RAID 5, 6, 7, 0+1, 1+0 and Summary
RAID 5: Block interleaved Distributed Parity
RAID 5 is perhaps the most popular form of RAID on the market (more so for businesses than end users). It utilizes a striped system with the dedicated parity drive replaced by a distributed parity algorithm, which allows parity data to be spread across the drives in the array. This has the effect of removing the bottleneck effect of a single parity drive, allowing increases in system performance, although the performance cost due to the complexity of the parity operations is still present. Fault tolerance is still maintained by keeping parity data from a given block of data on a wholly different drive to that on which the original data is stored.Due to the additional complexity of RAID 5 a fairly high end system and RAID controller are required. A minimum of 3 drives are required, preferably identical. Without a dedicated controller card and a well specified system RAID 5 can severely slow down a system due to the amount of calculations required for distributed parity.
RAID 6: Independent Data Disks with Double Parity
Based on RAID 5 this variant is designed solely to improve data redundancy with parity data written to two separate disks. Where most RAID types can tolerate the loss of a single drive from the array, RAID 6 is able to recover from the loss of two drives failing. Some performance is lost over RAID 5 due to the additional calculations although random read times may be slightly improved as data is spread over an additional disk.A minimum of 4 drives are required for this type of array as is a specialized (read expensive) dedicated controller card. For this reason RAID 6 is infrequently used, the chances of 2 disks failing simultaneously being so slim except in situations where the entire array fails in which case no amount of error correction will help.
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.
In summary
Although new RAID solutions are in the works, particularly as the bandwidth offered by PCI-E is taken advantage of, the average desktop user will be mainly interested in RAID5 or RAID0+1/1+0. Falling prices of hard drives makes a large array possible for a fairly low cost making it inexpensive and easy to enjoy the benefits of a RAID system.Ed's note: Some would suggest that desktop users may be most interested in RAID 0, providing improved performance without having to purchase as many disks. Of course, the biggest issue with this is that, as we've already explained, a single drive failure will result in data loss. With many consumer storage chipsets supporting RAID 1, 1+0 & 0+1, and increasingly more supporting RAID 5 (admittedly usually through software, taking up extra CPU time) and the aforementioned falling hard drive prices, a truly redundant storage system is more appealing than ever. One final point to make is that RAID is no substitute for performing backups. If something disastrous happens to your computer, you could lose your entire RAID array, so make sure you have your most precious data backed up and kept somewhere safe!
