code: File getValidateBlockFile(int namespaceId, Block b, boolean checkSize)
        throws IOException {
      //Should we check for metadata file too?
      DatanodeBlockInfo blockInfo = this.getDatanodeBlockInfo(namespaceId, b);
      File f = null;
      if (blockInfo != null) {
        if (checkSize) {
          blockInfo.verifyFinalizedSize();
        }
        f = blockInfo.getBlockDataFile().getFile();
        assert f != null;
  
        if(f.exists()) {
          return f;
        }
     
        // if file is not null, but doesn't exist - possibly disk failed
        datanode.checkDiskError();
      }
      
      if (InterDatanodeProtocol.LOG.isDebugEnabled()) {
        InterDatanodeProtocol.LOG.debug("b=" + b + ", f=" + ((f == null) ? "null"
            : f));
      }
      return null;
    }
docs: Find the file corresponding to the block and return it if it exists.