In fact, just this month (January 2010), ACM Queue published an article entitled, "Triple-Parity RAID and Beyond" by Adam Leventhal. Note Figure 1 in the document ("Comparison of RAID-5 vs. RAID-6 Reliability") is actually a NetApp-created chart from a few years back:
(SIDE NOTE: extra credit if you can calculate the probability of data loss with 2TB SATA disks under RAID-5! )
Back to the patent now: So NetApp techies are familiar with "diagonal parity" in RAID-DP (if you're not, read more here: http://media.netapp.com/documents/wp_3298.pdf). With this patent, NetApp takes it one step further with a novel approach to triple parity encoding using anti-diagonal parity sets -- quoting:
"…the anti-diagonals have a slope that is orthogonal to the diagonals, providing a uniform stripe depth and an amount of parity information equal to three disks worth" (minimum required to reconstruct from a triple disk failure).
Also of interest is the technique for triple parity reconstruction (Section F). Again, reference the above patent for details.
