Disk mounted to VM 3x slower

[Marx]

Hi
I've measured disk speed when mounted directly to proxmox:

root@silver:~# dd if=/dev/zero of=/mnt/xx/output.img bs=8k count=256k
262144+0 records in
262144+0 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 13.7445 s, 156 MB/s

Then I've run OMV VM and passed-through whole disk, mounted inside VM, and measured speed again:

root@omv:~# dd if=/dev/zero of=/srv/dev-disk-by-label-seag1/output.img bs=8k count=256k
262144+0 records in
262144+0 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 35.8209 s, 60.0 MB/s

As disk was exclusively mounted in both examples, I was expecting to see similar speed. Maybe with VM it can be a little slower because of VM overhead, but it shouldn't be almost 3x times difference. What can be a cause of such behavior? How can I fix it?

Edit: I've enable write back cache and speed increased

root@omv:~# rm /srv/dev-disk-by-label-seag1/output.img
root@omv:~# dd if=/dev/zero of=/srv/dev-disk-by-label-seag1/output.img bs=8k count=256k
262144+0 records in
262144+0 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 11.959 s, 180 MB/s