This is what the book says:
"-Write performance will suffer when a RAID group spans more than one FC-AL or SCSI adapter. -Keep all drives assigned to a RAID group on the same adapter whenever posible. -There is an approximate 10% decrease in write performance when the filer attempts to write to a RAID group spanning two adapters. This is due to inherent limitations in the PCI bus."
Bruce Sterling Woodcock wrote:
Unless I misunderstand some of the bus issues involved, the 10% performance hit would only occur on writes, and since writes are grouped and it is unlikely that you are writing to the drives ever second, you'll never notice it.
Reads come off individual disks as needed, so it doesn't matter which controller they are on. (I guess if you needed to read a whole stripe at once there's a potential hit, but I'm not sure that necessary.) The point is, with two volumes you're using both controllers anyway.
Bruce