code: private void addToInvalidates(Block b, boolean ackRequired) {
      StringBuilder sb = new StringBuilder();
      for (Iterator<DatanodeDescriptor> it =
        blocksMap.nodeIterator(b); it.hasNext();) {
        DatanodeDescriptor node = it.next();
        addToInvalidatesNoLog(b, node, ackRequired);
        sb.append(node.getName());
        sb.append(' ');
      }
      if (isInitialized && !isInSafeMode()) {
        // do not log in startup phase
        NameNode.stateChangeLog.info("BLOCK* NameSystem.addToInvalidates: "
                + b.getBlockName() + " is added to invalidSet of " + sb);
      }
    }
docs: Adds block to list of blocks which will be invalidated on
  all its datanodes.