Jon wrote: > I'm going to bed soon, promise. Just one more question about RAID5: In a > 3 disk RAID5, the parity bit can be used to derive the missing > information if one of the disks fails. I think. > > The rather simplistic example in my book has is the following (each > column represents one disk) > > 22 12 34 > 65 68 3 > 13 9 4 > > the parity bits are 34, 68, and 13. > > So, my question is - say, for example, that disk 3 goes down. I am > therefore left with > > 22 12 > 65 68 > 13 9 > > How does the system know when to add and when to subtract to get the > missing values? Like how does it know not to do 22-12 and end up with 10 > instead of 34? > > Or 65 + 68 = 133 instead of 68-65 = 3?
AFAIK Redundancy is based on XOR instead of ADD. It's a mathematical principle that given 3 number b1, b2 and b3, if "b1 XOR == b3", then any one is the XOR of the other two numbers, which means "b1 XOR b3 == b2" and "b2 XOR b3 == b1". That's how RAID5 reconstruct missing data. -- Eric _______________________________________________ clug-talk mailing list [email protected] http://clug.ca/mailman/listinfo/clug-talk_clug.ca Mailing List Guidelines (http://clug.ca/ml_guidelines.php) **Please remove these lines when replying
