Disk write speed drops to zero in 16.04, but not in 14.04

Hi,

I'm using a Dell R410 2x Xeon E5620, 24GB RAM, 2x2TB 7200rpm, disk controller is a PERC H200, hardware RAID0. Ubuntu server 16.04 fresh install fully updated.

Problem: When writing a constant speed of 100MB/s, write speed drops to 0MB/s once every 20-25 second.

At first I thought that I had defective hard drives, but SMART-status check is OK. Still thought something was wrong with hard drives, since this behavior is not normal.

Double-check disk controller:

/opt/dell/srvadmin/bin/omreport storage vdisk | grep 'Disk Cache Policy'
Disk Cache Policy : Enabled

I decided to test a 10G file with wget (from local server) -> https://i.imgur.com/q05YOn0.gif (gif is like 2x speed)

This gif shows the problem pretty clearly. Same wget with -O /dev/null, rock solid 110MB/s the whole 10G.

Next test with a dd:

dd if=/dev/zero of=test bs=64k count=256k conv=fdatasync

17179869184 bytes (17 GB) copied, 61.1649 s, 281 MB/s

OK, that seems fine. Whats going on?

with iotop during "dd if=/dev/zero of=test bs=64k count=256k conv=fdatasync" -> https://i.imgur.com/Nt1qHuU.gif

Wait what, look at top right corner: "Total DISK WRITE:", that doesn't seem healthy! Jumping around in speed with an "base average" of 30-40MB/s. Should be a steady average of 280MB/s!

wget to RAM, no problem.
wget to disk, problem.

I'm just guessing it's disk-related. It. must. be.

I kind of accidentally re-install the whole system with 14.04 instead of 16.04, and what do you know, everything is working just as it should!

Top right corner: "Total DISK WRITE:"

16.04 https://i.imgur.com/Nt1qHuU.gif (average of 30-40MB/s, peaking to 500+ jumping back and forth).
14.04 https://i.imgur.com/KIWKDGG.gif (rock-solid average 270MB/s)

wget 10G file 110MB/s to disk rock solid, no dips at all.

What is going on? Seems to be a Linux/kernel/driver related problem? I tried re-install 16.04, same problem occurs again, back to 14.04, working just fine.

Help?