Virtualization is causing customers to rethink their backup strategies, with technology that combines pieces of traditional and well-understood enterprise backup with some pieces that are unique in the virtualized world.

In the past, traditional enterprise backup in the vast majority of shops has included spinning disk for short-term and intermediate data use, archival tape for long-term storage, and software such as IBM Tivoli and HP StorageWorks.

But some say that's no longer enough in a virtualized world.

"You definitely can't take a wait-and-see approach with backup, especially now that more and more companies are using server virtualization in critical production environments," says Stephanie Balaouras, a senior analyst for virtualization strategies at Forrester Research. "Backup is going to become a major challenge if companies haven't explored their options."

Traditional backup systems have a one-to-one relationship with servers. These tried-and-true backup systems and associated software already support storage-area networks (SAN), fiber optics, and the latest operating system and server hardware updates. But they are not geared specifically for the complex world of virtualization, which involves multiple guest operating systems on the same box.

Dave Russell, Gartner's vice president of research for servers and storage, outlined three popular strategies for virtualization backups. The most common is putting software agents on each virtual machine (VM) and then using traditional enterprise backup software. A second approach is to create an image of the VM and either use a storage service hosted elsewhere or take daily snapshots of the logical unit number (LUN).

A third strategy is to use VMware consolidated backup (VCB) that incrementally archives the VM -- meaning it copies only what has changed since the last backup. In this way, companies can restore a single file, even from one of 30 guest operating systems that all reside on a single physical server.

"Most companies gravitate toward the backup agents and traditional backup software, which they are used to doing with a physical server, and it feels very natural and easy," says Russell. "But this approach has proven to be cost prohibitive because of the number and scale of VMs and the licensing required."

Backup agents are included with VMware and other virtualization products to help administrators integrate VMs into the traditional backup process. The main advantage is cost: The agents are free or add a relatively minimal fee. On the downside, agents force administrators to use a fairly simplistic approach: Admins can archive an entire virtualized server, but not pick and choose volumes or guest operating systems. Nor can server administrators restore specific portions of data, or substantiate (verify the data integrity) of VM volumes.