code: void deleteAsync(FSDataset.FSVolume volume, File blockFile,
        File metaFile, String blockName, int namespaceId) {
      DataNode.LOG.info("Scheduling block " + blockName + " file " + blockFile
          + " for deletion");
      ReplicaFileDeleteTask deletionTask = new ReplicaFileDeleteTask(volume,
          blockFile, metaFile, blockName, namespaceId);
      execute(volume.getCurrentDir(), deletionTask);
    }
docs: Delete the block file and meta file from the disk asynchronously, adjust
  dfsUsed statistics accordingly.