I got it to write to the disk by adding this paramter to *.conf: cache=directsync.
The whole config for the disk is now:
virtio0: /dev/disk/by-id/md-uuid-8dc1c44f:0ce6e1f3:f83efd32:7761571d,cache=directsync,size=1953382464K
However, it still gets cached to the RAM and does not flush it by using...