Raiding for an Explanation of RAID Page 4
RAID 0/1 & Spanning
Be aware, however, that to use a RAID 0/1 array you must have at least four hard disks (two groups of two striped disks). More disks (e.g., 8 or 16) can of course be used, should the organization desire. Again, it is best to use identical disks, as not doing so will result in a loss of performance or disk capacity.
Spanning is another function available on some RAID controllers. Spanning is a somewhat minor feature, as it does not boost either performance or reliability, but just adds convenience. Basically, two drives, perhaps of different sizes and speeds, can be joined to form one large one. Again, this is a somewhat moot point, as some operating systems (e.g., Win NT) can do this on their own; however, its at least worth mentioning.