From the manual, I found that For RAID 6 logical drives, choose a Codec Scheme from the dropdown menu.P+Q and Q+Q are available.
I think the safety should be similar since both P+Q and Q+Q provide 2 HDDs failure tolerance.
I wonder if there will be performance difference (higher write spead) with Q+Q? since the raid controller only need to calculate one (Q) instead of two (P+Q) parity. Am I correct?