On Wed, 10 Jun 2015 15:27:07 -0700
Ming Lin [off-list ref] wrote:

Hi NeilBrown,

As you may already see, I run a lot of tests with 10 HDDs for the patchset
"simplify block layer based on immutable biovecs"

Here is the summary.
http://minggr.net/pub/20150608/fio_results/summary.log

MD RAID6 read performance is OK.
But write performance is much lower than HW RAID6.

Is it a known issue?

Did you tune the stripe_cache_size for the array? Try 32768.
https://peterkieser.com/2009/11/29/raid-mdraid-stripe_cache_size-vs-write-transfer/

-- 
With respect,
Roman