XenServer and Virtual Iron
While open-source Xen-based hypervisors are offered by Citrix Systems Inc., Novell Inc., Oracle Corp., Red Hat Inc., Sun Microsystems Inc. and other Linux distributions, the greatest traction has been garnered by Citrix XenServer and Virtual Iron Software Inc.'s Virtual Iron.

Like Hyper-V, Xen runs on Unix-like OSes, mostly Linux; but contrary to Hyper-V, VM files are written to raw disks, challenging data protection tools that monitor and back up file changes. "We needed to add a volume block-level filter driver to replicate Xen virtual image files," says Roudebush.

Live migration of running virtual machines is part of the Xen hypervisor, but it's not based on a virtual file system like VMFS. Instead, it depends on NFS mounts for shared storage while VMs are moved from one host to another. To move single virtual servers, XenServer assigns a LUN to each virtual disk image and leverages features in storage systems to back up, snapshot and clone these volumes.

Among the various Xen hypervisors, Virtual Iron has the most extensive management tools, attempting to match what VMware offers. "We come into play for customers for whom VMware is too expensive; for a third of the price, we provide similar management tools as VMware," claims Chris Barclay, director of product management for Virtual Iron.

Virtual server backup choices
Depending on the hypervisor in use, IT infrastructure in place...

Traditional backup and recovery: For small- to midsized firms with a small number of virtual servers, backing up VMs and restoring them in the DR site in case of a disaster is a frequently used option. As the restore can be done on almost any hardware that runs the hypervisor software, hardware requirements for restoring physical servers become a non-issue. Being able to restore multiple VMs to a single host further reduces hardware requirements for the secondary site and significantly lowers the overall cost of DR.

In its simplest form, virtual servers can be backed up by installing backup agents in the VMs, but that adds overhead and will likely impact server performance while backups are running. Companies that run VMware have the option to deploy VMware's Consolidated Backup, which removes the backup load from VMs. Besides backing up each VM, backups can be taken at the hypervisor level, eliminating the need to install agents on each VM, but with the disadvantage that it only allows restoring at the VM level.

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synchronous vs. asynchronous replication (PDF).

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