Many storage controllers allow RAID levels to be "nested". That is, elements of a RAID may be either individual disks or RAIDs themselves. Nesting more than two deep is unusual. How nested RAIDs are denoted can get confusing, but the following explanation may help you understand their configurations and benefits.

As there is no basic RAID level numbered larger than 9, nested RAIDs are usually described by combining the numbers indicating the RAID level, sometimes with a "+" in between. For example, RAID 10 which is usually denoted RAID 1+0, consists of several level 1 arrays of physical drives, each array being one of the "drives" of a level 0 array striped over the level 1 arrays. Some examples include:

• RAID 0+1: Striped sets in a mirrored set, with a minimum four disks and an even number of disks. This provides fault tolerance and improved performance, but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror, the data on the RAID system is lost.


• RAID 1+0: Mirrored sets in a striped set, with a minimum four disks and an even number of disks. This also provides fault tolerance and improved performance with increased complexity, but the key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. In a failed disk situation, RAID 1+0 performs better because all the remaining disks continue to be used. The array can sustain multiple drive losses so long as no mirror loses both of its drives.


• RAID 5+0: Stripe across distributed parity RAID systems.


• RAID 5+1: Mirror striped set with distributed parity. (Some manufacturers label this as RAID 53).