[clug-talk] RAID Question

[Jon]

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?

Pointers, thoughts, and links welcome.

Thanks!

J

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