[server] Write speed is fluctuating on all disks

Hi all,

I have some problems with my write speed on all disks.
First of all some informations:
- New to Ubuntu
- Ubuntu 16.04.1 LTS
- ProLiant MicroServer Gen8 (819185-421)
- Intel(R) Celeron(R) CPU G1610T @ 2.30GHz
- 4GB RAM
- 4 disks (1x 300GB, 3x 2TB Western Digital Red, 2 of the 2TB disks are new)
- Fresh installation on new server, it occurs since the begiing

When I copy something to the server via samba from a Win10 Pc the speed is fluctuating from 110 MB/s to 5-6 MB/s. In the example I copied 18 Files with ca 3 GB.
ToServer.JPG
The other way (from server to client) the speed is constant at 110 MB/s.

The same thing happens when I copy something via rsync localy on the server.
Copy from "Disk1" to "Disk2" via rsync -> fluctuating from 5 to 100 MB/s
Copy from "Disk1" to "OS Disk" via rsync -> fluctuating from 5 to 100 MB/s
Copy from "OS Disk" to "Disk 1" via rsync -> fluctuating from 5 to 100 MB/s

When I check iotop during the copy process, I saw that kworker and jbd2 use 99% of I/O eveytime when the transfer rate is going down

My fstab.conf:

/dev/mapper/srv01--vg-root /               ext4    errors=remount-ro 0       1
# /boot was on /dev/sda1 during installation
UUID=219329f1-bb1c-4226-bde5-702b80657af8 /boot           ext2    defaults        0       2
/dev/mapper/srv01--vg-swap_1 none            swap    sw              0       0
/dev/sdb /data/movies                                     ext4  errors=remount-ro 0 1
/dev/sdc /data/data                                       ext4  errors=remount-ro 0 1
/dev/sdd1 /data/backup                                    ext4  errors=remount-ro 0 1

With the old server that I had before the write speed was always around 100 MB/s. Somebody has an idea what could be the cause?

Thank you and when you need additional info let me know

Best
Speed