Re: RAID-6: help wanted

["H. Peter Anvin" <hpa@zytor.com>]

Jim Paris wrote:
> 
> I think they can both be updated with read-modify-write:
> 
> P' = P + D_n + D_n'
> Q' = Q + g^n * D_n + g^n * D_n'
> 
> However, the multiplications by g^n for computing Q' could be killer
> on your CPU, so it's a tradeoff.  Since we're updating e.g. 128k at
> once for a single value of n, it's possible that it could be done 
> in such a way that it's not too intensive (or cache-thrashing).
> Or perhaps you have so many disks that it really is worth the time
> to do the computation rather than read from all of them.  
> 

It's not a matter of cache-trashing, it's a matter of the fact that very 
few CPUs have any form of parallel table lookup.  It could be done with 
dynamic code generation, but that's a whole ball of wax on its own.

	-hpa