[libvirt-users] virtual drive performance

[Gianluca Cecchi]

On Mon, Jun 19, 2017 at 12:38 PM, Dominik Psenner <dpsenner at gmail.com>
wrote:

> When running a virtualized host for example with virtual box we don't see
> such an impact. What does virtual box do differently to improve virtualized
> IO and could that help libvirt/qemu/kvm?
>
>
Hello,
I would jump here to try to put some information
Even in virtualbox, at virtual storage chapter
https://www.virtualbox.org/manual/ch05.html

you get:
"
In general, you should avoid IDE unless it is the only controller supported
by your guest. Whether you use SATA, SCSI or SAS does not make any real
difference. The variety of controllers is only supplied for VirtualBox for
compatibility with existing hardware and other hypervisors.
"

In fact, just tried with a not so new version of virtualbox (4.3.6 I don't
use it very often), if I create a VM with Windows 2012 as OS, by default it
sets the os disk on top of a virtualized sata controller, that should be
more efficient.
Or are you saying that you explicitly configured Virtualbox and selected
IDE as controller type for the guest?

Indeed I verified that in new version of virt-manager, when you configure a
windows 2012 R2 qemu/kvm VM instead, it does set IDE as the controller and
so the performance problems you see. Probably you choose the default
proposed?
In the past I also tried IDE on vSphere and it had the same performance
problems, because it is fully virtualized and unoptimized

You should set SCSI as controller type in my opinion if you have a recent
version of libvirt/qemu/kvm

That said, I don't know what is the level of support for W2016 at time with
virtio and virtio-scsi drivers.
You can download iso and virtual floppy images here:
https://fedoraproject.org/wiki/Windows_Virtio_Drivers

The method could be to add a new disk with the desired controller to the
guest: virtio or virtio-scsi
Then configure it using iso or vfd images
Then shutdown the guest and set also the boot disk with the same virtio or
virtio-scsi controller and try to boot again.
Having installed the drivers it should auto reconfigure (not tried with
w2012 and w2016)
If all goes well shutdown guest again and remove the second disk

Also, you can try to install a new guest and change the controller and
provide the installation process the vfs image file.
Try it on a test system to see if it works and if it gives you the desired
performance.
HIH a little,
Gianluca
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