If you care only about the hypervisor technology, then choosing between Citrix XenServer and open source Xen is easy. Citrix XenServer and the different open source Xen vendors are all using the same hypervisor. There are some minor differences, but as Xen itself is an open source project, the new features that are added by Citrix Systems Inc. today will be added to the open source Xen products of tomorrow, and vice versa. Hence, there is no real difference between the two approaches.
However, if you ask about management of the Xen environment, there are distinct differences. In short, every vendor has its own products and services for management. In Citrix XenServer, the management features are very well developed. Citrix XenServer comes with XenCenter, which is the default management environment that was clearly inspired by VMware Inc.'s Virtual Center (now known as vCenter Server). Using XenCenter, you can easily manage virtual machines (VMs) and virtualization hosts. The tool is also scalable, and if you're managing huge environments, you can also use pools. Pools allow you to manage groups of hosts and resources.
Citrix XenServer is free and comes with some important base functionality, such as the option to perform live migration of VMs. If you're looking for more advanced functions, you need to download Citrix XenCenter. With XenCenter, you'll get advanced provisioning tools and the option to implement high availability -- VMs are automatically restarted if they, or the host they were running on, go down unexpectedly.
Which open source Xen hypervisor?
There are many open source Xen products. Some of them are free and come with no support at all, and others are part of an enterprise Linux distribution. Until recently, there were three major players in this area: Red Hat Inc., Oracle Corp. and Novell Inc. For an enterprise environment, these are currently the only Xen implementations that are of interest.
Of these three, Red Hat is probably not the most likely choice for virtualization. Red Hat recently switched from Xen to KVM as its default virtualization platform. Although Red Hat still supports Xen, and you can manage it very well from Red Hat Enterprise Virtualization for Servers, it clearly is not the direction the company wants to go in the future. Therefore, Red Hat doesn't seem to be a worthy alternative for Citrix XenServer.
The next major Xen player is Oracle. Oracle, however, is hardly considered an open source Xen player, even though it does have its own Linux distribution. In Oracle's Linux distribution, which, in practice, is used in combination with another proprietary Oracle product, a Xen stack is integrated. Even if Oracle's Linux virtual machines are all managed from a very nice virtualization tool, because of the way the Oracle VM product is embedded in the proprietary Oracle product, Oracle probably isn't a good alternative for Citrix XenServer either.
Then there's Novell. In 2008, Novell proudly announced that it wanted to be a major player in the virtualization market, but this is hardly the case at the moment. At Novell's recent annual user conference, BrainShare, which was in Salt Lake City this past March, there wasn't a single announcement with regard to Xen. If you take a closer look, Novell doesn't seem to have put much recent effort into their virtualization platform. Xen in SUSE Linux Enterprise Server is still managed with basic open source tools, such as the System Center Virtual Machine Manager, which is not exactly a scalable tool. So from that perspective, you can drop Novell as an open source Xen candidate as well.
If your customer is still bent on a Novell product, consider its PlateSpin platform. Novell purchased PlateSpin in 2008, but since then, nothing has been done to properly integrate Novell's own virtualization solution. Therefore, PlateSpin is a perfect tool to manage VMware or Citrix XenServer environments, but it isn't fit for management of SUSE Xen at all.
There seems to be only one possible conclusion at this time. If you're looking for a decent enterprise-level virtualization offering, Citrix XenServer is the way to go. If you don't need high availability, it is free, easy to manage and a very scalable enterprise virtualization product.
About the expert
Sander van Vugt is an independent trainer and consultant living in the Netherlands. Van Vugt is an expert in Linux high availability, virtualization and performance and has completed several projects that implement all three. He is also the writer of various Linux-related books, such as Beginning the Linux Command Line, Beginning Ubuntu Server Administration and Pro Ubuntu Server Administration.
This was first published in May 2010