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Forum Discussion
Xraid 5 Parity and Stripping how much capacity?
I've been trying to get my head round this one. Take a 4 drive nas for instance with four 500gb drives. With xraid5 the volume availanle is 1500gb. That's where I'm stumped and I know it's because of ...
Maybe it will help if we use bits (0 or 1) instead of bytes and blocks.
xor gives you a "1" if the two bits are different, and a "0" if the two bits match.
So imagine that
d1 = 0
d2 = 1
d3 = 1
then p = 0.
Imagine that d3 gets erased/lost
d3 = d1 xor d2 xor P -> 0 xor 1 xor 0 -> 1
if d2 is lost, then
d2 = d1 xor d3 xor P -> 0 xor 1 xor 0 -> 1
if d1 is lost, then
d1 = d2 xor d3 xor P -> 1 xor 1 xor 0 -> 0
What is happening here is a mathematical equation (algebra) is being set up between the blocks. If only one of the 4 blocks is lost, the NAS can solve the equation.
xor gives you a "1" if the two bits are different, and a "0" if the two bits match.
So imagine that
d1 = 0
d2 = 1
d3 = 1
then p = 0.
Imagine that d3 gets erased/lost
d3 = d1 xor d2 xor P -> 0 xor 1 xor 0 -> 1
if d2 is lost, then
d2 = d1 xor d3 xor P -> 0 xor 1 xor 0 -> 1
if d1 is lost, then
d1 = d2 xor d3 xor P -> 1 xor 1 xor 0 -> 0
What is happening here is a mathematical equation (algebra) is being set up between the blocks. If only one of the 4 blocks is lost, the NAS can solve the equation.
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