long getUnallocatedRawCapacityInBytes(Disk disk) {
    return disk.getRawCapacityInBytes() - getAllocatedRawCapacityInBytes(disk);
  }
How many data nodes to put in a random sample for partition allocation. 2 node samples provided the best balance
of speed and allocation quality in testing. Larger samples (ie 3 nodes) took longer to generate but did not
improve the quality of the allocated partitions.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/StaticClusterManager.java