code: public File getBlockFile(int namespaceId, Block b) throws IOException {
      File f = validateBlockFile(namespaceId, b);
      if (f == null) {
        if (InterDatanodeProtocol.LOG.isDebugEnabled()) {
          InterDatanodeProtocol.LOG
              .debug("b=" + b + ", volumeMap=" + volumeMap);
        }
        throw new IOException("Block " + b + ", namespace= " + namespaceId
            + " is not valid.");
      }
      return f;
    }
docs: Get File name for a given block.