Of course it's the old adage: Wherever it makes sense. And where does it make the most sense? Well, its whatever...
area you are trying to address.
In a high-end system, you might have some functions of
RAID being performed in an external
array. Or, RAID could be performed in an enterprise, midmarket or even an entry-level array in hardware with hardware offload or assist. You might have RAID adapters in servers that are actually striping data across multiple RAID controllers for high throughput, high performance or high IOPS. In some instances you might have servers using software host-based RAID across external RAID arrays.
The best place for RAID could be in several different places, depending on your particular needs.
Check out the entire RAID FAQ guide.