RAID1 – MDADM Mirror – Do not perform parallel reading as expected?

dd if=/dev/md2 bs=1048576 of= /dev/null count=25000
dd if=/dev/md3 bs=1048576 of=/dev/null count=25000
dd if=/dev/md4 bs=1048576 of=/dev/null count=25000
dd if=/dev/md5 bs=1048576 of=/dev/null count=25000
dd if=/dev/md6 bs=1048576 of=/dev/null count=25000< /pre> 
When those people are observing the output of iostat -xm 1 (sample output included below-the mirror is composed of sda, sdb and sdc).
 
 
Device : rrqm/s wrqm/sr/sw/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
sda 100669.00 0.00 10710.00 0.00 435.01 0.00 83.18 33.28 3.11 0.09 100.00
sdb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
md1 0.00 0.00 19872.00 0.00 77.62 0.00 8.00 0.00 0.00 0.00 0.00
md2 0.00 0.00 18272.00 0.00 71.38 0.00 8.00 0.00 0.00 0.00 0.00
md5 0.00 0.00 18272.00 0.00 71.38 0.00 8.00 0.00 0.00 0.00 0.00
md7 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
md6 0.00 0.00 18240.00 0.00 71.25 0.00 8.00 0.00 0.00 0.00 0.00
md4 0.00 0.00 18208.00 0.00 71.12 0.00 8.00 0.00 0.00 0.00 0.00
md3 0.00 0.00 18528.00 0.00 72.38 0.00 8.00 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

We There is a three-way RAID 1 mirror driven by mdadm. I think I have read that mdadm should take multiple simultaneous read requests and distribute them to different drives in the mirror (parallelized reads) to improve read performance, but in When we test and observe the output of iostat -xm 1, it only shows that /dev/sda is being used, even though the I/O of that device is being saturated from 5 different md devices.

Am I misunderstanding something? Does mdadm need to be configured differently? Does our version (CentOS 6.7) not support this? I am not sure why it behaves like this.

Benchmark settings-run the following commands at the same time:

dd if=/dev/md2 bs=1048576 of= /dev/null count=25000
dd if=/dev/md3 bs=1048576 of=/dev/null count=25000
dd if=/dev/md4 bs=1048576 of=/dev/null count=25000
dd if=/dev/md5 bs=1048576 of=/dev/null count=25000
dd if=/dev/md6 bs=1048576 of=/dev/null count=25000< /pre> 
When those people are observing the output of iostat -xm 1 (sample output included below-the mirror is composed of sda, sdb and sdc).
 
 
Device : rrqm/s wrqm/sr/sw/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
sda 100669.00 0.00 10710.00 0.00 435.01 0.00 83.18 33.28 3.11 0.09 100.00
sdb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
md1 0.00 0.0 0 19872.00 0.00 77.62 0.00 8.00 0.00 0.00 0.00 0.00
md2 0.00 0.00 18272.00 0.00 71.38 0.00 8.00 0.00 0.00 0.00 0.00
md5 0.00 0.00 18272.00 0.00 71.38 0.00 8.00 0.00 0.00 0.00 0.00
md7 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
md6 0.00 0.00 18240.00 0.00 71.25 0.00 8.00 0.00 0.00 0.00 0.00
md4 0.00 0.00 18208.00 0.00 71.12 0.00 8.00 0.00 0.00 0.00 0.00
md3 0.00 0.00 18528.00 0.00 72.38 0.00 8.00 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Perform the test again, but change it so that Perform all five reads on one MD device (e.g. /dev/md2), and you should see that they are being distributed.
A single read operation can only read from one drive in the mirror. It will start from the first disk assigned to the mirror, which in this case looks like /dev /sda. Since you have configured 5 MD devices and are performing a single read operation from each device, they all come from /dev/sda.
I recommend not to configure multiple MD devices, only You need to use a single device that spans the entire SSD.
Or, change your test method to force it to perform tasks for several different drives. Take a look at bonnie++, it's very beautiful.