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DR for virtualized servers

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DR strategies for virtual machines
The reason virtual servers are easier to protect than physical servers lies in part in how the virtual machine's (VM's) hypervisor is architected. A hypervisor is a virtualization platform where multiple virtualized systems (so-called guest OSes) run on a single physical machine, known as the host. The VMs run their own OSes but share the underlying physical machine resources, from CPU and memory to I/O devices and storage. While physical servers are inseparably attached to a physical machine, virtual servers are stored as files or VM images on the host system. Because the VM image contains everything about the virtual server, the VM can be moved among physical machines by copying the relatively 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 DR site.

In comparison, DR of physical servers where primary servers are configured to fail over to secondary servers is a much bigger and more costly challenge. The primary and secondary machines require the same or at 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

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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," explains 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," says Mark Bowker, analyst at Enterprise Strategy Group (ESG), Milford, MA. "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 DR servers can be used for noncritical apps during normal operations. Instead of DR servers being idle 99%-plus of the time, they can be used for apps 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," says Peter Allen, director of IT operations at Nixon Peabody LLP in Rochester, NY, an international law firm with 18 locations worldwide. 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 DR 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," says 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 was first published in December 2008

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