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The future of virtual machine backup

The data center will have a very different look five years from now. Here's a prediction: lots and lots of virtual machines running on VMware, the open-source Xen, Microsoft's Virtual Server or some other yet-unknown platform.

In 2004, total market revenue for virtual machine software grew at 30%, says Dan Kusnetzky, VP of IDC's system software research, with VMware enjoying the lion's share of the market. In the coming years, he says, "open source and a highly competitive market will drive down the price" of virtual machine software—and thus of revenue growth—but "it will not in any way slow down the adoption."

Storage managers responsible for data protection may be wondering whether virtual machine backup happens any differently than it does on single operating system/single server systems.

In theory, there's nothing about a virtual machine that says you can't back it up the old-fashioned way: by installing a backup agent that traverses the file system. The problem some VMware shops have had with that traditional file-level backup is performance, says Karthik Rau, senior director of product management at VMware. "In a typical virtual environment, you can sometimes have 10 or 20 virtual machines running on a host," says Rau, and adding more CPU overhead simply isn't an option.

Version 3 of VMware's ESX Server, due in the first half of 2006, will address the performance issue with a feature called "consolidated backup," says Rau. Much like serverless backup today, ESX Server will communicate to the backup server where the virtual disks are located on a NAS device, and allow the backup server to copy that data directly.

In the meantime, for shops that can't afford the performance hit of a backup agent, you can take periodic snapshots of a virtual machine and replicate them offsite. In the case of a disaster, simply mount the snapshot on an alternate ESX Server, and you're up and running.

The downside to snapshot-based backup, says Rau, is that while you get the recoverability, "you don't get the file-level granularity." To recover a single file, you have to recover the entire virtual machine.

Tom Dugan, CEO at backup and disaster recovery service provider Recovery Networks, is a VMware user who uses Asigra software to take image backups of his virtual machines. Unlike other backup packages, Asigra is agentless, so it doesn't impose an additional CPU load and doesn't require organizations to revalidate their servers. Furthermore, Asigra backs up only changed blocks rather than the whole file, keeping backup traffic to a minimum.

While Asigra's backup doesn't provide for file-level granularity either, it does deliver on the hardware-independence promise of virtual machines. With traditional backup, notes Dugan, "I [can] back up, let's say a Dell 2650 server, [but] I can't just restore it to a Compaq machine."


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