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RAID-5 or RAID-6 - is RAID-5 really that bad?

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http://serverfault.com – I need to decide myself between RAID5 and RAID6. The servers have a hardware-RAID controller and 6 drives each. The drives are RE3 enterprise western digital 1TB drives. The data sheet says MTTF = 1.2Mio hours, Bit error rate = 1/10^15 On another Server there are even 6 Seagate SAS Drives (172GB each) with MTTF = 1.6Mio hours, Bit error rate = 1/10^16. When doing the Math I get quite comfortable numbers for this setup (about 110 years to data-loss) with the SAS-drives even more. However this uses the manufacturer data. Is this realistic? Here are the formulas (on the last slides, it's i (HowTos)