Get inside Virtualisation

Xen
Key Features Enterprise-capable solution, free, supported by Citrix
Key Drawbacks Not the same product as XenServer and seems to be a generation behind the Citrix commercial offering

Xen is a very close competitor with VMware on the server virtualisation playing field. Citrix owns the Xen code but still maintains the free, open source xen.org site. Its own XenServer is free as well. This is good news for small and medium-sized businesses that don’t need enterprise-level management tools. Xen is a Type 1 hypervisor and competes directly with VMware’s ESX/ESXi products.

Xen is really a tale of two separate projects: xen.org and Citrix XenServer. The xen.org version is a group of packages that you install to an existing operating system to re-create that system as a Type 1 hypervisor with a Xen-aware Linux kernel, some libraries and other tools. One of those tools is the virt-manager, which is your graphical virtual machine management interface.

You may also work at the command line, similar to the procedure shown in the QEMU section. Since most Xen distributions include QEMU, you can create and modify VMs in the same way that you would with a standalone version of QEMU. Xen also uses the QEMU device model and some of its device emulation code. While QEMU is not coupled with Xen as tightly as it is with KVM, you might say that Xen is QEMU compatible.

XenServer
Key Features Enterprise virtualisation solution, firm competitor with VMware, free, Citrix support
Key Drawbacks The Citrix Essentials management suite is an expense and almost required for larger implementations

On the other side of the Xen fence is XenServer, which is the commercial version of the Xen hypervisor. Unlike the xen.org version, XenServer arrives as an ISO file that contains a fully configured, ready-to-install-and-run Type 1 hypervisor. You have command-line utilities to manage your environment with this commercial version, but you have two other options as well: the xsconsole and the XenCenter graphical application. The xsconsole allows you to configure the Xen host system and to perform some light VM management. XenCenter is a remote-only management application and it’s only available for Windows systems, which causes a bit of angst for the Linux purist. This graphical application provides an intuitive interface into your Xen-centred world.

Xen and VMware both provide magnificent tools for working with virtual machines. Not only can you create, modify and delete virtual machines using their management applications, you can also manage pools (collections of VMs), templates and storage objects. And, the most exciting feature of true enterprise virtualisation is the ability to move a running virtual machine from one Pool or Xen host to another with no service interruption. Using the graphical application, XenCenter, makes such a move as easy as a drag-and-drop operation.

These ‘motions’ as they’re called, make balancing workloads between Xen hosts an easy task. Properly configured Xen hosts do some dynamic VM motions on their own, but you – as the administrator – may override this activity and ‘pin’ particular VMs to a specific Xen host. Xen and XenServer provide you with free, enterprise-level virtualisation solutions that are high on performance, low on headaches and right for your budget.

Conclusion
Before locking yourself into an expensive and proprietary virtualisation solution, you should explore each of these free technologies more thoroughly for yourself. Virtualisation is the perfect technology for server consolidation projects, cloud-based solutions and repurposed hardware. You can lower your power consumption, cooling requirements and hardware costs by converting to a virtualised infrastructure. Without paying thousands in licensing fees, you’ll realise the savings afforded by going virtual.