I'm running a VM using libvirt+KVM and I have a disk performance issue.

The host is the following:

4 cores Intel Xeon 5140@2.33 GHz, 16 GB of RAM, SATA HDD, OS Debian Wheezy, libvirt 0.9.12-11, QEMU-KVM 1.1.2+dfsg-2.

The guest:

1 CPU, 2 GB RAM running Debian 7.0, image in compressed qcow2 format.

When I try do run "dd if=/dev/zero of=io.test bs=32768k count=40" I get around 500 MB/s on bare metal, while only around 30 MB/s inside the VM.

I'm trying to get something more out of the virtualization layer, I hope that there's room for improvement.

I'm using virtio, I aready set cache='none' and io='native' in the domain definition. Both host and guest are using deadline as I/O scheduler. The VM uses an ext4 filesystem, while the image is saved on an ext3 disk. I mounted the host and guest filesystems specifying nodiratime and noatime options. Even if I convert the image to raw format nothing changes.

I didn't mess with iotune nor blockio.

Is there something that I overlooked or any other suggestion?

Thanks in advance for your help.

Matteo

--
A refund for defective software might be nice, except it would bankrupt the entire software industry in the first year.

Andrew S. Tanenbaum, Computer Networks, 2003, Introduction, page 14

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