On 20/03/17 02:14, Youenn Gestin wrote:

Hello,

I'm encountering a drop of write performance after creating a mirror 
with two whole disk ( sdc and sdd ) and i'm not able to find where it 
come from:

Perf before :
abyss ~ # dd bs=1M count=2024 if=/dev/zero of=/dev/sdd conv=fdatasync
2024+0 records in
2024+0 records out
2122317824 bytes (2,1 GB, 2,0 GiB) copied, 14,1238 s, 150 MB/s
abyss ~ # dd bs=1M count=2024 if=/dev/zero of=/dev/sdc conv=fdatasync
2024+0 records in
2024+0 records out
2122317824 bytes (2,1 GB, 2,0 GiB) copied, 14,7046 s, 144 MB/s

Creating the mirror :
abyss ~ # mdadm --create --verbose /dev/md/md6to --level=1
--raid-devices=2 /dev/sdc /dev/sdd
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
mdadm: size set to 5860391488K
mdadm: automatically enabling write-intent bitmap on large array
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md/md6to started.

Perf after :
abyss ~ # dd bs=1M count=2024 if=/dev/zero of=/dev/md/md6to 
conv=fdatasync
2024+0 records in
2024+0 records out
2122317824 bytes (2,1 GB, 2,0 GiB) copied, 21,1528 s, 100 MB/s 

Two possible thoughts:
1) Did you wait until the two drives had completed the initial sync 
before testing? "cat /proc/mdstat" will show this.
2) As noted, "mdadm: automatically enabling write-intent bitmap on large 
array" try to disable this to see if you get the results you were expecting.

Other than that, I'm not sure, but please reply, and if it isn't so 
simple, then someone else might jump in.

Regards,
Adam

-- 
Adam Goryachev Website Managers www.websitemanagers.com.au