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Red Hat Enterprise Linux 9 Essentials Book now available.

Purchase a copy of Red Hat Enterprise Linux 9 (RHEL 9) Essentials

Red Hat Enterprise Linux 9 Essentials Print and eBook (PDF) editions contain 34 chapters and 298 pages

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15. Virtualization

15.1. Kernel-based Virtual Machine

Red Hat Enterprise Linux 6 includes full support for the Kernel-based Virtual Machine (KVM) hypervisor on the AMD64 and Intel 64 architectures. KVM is integrated into the Linux kernel, providing a virtualization platform that takes advantage of the stability, features, and hardware support inherent in Red Hat Enterprise Linux.

15.1.1. Memory enhancements

  • Transparent Hugepages increase the memory page size from 4 kilobytes to 2 megabytes. Transparent Hugepages provide significant performance advantages on systems with highly contended resources and large memory workloads. Additionally, Red Hat Enterprise Linux 6 provides support for utilizing Transparent Hugepages with KSM.
  • Extended Page Table age bits enables a host to make smarter choices for swapping memory under memory pressure and allows swapping of Transparent Hugepages by breaking the extended pages into smaller pages.

15.1.2. Virtualized CPU features

  • Red Hat Enterprise Linux 6 supports up to 64 virtualized CPUs for a single virtualized guest.
  • CPU extensions present on the host processor can now be utilized by virtualized guests. Support for these instruction sets allow virtualized guests to take advantage of modern processor instruction sets and hardware features.
  • The new x2apic virtual Advanced Programmable Interrupt Controller (APIC) improves virtualized x86_64 guest performance by allowing direct guest APIC access and removing the overhead of emulated access.
  • New user space notifiers allow the caching of CPU registers, avoiding the computationally expensive actions of preserving register states of unused components during context switches.
  • Read copy update (RCU) kernel locking is now used for enhanced symmetric multiprocessing support. RCU kernel locking provides greater performance for networking functions and multi-processing systems.

15.1.3. Storage

  • Indirect ring entries (spin locks) for the para-virtualized (virtio) driver improve block I/O performance and allows more concurrent I/O operations.
  • Virtualized storage devices can now be added and removed (hot plugged) from guests during runtime.
  • Support for block alignment storage topology awareness. Underlying storage hardware features and physical storage sector sizes (for example, 4KB sectors) are presented to guests. This feature requires compatible storage device information and commands. Guest topology awareness allows virtualized guests to optimize file system layouts and improve performance of applications using I/O optimizations.
  • Performance enhancements for the qcow2 virtualized image format.

15.1.4. Networking

  • MSI-X support which increases the number of interrupts available to network devices. MSI-X support increases the performance of compatible hardware.
  • Virtualized network devices can now be hot plugged and hot removed from running guests. Network boot using gpxe for more advanced PXE network booting.

15.1.5. Kernel SamePage Merging

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.

15.1.6. Device Assignment

Assignment devices can now be hot plugged and hot removed from running guests.

15.1.7. virtio-serial

The para-virtualized serial device (virtio-serial) provides a simple communication interface between the host's user space and the guest's user space. virtio-serial can be used for communication where networking is not be available or unusable.

15.1.8. sVirt

sVirt is a new feature included with Red Hat Enterprise Linux 6.0 that integrates SELinux and virtualization. sVirt applies Mandatory Access Control (MAC) to improve security when using virtualized guests. sVirt improves security and hardens the system against bugs in the hypervisor that might be used as an attack vector for the host or to another virtualized guest.

15.1.9. Migration

  • Guest ABI stability provides enhanced migration support. Guests PCI device numbers are preserved during migration and identical PCI device positions are presented after migrating the guest.
  • Migration now accounts for CPU models. CPU models allow guests to take advantage of new processor instruction sets. Guests can be migrated to hosts with a compatible CPU model.
  • Enhancements to the migration protocol.

15.1.10. Guest Device ABI Stability

As part of the new qdev device model, the guest ABI is now stable and will be kept consistent for newer releases. The devices and device arrangements on guests will remain consistent in future updates. This feature resolves issues with some operating system activation processes.


Red Hat Enterprise Linux 6 includes components providing functionality for the Simple Protocol for Independent Computing Environments (SPICE) remote display protocol. These components are only supported for use in conjunction with Red Hat Enterprise Virtualization products and are not guaranteed to have a stable ABI. The components will be updated to synchronize with functional requirements of Red Hat Enterprise Virtualization products. Migration to future releases may require manual operations on a per-system basis.

15.2. Xen

Red Hat Enterprise Linux 6 is supported as a Xen guest for the x86 and the AMD 64 and Intel 64 architectures. The para-virtualized operations (pv-ops) are included in the Red Hat Enterprise Linux 6 kernel. The default Red Hat Enterprise Linux 6 kernel can be used as a Xen para-virtualized guest and as a Xen fully virtualized guest on Red Hat Enterprise Linux 5 hosts. Red Hat Enterprise Linux 6 includes the para-virtualized drivers for fully virtualized Xen guest installations.
Red Hat Enterprise Linux 6 is not supported as a Xen host.

Further Reading

The Virtualization Guide details the process to install, configure and manage the virtualization technologies in Red Hat Enterprise Linux 6.

15.3. virt-v2v

Red Hat Enterprise Linux 6 features the new virt-v2v tool, enabling system administrators to convert and import virtual machines created on other systems such as Xen and VMware ESX to KVM. virt-v2v provides a migration path for Xen guests running on a Red Hat Enterprise Linux 5 hypervisor.

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