Re: raid6 and parity calculations

[Andre Noll <maan@systemlinux.org>]

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On Wed, Sep 15, 15:55, Michael Sallaway wrote:
> As a sort-of follow up question, would anyone know if the data size of
> a Q calculation affects the result at all? eg. if I do a 64kb Q
> calculation on 10 drives of data, would that be the same as doing 16x
> 4kb Q calculations on sequential blocks of the same data, then
> concatenating it together? (I can't remember what that operation
> property is called....?)

Yes, the result would be the same. In fact, byte n of Q depends only
on byte n of the 10/16 data drives.

> I've been reading the maths of RAID6 PDF
> (http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf), but I'm a
> bit too rusty to understand Galois fields, and if the data size
> matters. I presume the data ordering is also critical for a Q
> calculation, correct? (eg. drives have to be d0 -> d10 in order, not
> just random).

Right, order matters.

> And, in contrast, for the P calculations, data size and input order
> makes no difference, correct? (since it's just a simple bitwise XOR of
> all the inputs).

Also correct.

Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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