Beyond basic RAID - Storage Technology Magazine
Most people know their RAID 0, 1 and 5, but beyond that, things can get pretty hairy, pretty fast. Below are examples of some of the more exotic RAID levels you may come across.

  WHAT PROS CONS
RAID 6 Like RAID 5 (block level striping and distributed parity), but with dual parity for each data block Improved fault tolerance over RAID 5 - can handle the failure of any two drives in the array Slower writes than RAID 5
RAID 30 Striping (RAID 0) across RAID 3 (byte-level striping

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with dedicated parity disk) sub-arrays
RAID 0-like speeds, with increased fault tolerance Complex, expensive to implement
RAID 51 Mirroring combined with block striping and distributed parity Maximum fault tolerance and availability through mirroring and distributed parity Low storage efficiency

This was first published in October 2002