private List<ServerName> getServersFromRack(String rack) {
    return this.rackToRegionServerMap.get(rack);
  }
Place secondary and tertiary nodes in a multi rack case. If there are only two racks, then we
try the place the secondary and tertiary on different rack than primary. But if the other rack
has only one region server, then we place primary and tertiary on one rack and secondary on
another. The aim is two distribute the three favored nodes on >= 2 racks. TODO: see how we can
use generateMissingFavoredNodeMultiRack API here
@param primaryRS   The primary favored node.
@param primaryRack The rack of the primary favored node.
@return Array containing secondary and tertiary favored nodes.
@throws IOException Signals that an I/O exception has occurred.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/favored/FavoredNodeAssignmentHelper.java