Re: Can we build a proper email cluster? (was: Re: Why is debian.org email so unreliable?)

[Arnt Karlsen <arnt@c2i.net>]

On Sat, 16 Oct 2004 14:00:57 +0200, Marcin wrote in message 
<[🔎] 20041016120057.GA2287@melina11.ds11.agh.edu.pl>:

> On Sat, Oct 16, 2004 at 09:29:32PM +1000, Russell Coker wrote:
> > On Fri, 15 Oct 2004 23:33, Arnt Karlsen <arnt@c2i.net> wrote:
> > > > On Fri, 15 Oct 2004 03:19, Arnt Karlsen <arnt@c2i.net> wrote:
> > > > > > Increasing the number of machines increases the probability
> > > > > > of one machine failing for any given time period.  Also it
> > > > > > makes it more difficult to debug problems as you can't
> > > > > > always be certain of which machine was involved.
> > > > >
> > > > > ..very true, even for aero engines.  The reason the airlines
> > > > > like 2, 3 or even 4 rather than one jet.
> > > >
> > > > You seem to have entirely misunderstood what I wrote.
> > >
> > > ..really?   Compare with your average automobile accident and
> > > see who has the more adequate safety philosophy.
> > 
> > If one machine has a probability of failure of 0.1 over a particular
> > time period then the probability of at least one machine failing if
> > there are two servers in the cluster over that same time period is
> > 1-0.9*0.9 == 0.19.
> 
> But do we really care about whether a "machine" fails? I'd rather say
> that what we want to minimize is the _service_ downtime.
> 
> With one machine, the possibility of the service being unavailable is
> 0.1. With two machines it's equal to the possibility of both machines
> failing at the same time, so it's 0.1*0.1 == 0.01, as long as the
> possibilites are independent (not sure if that's the right translation
> of the term).
> 
> Or am I wrong in the first sentence?
> 
> Otherwise, I'd say that the increase of availability is worth the
> additional debugging effort :-)

..email is a lot like Zeppeliner transportation, even if these services
stop, there is no loss other than propulsion, unlike with common jet
airliners promptly dropping outta the sky to ditch in the drink or
rocks, unless the aircrew manages to do another Gimli glide.

-- 
..med vennlig hilsen = with Kind Regards from Arnt... ;-)
...with a number of polar bear hunters in his ancestry...
  Scenarios always come in sets of three: 
  best case, worst case, and just in case.