10. Virtualization


The new host kernel networking backend, vhost, is a fully supported feature in Red Hat Enterprise Linux 6.1. vhost provides superior throughput and latency over the userspace implementation.


The qcow2 image format now supports caching of metadata. Additionally, support is added for live snapshots using external qcow2 images.

Block I/O latency improvements

ioeventfd is now available, providing faster notification of block I/O.

Kernel SamePage Merging (KSM)

The KVM hypervisor in Red Hat Enterprise Linux 6 features Kernel SamePage Merging (KSM), allowing KVM guests to share identical memory pages. Page sharing reduces memory duplication, allowing a host with similar guest operating systems to run more efficiently.

KSM in Red Hat Enterprise Linux 6.1 is Transparent HugePage aware. KSM has the ability to scan subpages inside hugepages and split them when merging is possible.
Additionally, KSM enablement can now be controlled on a per-VM basis.
PCI device assignment improvements

PCI configuration space access is improved, enabling a broader set of PCI devices to be device-assigned to guest VMs.

KVMClock Improvements

In Red Hat Enterprise Linux 6.1, the Time Stamp Counter (TSC) synchronization can now be automatically detected on guest bootup or when a host CPU is hot-plugged. Additionally, the TSC synchronization frequency is adjusted after a live migration.

QEMU monitor

Additionally, the new drive_del command allows libvirt to safely remove a block device from a guest.

General Updates and Improvements

  • The maximum display resolution on qemu-kvm is now 2560x1600 pixels
  • Red Hat Enterprise Linux 6.1 includes the ability to expose an emulated Intel HDA sound card to all guests. This update enables native sound support for many guests including the 64-bit version of Windows 7
  • QEMU char device flow control is enabled
  • Message Signaled Interrupts (MSI) implemented for the win-virtio-blk driver
  • A new standard interface for selecting/prioritizing the boot devices of the guest
  • Stability improvements for live migration
  • QEMU userspace static tracing
  • Virtual disk online dynamic resize feature
  • Forbid pci hot unplug of critical devices such as gpu, pci bus controller, isa bus controller