@Override
  public boolean isDown() {
    return isMarkedDown || getDataNodeId().getState() == HardwareState.UNAVAILABLE || (getDiskId() != null
        && getDiskId().getState() == HardwareState.UNAVAILABLE);
  }
@return true if the replica is down; false otherwise.
ambry-test-utils/src/main/java/com/github/ambry/clustermap/MockReplicaId.java