[Linuxppc-users] [EXTERNAL] Poor RAID6 performance (vpermxor8 0 MB/s)

[Anton Blanchard]

Hi,

> My RAID6 array has been suffering quite poor write performance
> (26MB/s), using RAID1 does not have that write performance loss.
> While I do know RAID6 causes a write performance hit, I would expect
> better.
> 
> I have noticed this in my dmesg kernel log:
> 
> [  255.514795] raid6: using algorithm vpermxor8 gen() 0 MB/s
> [  255.514800] raid6: using intx1 recovery algorithm

That's expected if you disable CONFIG_RAID6_PQ_BENCHMARK. With it enabled
you should see vpermxor8 can easily do over 10GB/sec:

[    0.671884] raid6: vpermxor8 gen() 17389 MB/s
[    0.841905] raid6: vpermxor4 gen() 15622 MB/s
[    1.011929] raid6: vpermxor2 gen() 12473 MB/s
[    1.181953] raid6: vpermxor1 gen()  7652 MB/s
[    1.351975] raid6: altivecx8 gen() 13851 MB/s
[    1.521997] raid6: altivecx4 gen() 15624 MB/s
[    1.692022] raid6: altivecx2 gen() 12475 MB/s
[    1.862045] raid6: altivecx1 gen()  7655 MB/s
[    2.032067] raid6: int64x8  gen()  5071 MB/s
[    2.202094] raid6: int64x8  xor()  2008 MB/s
[    2.372111] raid6: int64x4  gen() 10823 MB/s
[    2.542133] raid6: int64x4  xor()  3669 MB/s
[    2.712158] raid6: int64x2  gen()  8896 MB/s
[    2.882182] raid6: int64x2  xor()  3319 MB/s
[    3.052226] raid6: int64x1  gen()  5071 MB/s
[    3.222249] raid6: int64x1  xor()  2146 MB/s
[    3.222316] raid6: using algorithm vpermxor8 gen() 17389 MB/s

The kernel has selected the right algorithm for you, so it doesn't explain
your issues. Having said that, we don't have a VMX version of the recovery
algorithm. Can you double check that your array isn't in degraded state?

To further diagnose this, a couple of things would be useful:

1. Are we CPU bound or IO bound? A chunk of "vmstat 1" might help here

2. If we are CPU bound, where is the time being spent?
"perf record -a sleep 30; perf report" will give us some clues.

Thanks,
Anton

> Is that a bug?
> 
> Thanks
> _______________________________________________
> Linuxppc-users mailing list
> Linuxppc-users at lists.ozlabs.org
> https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.ozlabs.org_listinfo_linuxppc-2Dusers&d=DwICAg&c=jf_iaSHvJObTbx-siA1ZOg&r=azkGdnz7EYeVmhCvUFAXMMwzYZqpcBIElGi1DsA0tKU&m=A_KwMEnXPAHjRPVUQQi2pwHw9tCLKOLqGgvz0Ywuk5E&s=F4Gek9swz1H03YhoNSpsSqLryC0hV5hmQe7RxlXF7JY&e=