About XenServer Open Source Edition

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XenServer now Xen Project

The Open Source Hypervisor

 

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What Happened to the old XenServer Hypervisor ?

The Xen Hypervisor began as a research project led by Ian Pratt at the University of Cambridge. Xen was first released  in 2003 and continued to be a supported by XenSource. Citrix Systems completed its acquisition of XenSource on the 22nd of October 2007.

The Xen Project was moved to the xen.org domain.  Citrix uses the Xen brand itself for some proprietary products unrelated to Xen, including XenApp and XenDesktop.

the Xen Project was moved on April 15, 2013 under the auspices of the Linux Foundation as a “Collaborative Project”. The Linux Foundation launched a new trademark for “Xen Project” to differentiate the project from any commercial use of the older “Xen” trademark. A new website was then launched at xenproject.org as part of the transfer.

 

Virtualisation Types

Xen provides five options for running guest operating systems:

1)      HVM. Hardware virtual machines

2)      HVM with PV drivers

3)      PVHVM. Paravirtualisation with full hardware virtualization i.e. HVM with PVHVM drivers

4)      PVH. PV in an HVM container and

5)      PV. Paravirtualisation

 

Paravirtualisation are modified guests.

Xen provides a form of virtualization known as paravirtualisation where guest operating systems are modified to use a special hypercall ABI, instead of certain architectural features.

Paravirtualisation allows Xen to achieve high performance on its host architecture (x86) which is known to be problematic with traditional virtualization techniques.

Xen can run paravirtualised guests known as “PV guests” in Xen terminology, even on CPUs without any clear support for virtualisation.

Paravirtualisation does not need to emulate a full set of hardware and firmware services therefore making a PV system simpler to manage while reducing vulnerability to potentially malicious guests

On a 32-bit x86 Xen host the kernel code runs in Ring 0 while the hosted domains run in Ring 1 and Ring 3 applications.

 

Hardware-assisted virtualisation allows for unmodified guests.

CPUs that support virtualisation enable hypervisors to run unmodified guests including proprietary operating systems such as Microsoft Windows.

Traditionally this is known as hardware-assisted virtualisation but is often referred to as a “Hardware Virtual Machine” (HVM) in Xen.

HVM extensions provide additional modes offering a clear distinction between the “most-privileged” modes used by the hypervisor and the ”less-privileged” modes used by guest kernels and applications with “hardware” accesses under complete control of the hypervisor. Both root and non-root mode support Rings 0 to 3.

AMD and Intel have both contributed modifications for Xen to exploit their respective Intel VT-x and AMD-V architecture extensions.

HVM extensions also support new instructions to allow direct calls from a paravirtualised guest into the hypervisor. This is typically used for I/O or other operations needing high performance. These extensions allow HVM guests with slight modifications to benefit from the increased performance facilitated by  paravirtualised I/O.

Xen HVM’s device emulation is based on the QEMU project which was designed to provide I/O virtualisation to the virtual machines.

The system emulates hardware via a patched QEMU “device manager” called qemu-dm, a daemon running as a backend in dom0. This means that a virtualised machine can see an emulated version of a very basic PC.

PV-on-HVM disks and network drivers are used during normal guest operation in performance-critical environments so that the emulated PC hardware is primarily used for booting.

 

Virtual machine migration.

In a similar way to VMware’s V-Motion, administrators can migrate live virtual machines between physical hosts across a LAN ow WAN without down-time. During this procedure the memory of the virtual machine is iteratively copied to the destination VM without stopping its execution.

The flip between VMs requires a pause of only 60–300 ms to perform a final synchronization before the destination virtual machine begins executing, providing an illusion of seamless migration.  Similar technologies can be used to suspend running virtual machines to disk for resumption at a later date.

 

Host Operating Systems

Xen can be deployed on a dedicated virtualization platform or as an optional configuration for many standard operating systems.

Xen is available for the following operating systems.

  • Alpine Linux offers a dom0 system known as Busybox & UClibc, can run from removable media.
  • Arch Linux provides the necessary packages with detailed setup instructions on their Wiki.
  • Debian GNU/Linux since version 4.0 and many of its derivatives.
  • FreeBSD 11 includes experimental host support.
  • Gentoo has the necessary packages available to support Xen, along with instructions on their Wiki.
  • Mageia since version 4.
  • NetBSD which can function as domU and dom0.
  • OpenSolaris based distributions can function as dom0 and domU from Nevada build 75 onwards.
  • openSUSE 10.x to 12.x, only 64-bit hosts are supported since 12.1.
  • Qubes OS offers Xen built-in for secure desktop usage.
  • SUSE Linux Enterprise Server since version 10.
  • Solaris since 2013 with Oracle VM Server for x86.
  • Ubuntu since 12.04 & 8.04 but no dom0-capable kernel in 8.10 until 12.04.