Choice of Toolstacks

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One really great feature of the Xen Project hypervisor is that there are several choices of toolstacks that can be used to manage it. Before the introduction of libxenlight, there was a complicated and inefficient toolstack situation. The issue was that xend, xapi, libvirt, etc. all needed to manage many common low-level operations, which led to code duplication, inefficiences, bugs, and wasted effort (e.g fixing the same bug in all of the toolstacks). Further, the default toolstack at the time (xend) was difficult to understand, modify, and extend.

ToolStacks.png

Each toolstack exposes an API, which will run different tools. The figure below gives a very brief overview over the choices you have, which commercial products use which stack and examples of hosting vendors using specific APIs.

Libxenlight (libxl)

Libxenlight is a small lower-level library, written in C. It is simple to understand; and easy to modify, and extend. Libxl is (or will be) shared by all Xen Project toolstacks. It was created to provide a simple and robust API for all the toolstacks to use (instead all of the toolstacks each implementing this common functionality). It was also created to be a common codebase to do Xen operations.

libxl Design Principles

  • Implement mechanisms, not policies
  • Stateless
  • Hide xenstore, libxenctrl, and libxenguest from higher levels
  • Be as simple as possible

Also see

Choice of Toolstacks

This section describes the various toolstacks (and the console that is used to run it).

Default / XL

xl is a lightweight command line toolstack built using libxenlight. xl is shipped along with Xen and is the default toolstack as of Xen Project 4.1.

xl was designed to be backwards compatible with the xm CLI for xend. It provides a straightforward command line interface to Xen's facilities for guest creation and management.

Also see:

XAPI / XE

The Xen Project management API (xapi) is the default toolstack for XenServer and XCP-ng. It is also used in some installations of CloudStack. It has Free Software licence. The now-abandoned XCP project was an attempt to provide a community-developed, Open Source style distribution of XAPI which would be available in distros.

libvirt / VIRSH

Libvirt is a virtualization API/toolkit used to manage various virtualization technologies. Originally designed to interface with xm, there is a libxenlight port of libvirt.

Also see:

Default / XEND (Deprecated in Xen 4.1; Removed in 4.5)

XEND is the previous toolstack and continues to be included as part of the Xen source releases. However as of Xen 4.1 XEND is now deprecated and will likely be removed in a future release. It is recommended that new deployments use one of the other toolstacks discussed above.

xl was designed to be command line compatible with the xm CLI to xend and this should provide the easiest upgrade path.

Also See

Command Line Interface (CLI) Tools

The table shows the command line tools to use with each of the toolstacks.

Toolstack xl XAPI libvirt xend
CLI tool XL xe virsh XM

Note: xl does not have a toolstack/cli split since the toolstack is the cli. Hence it appears under both headings.

Note: xm has been removed in Xen 4.5.

Summary

The power and flexibility of the management of Xen Project is an important feature. Users have a choice to use xl, XAPI, libvirt, or another toolstack to manage their Xen system(s).

  • xl is very lightweight and is a great choice for develpers or users who are most comfortable with Unix like CLI tools.
  • XAPI is a powerful toolstack that provides domain life-cycle management, pool-awareness (multiple Xen systems added to a central logical pool), and many other advanced features available with Xen Project software.
  • libvirt is a generalized virtualization layer for managing various virtualization technlogies with a common toolkit (or toolstack).
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