[Qemu-discuss] virtual disks perfermonca <=2MB/s

[Mimiko]

Hello.

I've setup qemu / kvm on Debian Wheezy to host some Debian Jessie guests.
I create disk like this:

virsh vol-create-as --pool default --name root.vhd --capacity 50G --format vpc

Then create a virtual with this:
virt-install --connect qemu:///system \
--ram 2048 \
--vcpus=4 \
--cpuset=auto \
--pxe \
--os-type linux \
--boot network,hd,menu=on \
--disk vol=default/root.vhd,device=disk,bus=virtio,cache=writethrough \
--network=bridge:br0,model=virtio \
--noautoconsole \
--hvm \
--autostart \
--check-cpu

After setting up the system I've got poor disk performance.
Using

time dd if=/dev/zero of=/tmp/test oflag=direct bs=64k count=16000

I get 1048576000 bytes (1.0 GB) copied, 397.473 s, 2.6 MB/s

I've changed cache to writeback and got 20MB/s with this test. But copying a 
file to this virtual using samba gives me no more of 2 MB/s.

I've tested network connectivity with iperf and got around 750Mbit/s, which I 
think is satisfactory.

I will not want to use raw devices, as it is not self expanded and will use 
whole space regardless of how much real data occupy.

libvirtd (libvirt) 0.9.12.3

Is there a way to increase disk speed to at least 50MB/s ?