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

[John Robinson <john.robinson@anonymous.org.uk>]

On 02/04/2012 23:48, Jim Kukunas wrote:
> On Sat, Mar 31, 2012 at 12:38:56PM +0100, John Robinson wrote:
[...]
>> 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: 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?
>
> Out of curiosity, are you running with CONFIG_PREEMPT=y?

No. Here's an excerpt from my .config:

# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_BKL=y
CONFIG_PREEMPT_NOTIFIERS=y

But this is a Xen dom0 kernel, 2.6.18-308.1.1.el5.centos.plusxen. Now, a 
non-Xen kernel (2.6.18-308.1.1.el5) says:
raid5: automatically using best checksumming function: generic_sse
    generic_sse: 11892.000 MB/sec
raid5: using function: generic_sse (11892.000 MB/sec)
raid6: int64x1   2644 MB/s
raid6: int64x2   3238 MB/s
raid6: int64x4   3011 MB/s
raid6: int64x8   2503 MB/s
raid6: sse2x1    5375 MB/s
raid6: sse2x2    5851 MB/s
raid6: sse2x4    9136 MB/s
raid6: using algorithm sse2x4 (9136 MB/s)

Looks like it loses a chunk of performance running as a Xen dom0.

Even still, 11892 MB/s for XOR vs 9136 MB/s for XOR+Q - it still seems 
remarkable that the XOR can't be done several times faster than the Q.

Cheers,

John.