How to Backup Virtual Servers

As server virtualisation assumes a greater role in the enterprise, administrators face a proliferation of virtual machines residing on the same physical server. Each virtual machine uses a portion of the physical machine's processing, memory and I/O resources. Ideally, server virtualisation provides a means of increasing hardware utilisation.

But as more "logical" servers are consolidated into fewer "physical" computer systems, it's important to protect each virtual machine's data against failure or loss. Virtual servers are the key to providing this protection. This article examines how virtual server backup can be achieved using a mix of traditional backup techniques and specialised virtualisation tools. It also highlights important deployment issues.

What is virtual server backup?

A virtual machine is a complete logical environment existing as a separate entity on a physical server. Each virtual machine is treated and perceived as if it is physical. In fact, a user cannot tell the difference between a real and virtual machine. A data centre may host thousands of virtual machines running on only a fraction of that much hardware, and this presents a serious problem for storage or backup administrators. Data loss on a virtual server can be just as catastrophic as data loss on a physical server, so every virtual server must be backed up as part of a company's backup regimen.

Virtual server backups can be accomplished using a traditional approach with conventional backup software. The backup software is simply installed and configured on each virtual machine, and backups will run normally to any conventional backup target, including tape drives, virtual tape libraries (VTL) or disk storage. "That's probably the most popular way that people do it today because it's familiar," says Lauren Whitehouse, analyst with the Enterprise Strategy Group (ESG). "It ensures a consistent backup; it will give you the granular recovery that you're looking for, and it's application-specific."

However, applying traditional backup tactics to virtual server backups does have drawbacks. The most significant problem is resource contention. Backups demand significant processing power, and the added resources needed to execute a backup may compromise the performance of that virtual machine and all virtual machines running on the system. "Don't go for 100% utilisation," says Greg Schulz, founder and senior analyst at the Storage IO Group. Leave some server resources unused to accommodate backup tasks and stagger backup processes so that only one virtual machine is being backed up on any physical system at one time.

Backup process more costly in virtualised environments

There are far more installations when the backup software is installed on every virtual machine, and this can make your backup process far more costly. Also, traditional backups will copy programs and application data but do not necessarily capture the entire virtual machine state. This may be fine if your only goal is to preserve an application, such as a database, but a failed virtual machine may need to be recreated and reconfigured from scratch before the backup can be restored.

Virtualisation-specific tools, such as VMware Consolidated Backup (VCB) or Microsoft's Virtual Machine Manager (VMM), interface directly with their respective virtualisation platform and capture point-in-time snapshots of the entire VMware's Virtual Machine Disk (VMDK) or Microsoft's Virtual Hard Drive (VHD). Virtual server backup tools like, VCB or Virtual Machine Manager (VMM), can capture the entire virtual machine state quickly, and the virtual machine typically does not need to be quiesced or taken offline. Not only does this allow for fast, complete system restorations, but complete snapshots can also be uploaded to new virtual machines, allowing system administrators to "clone" virtual servers on demand.

The downside to virtual server files is a potential loss in granularity. With traditional backups, it is easy to restore a single application or data file. When there is one single VMDK or VMM file, you typically have to restore the entire snapshot in order to recover, even if only one file is lost or corrupted. "Some snapshot vendors have figured out how to take that image-level backup and break it down into the granular single files that people need to recover," Whitehouse says, "Not everyone has done that though."

Implementing virtual server backups

Storage space poses a particular challenge for virtual machine files. The virtual snapshot is always seen as a new file, so it is backed up in its entirety, regardless of how much data has actually changed since the last