These hardware is exactly the same, no changes in bios, reinstalled from scratch.

The versions in the "old ones" are:

RHEL 5.6
kvm-83-224.el5
libvirt-0.8.2-15.el5_6.1
kmod-kvm-83-224.el5
kvm-tools-83-224.el5
libvirt-0.8.2-15.el5_6.1
Kernel: 2.6.18-238.1.1.el5 #1 SMP Tue Jan 4 13:32:19 EST 2011 x86_64 x86_64 x86_64 GNU/Linux

I the ovirt "new" ones:

RHEL 6.5
qemu-img-rhev-0.12.1.2-2.415.el6_5.14.x86_64
qemu-kvm-rhev-0.12.1.2-2.415.el6_5.14.x86_64
qemu-kvm-rhev-tools-0.12.1.2-2.415.el6_5.14.x86_64
libvirt-0.10.2-29.el6_5.12.x86_64
libvirt-client-0.10.2-29.el6_5.12.x86_64
Kernel: 2.6.32-431.23.3.el6.x86_64 #1 SMP Wed Jul 16 06:12:23 EDT 2014 x86_64 x86_64 x86_64 GNU/Linux

I haven't gone into the details but the command line generated by our old software is like 2 lines of text while in the ovirt nodes for the same type of machine it is 10 :)

X

On Thu, Oct 30, 2014 at 11:07 AM, Gianluca Cecchi <gianluca.cecchi@gmail.com> wrote:
On Thu, Oct 30, 2014 at 10:46 AM, Xavier Naveira <xnaveira@gmail.com> wrote:
I haven't tested the performance differences with or without HT. We were running pure kvm-libvirt hosts on these machines and we're migrating them to oVirt and that's what triggered the problem with the Redhat 5 vms.


So a point is that the hw is the same and that the problem is only related to RH EL 5.x VMs.
Also no change in BIOS settings.
Did you reinstall from scratch?

What about the sw?
previously used version of Qemu/KVM and libvirt?
current version of Qemu/KVM and libvirt with oVirt?

Also, did you compare the qemu-kvm generated command line between plain Qemu/KVM and the one instantiated by oVirt?