Re: [PATCH 1/4] md: Factor out RAID6 algorithms into lib/

H. Peter Anvin
2009-07-17T15:40:34Z

Ric Wheeler wrote:
>>
>> I have seen the papers; I'm not sure it really makes that much
>> difference. One of the things that bugs me about these papers is that he
>> compares to *his* implementation of my optimizations, but not to my
>> code. In real life implementations, on commodity hardware, we're limited
>> by memory and disk performance, not by CPU utilization.
>>
>
> Fair enough - I thought that his coverage of the other open source
> friendly encodings beyond RAID6 was actually quite interesting.
>
> If you have specifics that you found unconvincing in his work, I am
> pretty sure that he would be delighted to hear from you first hand.
> James seemed to me to be very reasonable and very much a pro-Linux
> academic, so I would love to be able to get him and his grad students
> aligned in a useful way for us :-)
>

The main flaw, as I said, is in the phrase "as implemented by the
Jerasure library". He's comparing his own implementations of various
algorithms, not optimized implementations.

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.

-hpa