code: static long getGenerationStampFromSeperateChecksumFile(String[] listdir, String blockName) {
      for (int j = 0; j < listdir.length; j++) {
        String path = listdir[j];
        if (!path.startsWith(blockName)) {
          continue;
        }
        String[] vals = StringUtils.split(path, '_');
        if (vals.length != 3) {     // blk, blkid, genstamp.meta
          continue;
        }
        String[] str = StringUtils.split(vals[2], '.');
        if (str.length != 2) {
          continue;
        }
        return Long.parseLong(str[0]);
      }
      DataNode.LOG.warn("Block " + blockName +
                        " does not have a metafile!");
      return Block.GRANDFATHER_GENERATION_STAMP;
    }
docs: Find the metadata file for the specified block file.
  Return the generation stamp from the name of the metafile.