When binlog file size exceed max_binlog_size designed by configuration, it will call find_uniq_filename to generate the next binlog file name for next binlog file. In this case, user thread will scan all the files in the binlog directory, which can be found by log_bin_basename. It will cost a lot of time when there are a lot of binlog files exist. and as a result, all DML/DDL will be blocked by this user thread for some time.