protected void validateClusterName() {
    if (clusterName == null) {
      throw new IllegalStateException("HardwareLayout clusterName cannot be null.");
    } else if (clusterName.length() == 0) {
      throw new IllegalStateException("HardwareLayout clusterName cannot be zero length.");
    }
  }
Finds Disk by hostname, port, and mount path.

@param hostname of datanode
@param port of datanode
@param mountPath of disk
@return Disk or null if not found.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HardwareLayout.java