RAID 7 and RAID 0+1
RAID 7: Asynchronous, cached striping with dedicated parity
Unlike the variants listed above RAID 7 is not an industry standard, instead it is the trademarked solution of a single company, Storage Computer Corporation, who use it to describe their proprietary RAID design.
RAID 7 is based on RAID 3 and 4 but with enhancements to solve some of the issues surrounding their performance, the largest enhancement is the inclusion of a large amount of cache arranged on multiple levels and a special dedicated processor to manage the array asynchronously in real time. The additional hardware support allows the array to support many simultaneous processes in turn boosting both read and write performance whilst maintaining the integrity of the data and fault tolerance. Simply put, the additional hardware removes much of the burden placed onto the array by the parity calculations and parity drive.
However, despite the increase in performance RAID 7 remains a very expensive solution, produced by only one company and is vulnerable to power cuts which erase cached information.
RAID 0+1: Mirrored Stripes and RAID 1+0: Striped Mirrors
These derivatives combine the performance gains from a striped array with the redundancy of a mirrored array without the need for parity calculations; however the overheads involved for redundancy are still high.
RAID 0+1 has a mirrored configuration of a striped array; RAID 1+0 is a stripe across a mirrored array. Both require a minimum of four drives although falling prices of large IDE drives makes this less of an obstacle in the market of today as compared with a couple of years ago.
RAID 1+0 has greater fault tolerance and a better rebuild time post failure than RAID 0+1, although both types suffer from the loss of half of the available capacity for redundancy. Most often used where speed and reliability are paramount and the loss of efficiency in storage space due to mirroring are worth the gains in reliability and speed.