RAID-53: RAID by any other name
By Rick Cook
The first thing to understand about RAID-53 is that it has the wrong name.
RAID-53 is the accepted term for a series of RAID-5 arrays (striping with distributed parity) striped across a RAID-3 array (striping with separate parity). A better name for it would probably be "RAID-03," as it involves, in essence, striping (RAID-0) a series of RAID-3 (striping with a separate parity disk) arrays without additional parity. However, RAID-53 is the popular nomenclature.
The major advantage of RAID-53 over RAID-3 is a much higher transaction rate. The data read and write transfer rates are high in a RAID-3 array, but the transaction rate is no higher than that of a single disk. Also, RAID-3 only achieves that rate if the disk spindles in the array are synchronized. (The limiting factor in a RAID-3 array is the separate parity disk.) By striping the data across multiple RAID-3 arrays, the transaction rate is greatly improved, since it is no longer limited by the rate of the individual disks.
RAID-53 offers nearly the level of protection of RAID-10 (striping and mirroring) without the 100 percent overhead of RAID-10. Because it spreads the data over several arrays, it offers protection from failure of an array adapter. It also offers protection against failure of several drives, assuming the drives are part of different arrays.
It requires at least 5 disks to implement a RAID-53 array, and, because it uses byte-level striping like RAID-3, it doesn't use storage space very efficiently. This is its major disadvantage. Also, like RAID-3, RAID-53 requires synchronized drive spindles for best performance. This limits the kinds of drives that can be used to set up a RAID-53 array.
In fact, the need for synchronization has become the biggest drawback for RAID-53. Although spindle synchronization was fairly common on high-end hard drives a few years ago, there are very few drives available today to support it.
For these reasons RAID-53 is not a common configuration. Most enterprises prefer the simplicity of RAID-10 or the lower level of protection and performance offered by RAID-5 or RAID-3.
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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K 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.
23 May 2005
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