Re: [Qemu-discuss] Nested Qemu/KVM Performance Degradation

[Narcis Garcia]

I've not tried nested VMs, but in this situation I would make speed
tests inside each layer (disk i/o, RAM i/o, CPU) to get more information.


El 07/01/16 a les 17:21, Jan Teske ha escrit:
> Hello,
> 
> I am trying to run a Windows guest in a nested VM configuration using
> Qemu/KVM.
> 
> The setup is as follows:
> 
> - host:
>     - CPU: Intel(R) Xeon(R) CPU E5-2420 v2 @ 2.20GHz (6 cores)
>     - RAM: 16 GB
>     - OS: Ubuntu Server 14.04, 64bit
> - outer VM (running in normal virtualization on the host):
>     - Qemu/KVM running Ubuntu Server 14.04, 64bit
>     - cmdline:
>         $ qemu-system-x86_64 \
>                 -m 8192 \
>                 -cpu host \
>                 -drive file=/dev/mapper/vmray-nested,cache=none,if=virtio \
>                 -vnc :0 \
>                 -enable-kvm
>     - the virtual disk is an LVM volume, 200 GB in size
> - inner VM (running in nested virtualization inside the outer VM):
>     - Qemu/KVM running Windows 7 Pro, 64bit
>     - cmdline:
>         $ qemu-system-x86_64 \
>                 -m 2048 \
>                 -cpu host \
>                 -drive file=win7.img,cache=none \
>                 -vnc :0 \
>                 -enable-kvm
>     - for the virtual disk I tried both a qcow2 and a raw image, 50 GB
> in size
> 
> Both VMs run without producing any visible errors. However, I experience
> an extreme performance degradation in the inner VM. Installing Windows
> inside the inner VM takes roughly 30 times longer than performing the
> installation under normal (i.e., non-nested) virtualization.
> 
> This seems to be a problem of Qemu/KVM, not of the underlying hardware.
> I tested the same setup using VMWare Player instead of Qemu/KVM. In this
> configuration, the inner VM is not significantly slower than the outer
> VM. Finally, I also tried running VMWare Player as the outer hypervisor
> and Qemu/KVM as the inner hypervisor, which also provided me with
> acceptable speed. Hence, it seems to me that the problem lies either in
> Qemu/KVM's support for nested virtualization or in the hypervisor
> configuration I chose.
> 
> Can anyone help me spot the problem? And has anyone successfully run
> Qemu/KVM in a nested configuration before?
> 
> Best regards,
> Jan
> 
>