In the advanced administration and troubleshooting guide netapp indeed points out that a raidgroup should not have less than three data disks. Otherwise you will have a
It used to be (afaicr) that this was due to the way the writes were queued then flushed. Doing a burst across 3 drives was a performance optimization. I imagine that if you have 2 disks then you have unbalanced writes (eg: 1 write to disk A then 2 writes to disk B). It has been a while since I heard the detailed explanation of why 3 disks, so I may be mis-recollecting :)
all "critcal" data must be on mirrored filesystems, not on raid
Hmm. I am pretty sure that mirroring falls under one of the defined RAID levels :)
http://www.acnc.com/04_01_00.html
I suppose they mean "not parity protected" rather than raid.
On filers, if you set the raidsize to 2, you effectively get a mirrored volume.
Interesting. I had not really thought of it that way, but I guess you sort of do, in a twisted way. But the reality is that you are still relying on parity protection in this scenario, even if you can survive a 1 disk failure. So you are not doing real mirroring (eg: duplication of writes & reads). The cost and performance overhead of doing this (2 drive raidgroup) solution would make me want to further investigate this mandate.
Depending on the reason mirroring is being mandated, the features in the filer can be used to counter most, if not all of these reasons.
alexei