdf

The basic calculations are the same, but his equations are much better than mine.

-Bill

]]> By: Bill Highleyman http://www.edgeblog.net/2007/in-search-of-five-9s/comment-page-1/#comment-27553 Bill Highleyman Wed, 31 Oct 2007 20:24:45 +0000 http://www.edgeblog.net/2007/in-search-of-five-9s/#comment-27553 I really enjoyed your article on five 9s. High availability is a field that I specialize in. You might want to check out the Availability Digest, a free monthly digest of high availability issues, at www.availabilitydigest.com. Go to the Article Archives and look under The Geek Corner. There you will find a series of papers on calculating availabity using effectively the same equations you use, though in a different form. - Bill Highleyman I really enjoyed your article on five 9s. High availability is a field that I specialize in. You might want to check out the Availability Digest, a free monthly digest of high availability issues, at http://www.availabilitydigest.com. Go to the Article Archives and look under The Geek Corner. There you will find a series of papers on calculating availabity using effectively the same equations you use, though in a different form.

- Bill Highleyman

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