[tahoe-dev] Fwd: On the value of "proofs"...

[Shawn Willden]

On Thursday 29 January 2009 02:29:05 am Toby Murray wrote:
> On Thu, 2009-01-29 at 00:02 -0700, Shawn Willden wrote:
> > Of course, you're still left with estimating the probability that
> > Brian goes berserk in the machine room, and I don't know how you do that,
> > but assuming you can come up with a reasonable number, the math can
> > factor it in, correctly.
>
> What about the more realistic and estimable probability that there is a
> fire in the machine room or other such event (flooding etc.) that causes a
> similar failure?

Yep, those, too.  Those sorts of failures are actually easier to get a handle 
on, by looking at the number of similar data centers that have suffered some 
sort of catastrophic failures.  You can even factor in geography-specific 
factors like hurricanes, tornadoes, earthquakes, etc.

All of this is non-trivial and as you accumulate more and more guesses you 
start to get into a GIGO situation.  However, since what you want to find is 
a lower bound on reliability, you can estimate pessimistically so that most 
error goes in the direction of showing lower-than-practical reality.

My paper (lossmodel.pdf at http://tinyurl.com/bbuhal) includes a simplistic 
and contrived example of incorporating multiple sorts of failure modes, some 
group and some individual.  Section 2.5.

	Shawn.