code: private OMMapBufferEntry mapNew(final OFileMMap file, final long beginOffset) throws IOException {
      return new OMMapBufferEntry(file, file.map(beginOffset, file.getFileSize() - (int) beginOffset), beginOffset,
          file.getFileSize() - (int) beginOffset);
    }
docs: This method maps new part of file if not all file content is mapped.
  
  @param file
  that will be mapped.
  @param beginOffset
  position in file from what mapping should be applied.
  @return mapped entry.
  @throws IOException
  is thrown if mapping is unsuccessfully.