Andrew, many thanks for this quick and detailed reply.
In fact i should have enought space but it makes me crazy to loose 6 disks, this is really too much security for SMB enterprise. I prefer having more space.
You 're totally right about silent bit rot, i have totally forgot such concern, then raid dp without spare seems a better choice.
On dot 7-mode there were a "raid timeout option" that shutdowns the controller after an amount of time (24 hours by default)... i had it at a customer 🙂
I do not want such a thing on the system, for sure.
We are using ADP to avoid "wasting" system disks.
edit : i just read an excelent paper on the new ADP that cdot 9 uses : root-data-data (instead of 'old' ADP root-data). It seems good as i can use active/active controler AND have 2 parity and one spare for all the 20 disks !
I do not know if this can be used on a fas 2552 (it is writen 'not on entry fas') and if mixed hdd/ssd could be used.