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Disaster recovery strategies for virtual machines

As more companies turn to virtual machines for their backup, it's important and, and also easy, to equip them with a disaster recovery plan.

According to Symantec Corp.'s 2008 annual Symantec Disaster Recovery Research Report, 35% of virtual servers aren't covered in organizations' disaster recovery (DR) plans. In addition, only 37% of those surveyed back up all of their virtual systems. The primary reason cited for insufficient data protection and disaster recovery of virtualized servers is a lack of resources. IT departments that are already stretched to their limits don't have the time to put workable disaster recovery plans in place for many of their virtualized systems. And the tools for protecting physical and virtual servers differ in many cases, resulting in higher training, labor and software costs. Notwithstanding the challenges identified in the Symantec survey, protecting virtual server data and virtual machines (VMs) is simpler and more cost effective than protecting physical servers.

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The reason virtual servers are easier to protect than physical servers lies in part in how the virtual machine's hypervisor is architected. A hypervisor is a virtualization platform where multiple virtualized systems (guest operating systems) run on a single physical machine, known as the host. The virtual machines run their own OSes but share the underlying physical machine resources, from CPU and memory to I/O devices and storage. While physical servers must be attached to a physical machine, virtual servers are stored as files or VM images on the host system. The VM image contains everything about the virtual server, and the VM can be moved among physical machines by copying the hardware-independent VM image file and booting it on different hosts. This innate mobility benefit of virtual servers is the primary reason why a virtual server is easier to deploy on a disaster recovery site.

In comparison, disaster recovery of physical servers where primary servers are configured to fail over to secondary servers is a bigger and more costly challenge. The primary and secondary machines require the same, or at the least, very similar hardware. And for any type of automated failover, the physical machines need to be configured in some type of cluster configuration. Traditional cluster software like Microsoft Cluster Server forces a relatively static relationship where clustered server nodes are pre-assigned and designated as primary or secondary nodes.

"In traditional cluster software that has been used for high availability [HA] and DR of physical servers, nodes are tightly coupled, which limits scalability and increases both capital and operational expenditure," explained Jason Nadeau, group product manager for Symantec's Veritas Cluster Server (VCS).

Conversely, in a virtualized server environment, the primary host with the production VMs and the secondary DR host can be very different. "DR for virtual servers no longer requires matching hardware," said Mark Bowker, analyst at Enterprise Strategy Group (ESG) in Milford, Mass. "You can have 10 physical servers running VMs in your primary data center and four very different physical servers in the secondary data center." To take this a step further, you can bring up the VMs on any of your DR hosts or run multiple instances of a single production VM on more than one host in your DR site.

The flexibility gained through the inherent mobility of virtual servers is stunning and opens new possibilities to use server resources. In a virtual server environment, physical servers (VM hosts) designated as disaster recovery servers, can be used for noncritical applications during normal operations. Instead of DR servers being idle 99% of the time, they can be used for applications not required during a disaster.

"With our VMware ESX hosts, we have the option to leverage the DR servers in our secondary data center for DEV and TEST instances during normal operations," said Peter Allen, director of IT operations at Nixon Peabody LLP in Rochester, NY. Allen runs 17 ESX hosts with more than 100 virtual servers in the primary data center in Rochester, and a secondary data center in Ohio.

The benefits of virtualized servers for data protection and disaster recovery are becoming a primary impetus for an increasing number of IT managers to convert to a virtualized server infrastructure. "DR is replacing server consolidation as the main driver to deploy virtual servers," said Bowker. "Some people are deploying a single VM on a host for critical and resource-intense apps like Exchange to reap the DR benefits but ensure sufficient resources; the data protection advantages by far outweigh the slight virtualization overhead."

This article originally appeared in Storage magazine.

About this author: Jacob Gsoedl is a frequent contributor to "Storage" magazine.

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