RAID5 XOR speed vs RAID6 Q speed (was Re: AVX RAID5 xor checksumming)

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On 29/03/2012 22:44, Jim Kukunas wrote:
Based on xor_speed, the AVX implementation appears to be ~32% faster than the
SSE implementation on my i7 2600:

	generic_sse: 15088.000 MB/sec
	avx: 19936.000 MB/sec

I just noticed in my logs the other day (recent el5 kernel on a Core 2):

raid5: automatically using best checksumming function: generic_sse
   generic_sse:  7805.000 MB/sec
raid5: using function: generic_sse (7805.000 MB/sec)
raid6: int64x1   2635 MB/s
raid6: int64x2   3208 MB/s
raid6: int64x4   3020 MB/s
raid6: int64x8   2519 MB/s
raid6: sse2x1    5099 MB/s
raid6: sse2x2    5742 MB/s
raid6: sse2x4    8237 MB/s
raid6: using algorithm sse2x4 (8237 MB/s)

I was just wondering how it's possible to do the RAID6 Q calculation faster than the RAID5 XOR calculation - or am I reading this log excerpt wrongly?

It's probably academic, since the machine this is running on only has a maximum of about 4500 MB/s of memory throughput, and a lot of that would be consumed sending data to disc in amongst the calculations being done.



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