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

[Ric Wheeler <rwheeler@redhat.com>]

On 07/17/2009 11:40 AM, H. Peter Anvin wrote:
> 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

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 :-)

ric