@Override
  public KnownSizeRecordWriter writerForEntryHeader(@NotNull K key) throws IOException {
    KnownSizeRecordWriter keyWriter = keyDescriptor.writerFor(key);
    if (keyWriter.recordSize() != keyRecordSize) {
      throw new AssertionError("fixedRecordSize(=" + keyRecordSize + ") != keyWriter.recordSize(=" + keyWriter.recordSize() + ")");
    }
    return new NonNullValueEntryWriter(keyWriter, /* valueWriter: */ KnownSizeRecordWriter.NOTHING);
  }
@Override
  public KnownSizeRecordWriter writerForEntryHeader(@NotNull K key)