Alex Elsayed wrote: > Ric Wheeler wrote: > >> On 07/17/2009 11:49 AM, H. Peter Anvin wrote: >>> Ric Wheeler wrote: >>>>> >>>>> The bottom line is pretty much this: the cost of changing the encoding >>>>> would appear to outweigh the benefit. I'm not trying to claim the Linux >>>>> RAID-6 implementation is optimal, but it is simple and appears to be >>>>> fast enough that the math isn't the bottleneck. >>>> >>>> Cost? Thank about how to get free grad student hours testing out >>>> things that you might or might not want to leverage on down the road :-) >>>> >>> >>> Cost, yes, of changing an on-disk format. >>> >>> -hpa >>> >> >> Putting RAID6 behind us, we still might be interested in the other > encodings >> that are in: >> >> "A Performance Evaluation and Examination of Open-Source Erasure Coding >> Libraries For Storage" >> >> http://www.cs.utk.edu/~plank/plank/papers/FAST-2009.html >> >> since they give us even more flexibility.... > > Of course, there's also the fact that, using (essentially unchanged) the > current code for Reed-Solomon coding, it's completely doable to have > arbitrary NxM redundancy up to (N + M) < 256 disks (this limit is due to > the > current maximum of 8 for symsize [referred to as 'w' in the below paper] > in > rs_init. If increased to 16, the maximum number of disks would be 65535). > It's also space-optimal for all combinations of N (checksum) and M (data). > > http://www.cs.utk.edu/~plank/plank/papers/CS-96-332.html even describes an > implementation _very_ similar to the current code, right down to using a > table for the logarithm and inverse logarithm calculations. > > Also, (referencing the earlier-posted paper comparing open-source coding > techniques), Cauchy Reed-Solomon codes seem to maintain most of the > benefits > of the current system (including the ability to provide NxM redundancy, > while still retaining the property of being space-optimal), with > significant > performance gains. It also provides an optimization for the RAID6 case, so > once again the common case would get a benefit over less common cases (as > with Mr. Anvin's RAID6 optimization in the current system) > > However, I will have to dispute that the other methods provide more > flexibility - Cauchy Reed-Solomon codes are at best a horizontal move > there, > and the other systems are restricted to (or at very least, far more > effective in) RAID6 systems. Whoops, got N (data) and M (checksum) backwards. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
