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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Citrix XenServer management
The XenServer management console is a Windows application that connects to the master server in the farm. Like VMware's vCenter Server and Red Hat's RHEV Manager, and unlike Microsoft's Virtual Machine Manager, the XenServer console allows management of all hosts and VMs. There is also a management API and integration SDKs for a variety of development platforms. XenServer is based on Linux, so it's no real surprise that all management operations have a CLI counterpart that can be run on any host, and there are a few CLI commands that can come in handy when the GUI has problems conducting an operation. For example, more than once we found that the only way to disconnect an ISO image from a VM was via the command line due to stalled operations within the client management.

Overall, the management UI is straightforward, offering several different perspectives on the infrastructure, such as viewing all VMs by operating system type, current status, by folder, or even by a tag assigned. This makes finding specific VMs in a large implementation easier than paging through a huge list.

Internally, however, VMs are referred to by a unique UUID, which makes sense from an architectural standpoint, but requires looking up the UUID of a specific VM on which to run CLI operations. Thankfully, the CLI provides tab completion of these IDs, which does help considerably.

The XenServer console also handles all snapshotting and backup features, which are essentially the same thing. You can configure scheduled snapshots to occur on a per-VM basis, and you can select the number of snapshots to maintain for each host.

XenServer's high-availability and load-balancing features are quite functional, but require some supporting players and configuration. To enable high availability, a central storage LUN must be configured and available to each host, though it only needs to be a gigabyte in size at most. Each server maintains state information on this file system, which is used by the cluster to determine if a host is truly down or if a networking issue is disrupting normal communication. When I pulled a blade from the chassis, taking down a host that had been running six VMs, it took roughly five minutes before those VMs began booting on other hosts -- slightly longer than the other solutions, but still quite reasonable.

Load balancing requires that a specific Citrix-built VM be imported into the cluster. Inexplicably, this is a Windows Server 2008 VM and, thus, requires a Windows Server license, though it will hopefully move to a Linux-based VM soon. The cluster is then configured to communicate with the VM, which enables VM migration operations to be conducted as requested by the load monitoring services. Several easily modified parameters control how sensitive the cluster is to host load, and CPU, disk, and network utilization thresholds can be configured to fine-tune the load balancing. In my tests, load balancing functioned as you would expect, and as with manual control, the VMs migrate quickly and smoothly from host to host. There is no concept of a live storage migration in XenServer, as there is in VMware.

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Citrix XenServer's management console shows highly detailed information on host utilization .

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