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

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

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