View Full Version : KVM Performance

23rd May 2008, 04:38 PM
i'm using F9 with KVM for virtualisation.
I now have some questions.
I have read that there are paravirt devices available in that fedora version.
How can i use this?
Is it possible to use an other network card for the guest with virt-manager?
Standard is a 100MBit Broadcom, but i need a 1Gbit link.

Also a little bit slow is hdd access.
There, the problem seams to be somewhere in the host os
on a physical partition i get 55MB/s on logical only 20MB/s.
Is there a way to get better performance from lvm?


3rd June 2008, 05:53 AM

The disk speed loss is normal for file based file system images i.e. the image is on the hosts file system. The file based image add overheads to disk access as access also has to go via the hosts file system and disk systems.

This is also the case for other virtualization solutions like Xen.

The disk performance can be improved by giving the VM a native raw disk/block device. The logical volume manager is handy for this. What you do is set aside a partition on a disk which is not used directly by the host machine. This give the VM a more direct path to it disk thus improving performance as it is not going via the hosts file system. In the enterprise space this is normally how it is done. The VM have volumes directly connected form a SAN.

IOMMU when/if implemented in CPUs will also improve the performance of IO systems and virtual machines.

Using TUN/TAP devices for network can improve network performance. This is not normally the default for simple qemu-kvm VMs. It does require some setup.