code: public Map<File, StorageLocationType> getImages(long txid) {
      Map<File, StorageLocationType> map = new HashMap<File, StorageLocationType>();
      for (Iterator<StorageDirectory> it =
        dirIterator(NameNodeDirType.IMAGE); it.hasNext();) {
        StorageDirectory sd = it.next();
        File fsImage = getStorageFile(sd, NameNodeFile.IMAGE, txid);
        if(sd.getRoot().canRead() && fsImage.exists()) {
          map.put(fsImage, getType(sd));
        }      
      }
      return map;
    }
docs: Return all images for given txid, together with their types
  (local, shared, remote).