On Wed, Jun 10, 2015 at 11:02 PM, Ming Lin [off-list ref] wrote:

On Wed, Jun 10, 2015 at 10:39 PM, Markus Stockhausen
[off-list ref] wrote:

quoted

quoted

Von: linux-raid-owner@vger.kernel.org [linux-raid-owner@vger.kernel.org]" im Auftrag von "Roman Mamedov [rm@romanrm.net]
Gesendet: Donnerstag, 11. Juni 2015 02:27
An: Ming Lin
Cc: linux-raid@vger.kernel.org; Neil Brown
Betreff: Re: write performance of HW RAID VS MD RAID

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

quoted

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/

+1 for giving an increased cache size a try.

Will try it.

quoted

From the numbers I anticipate that you are doing sequential
read/write tests. Otherwise I would expect a write penalty for
the HW RAID setup too.

Yes,

[global]
ioengine=libaio
iodepth=64
direct=1
runtime=1800
time_based
group_reporting
numjobs=48
gtod_reduce=0
norandommap
write_iops_log=fs

[job1]
bs=640K
directory=/mnt
size=5G
rw=write

I tested xfs write for RAID6 stripe size 64k with different stripe_cache_size.

stripe_cache_size       throughput(MB/s)
--------------------------       --------------------------
256                            181.7
512                            185.1
768                            178.3
1024                          194.4
2048                           227
4096                           247
8192                           300.3
16834                         312.4
32768                          304

While xfs write for HW RAID6 throughput is 753.16 MB/s