A simple script to demonstrate how Raid 5 uses parity to recover data.

raid5

#!/usr/bin/env ruby

$drives   = {
  :a      => 'Example 1.',
  :b      => 'abcdefghijklmnopqrstuvwxyz',
  :c      => '0123456789',
  :d      => '3.14159265',
  :parity => ''
}

def generate_product(letter = :parity)
  drives   = $drives.select{ |l| l != letter }
  capacity = drives.values.map{ |data| data.length }.max
  product  = ''
  
  capacity.times do |position|
    byte = drives.inject(0) do |sum, drive|
      sum ^ drive[1].ljust(capacity, ' ')[position].ord
    end
    product += byte.chr
  end
  
  product.strip
end

def print_drives
  $drives.each do |letter, data|
    if letter != :parity
      printf "%-7s: '%s'\n", letter, data
    else
      printf "parity : "
      p data
      puts
    end
  end
end

puts "\nGenerate parity"
$drives[:parity] = generate_product
print_drives

print "Enter a drive to erase (letter): "
drive = gets.strip.to_sym

puts "\nErase drive '#{drive}'"
$drives[drive] = ''
print_drives

puts "Recover drive '#{drive}'"
$drives[drive] = generate_product drive
print_drives