Re: How to calculate Availability?

From	Richard Heathfield <invalid@see.sig.invalid>
Organization	Go, ion artisan!
Subject	Re: How to calculate Availability?
Newsgroups	comp.programming
References	<da85e527-9f82-4335-864f-86b8f254dc4f@googlegroups.com>
Message-ID	<2Zgpw.273362$td3.59632@fx34.am4> (permalink)
Date	2015-01-01 19:10 +0000

Show all headers | View raw

xeon Mailinglist wrote:

> I have this question, and I am trying to understand how I can calculate
> the availability:
> 
> The company has two front-end web servers that provide some redundancy in
> order to ensure service availability. When a client accesses the service,
> any of the servers can be contacted (this is transparent for the client),
> and if one server is not available, the other will be used.
> 
> a) If the 2 front-end web servers have a reliability of 0.9, what is the
> availability of the on-line store (measured in terms of the probability
> that the service will be up and    running from a client perspective)?
> (Consider that all the other servers do not crash).
> 
> My answer is 0.9^2*100. Is it correct?

Server A has an uptime probability of 0.9. Server B has an uptime 
probability of 0.9.

There are four possibilities.

Server A is up AND Server B is up: probability is p.
Server A is up, but Server B is down: probability is q.
Server A is down, but Server B is up: probability is r.
Server A is down, and server B is down: probability is s.

We know that p + q + r + s = 1.0.

Your answer is p, but the answer you need is actually p + q + r. Or, if you 
prefer, 1.0 - s. They amount to the same answer.

I'm deliberately stopping just short of giving you the numerical answer, in 
the hope that you can take it from here.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

Back to comp.programming | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

How to calculate Availability? xeon Mailinglist <xeonmailinglist@gmail.com> - 2014-12-29 08:18 -0800
  Re: How to calculate Availability? Geoff <geoff@invalid.invalid> - 2014-12-29 09:08 -0800
    Re: How to calculate Availability? xeon Mailinglist <xeonmailinglist@gmail.com> - 2014-12-29 09:29 -0800
      Re: How to calculate Availability? Geoff <geoff@invalid.invalid> - 2014-12-29 10:08 -0800
  Re: How to calculate Availability? Richard Heathfield <invalid@see.sig.invalid> - 2015-01-01 19:10 +0000

csiph-web