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;
    for (auto & data_file : data_files)
    {
        if (data_file.marks.size() > num_marks_saved)
            data_file.marks.resize(num_marks_saved);
    }
}
void StorageLog::saveFileSizes(const WriteLock & )