How to disable buffering for a LVM partition under QEMU KVM

Question

I'm using Windows 10 under QEMU KVM on a Fedora 27 box. Disk for a VM is attached as a raw LVM partition with cache=none.

 <disk type='block' device='disk' >
   <driver name='qemu' type='raw' cache='none'/>
   <source dev='/dev/mapper/fedora-win10'/>
   <target dev='vda' bus='virtio'/>
 </disk>

Still when I load a VM my monitoring system is reporting high buffer usage (on a host). Consider following graph.

VM was running from ~21:30 till 22:50. All this time buffered memory (light blue on the graph, vmstat -s | grep "buffer memory") was quite high (3.5G) and drops to 260M right after VM shutdown.

As far as I understand guest OS will do its own buffering and there is no reason to do buffering on the host.

How can I disable buffering for VM partition entirely?