code: public File getFsImageName(StorageLocationType type, long txid) {
      File lastCandidate = null;
      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()) {
          if (isPreferred(type, sd)) {
            return fsImage;
          }
          lastCandidate = fsImage;
        }    
      }
      return lastCandidate;
    }
docs: Return the name of the image file, preferring
  "type" images. Otherwise, return any image.
  
  @return The name of the image file.