For those of you that are thinking about using using libvirt/kvm on Linux… here is a discussion on proposed best practices
Tag Archive: kvm
little annoyed that setting a ‘default’ connect string with virt-top and virsh is different:
virsh uses the environment variable VIRSH_DEFAULT_CONNECT_URI
export VIRSH_DEFAULT_CONNECT_URI=’qemu:///system’
virt-top uses the config file .virt-toprc
connect qemu:///system
for KVM, using an actual network bridge is desired for performance over NAT. There is a virtual bridge used by XEN (virbr0) that you won’t need.
To disable it:
$ virsh net-destroy default
$ virsh net-undefine default
$ virsh net-undefine default
# If Redhat based distro:
$ service libvirt-bin stop
# if Debian/Ubuntu based distro:
$ service libvirtd restart
$ ifconfig
A quick test (so take it with a grain of salt):
- Huge pages is slightly faster than not using huge pages (~10% with 4 winxp virtual machines copying 512MB from one memory location to another).
- KSM is slightly slower then not using KSM (~5% with 4 winxp virtual machines copying 512MB from one memory location to another).
So, at first glance it would appear that we can use a loose rule of thumb:
- to consolidate the maximum number of machines, use KSM as it will allow you to over commit the amount of memory on your box.
- Risk: if the memory pages are significantly different, you may start swapping in a very bad way. This is where monitoring comes in
- to give the best performance to a number of machines, use huge memory pages.. does not allow you to over commit
- Risk: if you don’t leave enough memory for the host os, you can crash your machine. This is also where monitoring comes in
This assumes that your host machine has a sound card! 
Add the args line in your /etc/qemu-server/<vm id>.conf file should give you sound:
args: -soundhw
On my Proxmox VE 1.8, I have several virtual audio devices available. I’ve had the best luck with ac97 with Windows XP/Vista and Win7.
red:/etc/qemu-server# kvm -soundhw ‘?’
Valid sound card names (comma separated):
pcspk PC speaker
sb16 Creative Sound Blaster 16
ac97 Intel 82801AA AC97 Audio
es1370 ENSONIQ AudioPCI ES1370
hda Intel HD Audio-soundhw all will enable all of the above
