Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

{
    if (!use_marks_file)
        return;
    size_t num_marks = num_data_files ? data_files[0].marks.size() : 0;
    if (num_marks_saved == num_marks)
        return;
    for (const auto & data_file : data_files)
    {
        if (data_file.marks.size() != num_marks)
            throw Exception("Wrong number of marks generated from block. Makes no sense.", ErrorCodes::LOGICAL_ERROR);
    }
    size_t start = num_marks_saved;
    auto marks_stream = disk->writeFile(marks_file_path, 4096, WriteMode::Append);
    for (size_t i = start; i != num_marks; ++i)
    {
        for (const auto & data_file : data_files)
        {
            const auto & mark = data_file.marks[i];
            mark.write(*marks_stream);
        }
    }
    marks_stream->next();
    marks_stream->finalize();
    num_marks_saved = num_marks;
}
void StorageLog::removeUnsavedMarks(const WriteLock & )