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5.3. Virtualization

Virtualization in Fedora 12 includes major changes, and new features, that continue to support KVM, Xen, and many other virtual machine platforms.
KVM and QEMU have gained a number of new features in this release. KVM guest memory usage and performance is improved by the addition of KSM and KVM Huge Page Backed Memory. The performance of the qcow2 image format is greatly improved. Support for both SR-IOV and NIC hotplug has been added. Finally, gPXE is now used in place of etherboot for guest PXE booting.
On the libvirt side, APIs have been added for storage management and network interface management. libvirt now also runs QEMU processes unprivileged.
A new library (libguestfs) and an interactive tool (guestfish) is now available for accessing and modifying virtual machine disk images.

5.3.1. Kernel Same Page Merging and Reduced Guest Memory Usage

Kernel SamePage Merging (KSM) allows identical memory pages to be merged by the kernel into a single page shared between one or more processes. This feature is leveraged by KVM to allow multiple, similar, guest virtual machines to have a reduced memory footprint. Because memory is shared, the combined memory usage of the guests is reduced.

5.3.2. KVM Huge Page Backed Memory

Enable KVM guests to use huge page backed memory in order to reduce memory consumption and improve performance by reducing CPU cache pressure. Users of KVM guests using huge page backed memory should experience improved performance with some savings in host memory consumption. The performance benefit is workload dependent.Using huge pages for guest memory does have a downside, however - you can no longer swap nor balloon guest memory.

5.3.3. KVM NIC Hotplug

Network interfaces may now be added to a running KVM guest using libvirt/virt-manager without the need to restart the guest.

5.3.4. KVM qcow2 Performance

The native disk image file format of qemu is qcow2. Qcow2 provides enhanced features over raw images, including: base images, snapshots, compression, and encryption.
Users wishing to protect guest machine data from host crashes commonly disable write caching on the host. Previously, this led to very poor performance for guests in qcow2 images.
The I/O performance of qcow2 disk images has been greatly improved. Users who did not use qcow2 because of the poor performance may consider to switch and take advantage of the additional features the format provides over raw disk images.

5.3.5. KVM Stable Guest ABI

KVM guests are presented with an emulated hardware platform or application binary interface that includes (e.g. a CPU model, APIC, PIT, ACPI tables, IDE/USB/VGA controllers, NICs etc.). When QEMU is updated to a new version, some aspects of this platform may change as new hardware capabilities are added. This is problematic for Windows guests where a guest ABI change may require a installation to be reactivated.
Guest virtual machines will now be presented with the same ABI across QEMU upgrades.

5.3.6. libguestfs Library for Manipulation of Virtual Machines

Added very late in the Fedora 11 development cycle, libguestfs is now an official feature in Fedora 12. libguestfs is a library for accessing and modifying guest disk images. Using Linux kernel and qemu code, libguestfs can access any type of guest filesystem that Linux and QEMU can.
The following tools are provided or augmented by libguestfs:
  • Bindings for OCaml, Perl, Python, Ruby, and Java programming languages.
  • guestfish - Provides an interactive shell for editing virtual machine filesystems and executing commands in the context of the guest.
  • virt-df - Displays free space on virtual machine filesystems
  • virt-inspector - Displays OS version, kernel, drivers, mount points, applications, etc. in a virtual machine.
  • virt-cat - "Cat" out any file from inside a virtual machine.

5.3.7. Network Interface management

Commonly used host network configurations, like bridges, bonds, VLAN's and sensible combinations thereof may now be created using the general-purpose network configuration library, netcf. Enhancements to the libvirt API expose this new functionality to remote managment hosts with libvirtd

5.3.8. Single Root I/O Virtualizaton

Single Root I/O Virtualization is a PCI feature which allows virtual functions (VF) to be created that share the resources of a physical function (PF). The VF devices are assigned to guest virtual machines and appear as physical PCI devices inside the guest. Because the guest OS is effectively driving the hardware directly, the I/O performance is on par with bare metal performance.

5.3.9. gPXE now Default for Guests

QEMU guests now make use of the more modern and currently maintained gpxe rather than the deprecated etherboot tool for PXE booting.

5.3.10. Virt Privileges

Changes have been introduced for QEMU/KVM virtual machines to improve host security in the event of a flaw in the QEMU binary.
  • Permissions on /dev/kvm have been updated to allow unprivileged users to utilize KVM hardware acceleration.
  • QEMU processes spawned by virt-manager on a local desktop install now run as the desktop user.
  • QEMU processes spawned by the privileged libvirtd daemon now run as an unprivileged account, user 'qemu', group 'qemu'.
  • libvirtd will change ownership of any disks assigned to a virtual machine at startup, to user 'qemu', group 'qemu', except for readonly/shared disks.
  • To revert to previous Fedora behaviour of running all QEMU instances as 'root', two config parameters are introduced in /etc/libvirt/qemu.conf. It is not recommended to change these.
For further details refer to:

5.3.11. Virt Storage Management

Fibre Channel N_Port ID Virtualization or NPIV allows the creation of multiple virtual N_Ports on a single physical host bus adapter. The libvirt node device APIs have been extended to create and destroy virtual adapters using NPIV.
The APIs permitting storage discovery and pool creation have been extended to discover and rescan storage on a per-SCSI-host basis. Administrators may now discover, configure, and provision storage for virtual machines without the need for multiple tools.

5.3.12. Other Improvements Libvirt Technology Compatibility Kit
Fedora now includes the libvirt Technology Compatibility Kit (TCK). The TCK is a functional test suite which provides detailed reports on functionality available for each libvirt driver and can be used to quickly identify failures or regressions in the development of Fedora's virtualization features. Virtualization Technology Preview Repo
The Virtualization Preview Repository has been created for people who would like to test the very latest virtualization related packages. This repo is intended primarily as an aid to testing and early experimentation. It is not intended for 'production' deployment. Xen Kernel Support
The kernel package in Fedora 12 supports booting as a guest domU, but will not function as a dom0 until such support is provided upstream. Work is ongoing and hopes are high that support will be included in kernel 2.6.33 and Fedora 13.
The most recent Fedora release with dom0 support is Fedora 8.
Booting a Xen domU guest within a Fedora 12 host requires the KVM based xenner. Xenner runs the guest kernel and a small Xen emulator together as a KVM guest.

KVM requires hardware virtualization features in the host system.

Systems lacking hardware virtualization do not support Xen guests at this time.