Re: SATA RAID5 speed drop of 100 MB/s

[Tejun Heo]

Jeff Garzik wrote:
> Carlo Wood wrote:
>> The dmesg output of 33480a0ede8dcc7e6483054279008f972bd56fd3 (thus
>> "before") is:
> [...]
>> And the dmesg output of 551c012d7eea3dc5ec063c7ff9c718d39e77634f (thus
>> "after") is:
> [...]
> 
> Your disk configurations are quite radically different between the two
> kernels (see attached diff for key highlights).
> 
> The new behavior of the more recent kernel (551c012d7...) is that it now
> fully drives your hardware :)  The reset problems go away, NCQ is
> enabled, and if you had 3.0Gbps drives (you don't) they would be driven
> at a faster speed.
> 
> Given that some drives might be better tuned for benchmarks in
> non-queued mode, and that a major behavior difference is that your
> drives are now NCQ-enabled, the first thing I would suggest you try is
> disabling NCQ:
>     http://linux-ata.org/faq.html#ncq
> 
> Other indicators are the other changes in the "ahci 0000:00:1f.2:
> flags:" line, which do affect other behaviors, though none so important
> to RAID5 performance as NCQ, I would think.
> 
> Turning on NCQ also potentially affects barrier behavior in RAID, though
> I'm guessing that is not a factor here.

Ah.. right.  That should have enabled NCQ.  Me slow today.  :-)

-- 
tejun
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