The 'other' RAID levels
By Rick Cook
RAID levels 1 through 5 are standardized and their characteristics are well-known. However, you may have heard of several other RAID level designations which are not "official" but which are still used. Knowing the definitions will allow you to choose between the various options you have.
RAID 10 refers to a combination of mirroring (RAID 1) and striping (RAID 0). It combines the high fault protection of disk mirroring (where every bit of data is written to two separate disks) with the speed of disk striping (where a file is split across multiple disks to improve performance). It is popular for applications where high reliability and high performance are needed. However, it is expensive because it uses so many disks (a minimum of four) and its scalability is limited.
RAID 53 combines striping by byte and parity on two disk sets. RAID 53 is fault-tolerant and fast, especially on large files, but it is expensive to implement and takes as much disk space as RAID 3, which has notoriously poor utilization of disk capacity.
RAID 7 is a Storage Computer Corp. trademark. This designates that it's a proprietary RAID architecture designed to maximize performance, especially on writes with multiple drives in the array. It includes an embedded, real-time operating system and a separate control channel manipulating a large central cache. While the drives, motherboards, memory and busses are standard, the RAID 7 system is proprietary. This, combined with its complexity, results in high cost. Because the central cache is so important, the system needs an uninterruptible power supply to prevent losing cache data in the event of a power failure.
RAID 6 is more theoretical than anything else, since apparently no one builds controllers for it. It is basically RAID 5 with a second parity calculation, and with results written across all disks in the array. The result is very high fault tolerance at the expense of slow writes and additional disks for the second parity operation.
Additional resources:
- For a good overview of the various RAID levels, see Advance Computer & Network Corp.'s Web site at: http://www.acnc.com/04_01_00.html.
- For more on RAID 7 architecture, see Intelligent Solutions' white paper on the subject at: http://www.intel-sol.com/products/scc/wp_raid7arch.html.
- For a list of the latest tips on SearchStorage, go to: http://www.searchstorage.com/searchStorage_Tips_Page/0,1800,,00.html.
About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
CORRECTION NOTE: Readers took us to task about Rick Cook's tip on the "other" RAID levels. Rick said that no one makes RAID 6 devices, which, we now find, is not correct.
Reader Laurence James sends the following:
We here at StorageTek have been selling RAID 6 disk subsystems for years. The Iceberg, RVA, SVA 9500 series is a high-end disk subsystem for multi-platform attachment based on RAID 6 disk architecture so it is not theoretical, it exists in large numbers.
Similarly, from reader Dennis Martin:
The recent article on RAID levels was a good overview but there was one error. In the comments on RAID 6, it says nobody builds such a device. StorageTek builds and sells a device that is RAID 6 (second parity, high fault-tolerance, etc.) along with some other enhancements. It is called the StorageTek 9500 SVA.
We appreciate this and the other feedback we got on this tip. We will discuss other aspects of RAID in future tips. If you have comments or suggestions on this and other tips we profile, please send us an e-mail mailto:mhope@techtarget.com.
