If you want to run a multi-petabyte storage system then you don't want to do it with Raid 5 or Raid 6 ; with modern disks' ~3% per year failure rate , that's 300 a year when you have 10000 disks and the odds start to get pretty good (relatively speaking) that you'll face permanent data loss at some point when you lose a third disk from an array while two are rebuilding. And of course monitoring and replacing disks in lots of small arrays is manpower-intensive, which to investors translates as "expensive." You probably don't want to go with triplication , either; disks are cheap, but not so cheap that you want to triple your hardware costs unnecessarily. While storing multiple copies of frequently used data is good, all your data probably isn't "frequently used." What is the solution? As it turns out, Raid is actually a special case of Reed-Solomon encoding , which lets you specify any degree of redundancy you want. You can be safer th...