Virtualization shoot-out: Citrix XenServer

Citrix tackles server virtualization with a fast hypervisor and enterprise features, but leaves a few rough edges

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The XenServer management console is not without its quirks. For instance, to assign ISO images to boot each VM for installation, you must define and link to specific NFS or CIFS ISO repositories. You can't simply map a DVD device to a local ISO on your PC or to an ISO somewhere on the iSCSI storage. Further, mapping that ISO isn't handled via the VM settings, but defined during VM creation. As a result, modifying it later was problematic, throwing errors when different ISOs were linked to an already configured VM.

This issue would also crop up when trying to start VMs linked to ISOs in a repository that was either offline or otherwise inaccessible, without a clear error message to that effect. This may sound like a minor problem, but it was frustrating to build a VM with a linked ISO, begin the installation, and then realize that you needed to reboot the VM to restart the installation, only to find that the VM wouldn't reboot but needed to be deleted and recreated.

There are other idiosyncrasies related to VM management, such as the fact that all VM operations are serialized. Although you can select multiple VMs to boot or shut down, they will do so one at a time, making bulk operations tedious and slow. Parallelization of these functions would be a significant time-saver.

By contrast, XenServer's instant cloning feature offers a fast, easy method to make many copies of a template. Rather than require any input, it simply begins building a new VM identical to the template with a single click. On the other hand, once you convert a VM to a template, you can't convert it back, so modifications of existing templates are a bit of a pain.

Like Microsoft Hyper-V, XenServer uses a balloon driver to handle dynamic memory allocation to VMs. This method of stretching physical memory resources is functional, but doesn't go as far as the memory management features in the VMware or Red Hat hypervisors.

Citrix XenServer performance
As you might expect, XenServer ran Linux VMs handily, but it also played very well with the Windows Server 2008 R2 VMs. I used both Red Hat Enterprise Linux 6 and RHEL 5.5 for testing, since RHEL 6 support is still labeled "experimental," but the Linux numbers reflected here were posted by RHEL 6 VMs. I did encounter some significant though sporadic issues with one RHEL 6 host that was built with two separate disks rather than one. The VM would occasionally lock up tight with what appeared to be a failure to access the second disk. An identical build with only a single LUN did not exhibit these problems.

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A linear view of each running XenServer host in the farm and the VMs attached to them.
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