Quick CRUSH example on how to store 3 replicas, two in rack number 1 and the third one in rack number 2.

Here the CRUSH rule:

rule 3_rep_2_racks {
   ruleset 1
   type replicated
   min_size 2
   max_size 3
   step take default
   step choose firstn 2 type rack
   step chooseleaf firstn 2 type host
   step emit
}

Hope that helps ;-)