[tahoe-dev] Monte Carlo simulator for emergent reliability

zooko zooko at zooko.com
Fri Sep 7 23:01:09 UTC 2007

Suppose you encode a file with K-out-of-N erasure coding, and you put  
each of the N shares on a separate server, and your servers have an  
average reliability of P.

What's the reliability of the file?

Here is a simple Monte Carlo simulator to answer that question:

#!/usr/bin/env python

import random

def r(k, n, p, tries=2**20):
     @param k: required shares
     @param n: total shares
     @param p: server reliability
     @param tries: number of computrons to sacrifice to the Gods of  
Monte Carlo in order to get an accurate answer
     succs = 0
     i = 0
     while i < tries:
         ups = 0
         for server in xrange(n):
             if random.random() < p:
                 ups += 1

         if ups >= k:
             succs += 1

         i += 1

     return float(succs) / tries

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