Virtualization is a hot topic. For a lot of good reasons

• being able to setup one or several sandbox machine with minimal hassle. It no longer required to assemble a temp box from spare hardware, that main characteristics are to typically generate supplementary heat, dust and noise.
  
• being able to snapshot the state of a virtual box, just in case something goes wrong and requires to revert to the last good state.
  
• being able to completely dissociate the software and the operating system from the hardware. The hardware real specs are not exposed to the virtual machines, and all virtual machines emulate the same virtual hardware.
  
• being able to migrate a virtual machine from one real box to another one, with little or not interruption of the service.
  
• being able to run a fill virtual machine as a user process on the host machine, so without specific privilege.
  
• performance now is a reality, provided that the guest architecture is the same than the host. The newer CPUs from Intel and AMD both provide virtualization support in their hardware, so that the guest has almost the same performance than the host.

Virtualization on Fedora

The virtualization in Fedora has been advertised early with the shipping of Xen. Xen requires that the host runs a specific kernel, that lags behind the most recent upstream kernel version. The Xen guys at RedHat made a terrific work to port the Xen changes to current kernel releases, but this looks like such a daunting task, that they have a hard time to follow upstream kernel.

A new solution has been developed, KVM, that has an invaluable advantage to be merged upstream, and to work with an unmodified kernel, just by loading a module into the system. KVM is an api exposed to userland via the /dev/kvm device, and that has been integrated into QEMU, so QEMU commands can work unmodified with KVM.

QEMU is an older emulator, not only for x86 architectures. Coupled with a kernel module (kqemu), it can provide a nice emulation speedup, compared to a pure userland emulation. This kernel module is not yet upstream. It was proprietary code until recently. Its license is now GPL.

RedHat also developed for Fedora an interesting framework, virt-manager and libvirt, that allows to manage virtual machines, in a way that is independent of the underlying virtualization technology available on the host. It can currently handle both Xen, and KVM.

QEMU features

QEMU simulates an HDD using a single disk partition or file on the host machine. It can use a disk image format allowing embedded snapshoted states of the virtual machine. Features include compressed storage, on-demand growing disk images. It is also possible to separate the disk image in two parts : a read-only disk image in one file (typically shared among several virtual machines), and a second writable disk image, specific to each virtual machine, that use the copy-on-write principle, to only store differences with the read-only base disk image.

QEMU networking is still a bit tricky to configure, if the virtual machine must be reachable on the real network. A bridge must be created, including the real network interface, and a tuntap virtual interface. This tuntap interface must be handled by a virtual network switch emulator, like vde2. Access to this virtual ethernet switch may be granted to a given user. This user can launch, and attach many QEMU instances to this switch. Each instance with a different MAC address, so that each one obtains a distinct IP address on the real network.

QEMU virtual machines management is easy if they are configured to launch a VNC server at startup.