As the number of servers being virtualized grows, backing up and protecting them becomes more of a problem. It's not enough for IT administrators to simply back up each virtual server and its data. Protection also is needed for the virtual server's image -- its operating system, configuration and settings -- and the metadata on the physical server that identifies the virtual server's relationship to networked storage.

A challenge for IT managers is choosing from among a variety of virtual server backup options, which include:

• Traditional agent-based backup software, which installs a software agent on each virtual machine to back it up.
• Serverless backup or consolidated backup, which offloads backup processing from virtual machines to a separate physical server.
• Snapshotting or cloning the virtual machines using software from a vendor such as Network Appliance or software included with the virtualization package to protect data and images.
• Writing scripts and executing them to quiesce (minimize the number of processes running on) the virtual machine, back up its contents and restore the virtual machine.
• A combination of agent-based software and cloning.

While virtualization can result in better utilization of server resources, backing up all the newly created virtual machines concurrently for a physical server can overwhelm the network and take resources from applications running in other virtual machines.

Since by virtualizing physical machines you increase the number of servers contending for a single bus, Chris Wolf, senior analyst at Burton Group, suggests that users only virtualize physical servers that contain a PCI-Express (PCI-e) bus.

"The problem, especially in virtualization, is that when you have a shared I/O channel for all your PCI devices, the bandwidth of the bus becomes a really important issue. Traditional PCI devices can severely slow you down when you talk about six to 10 virtual machines sharing the same bus," Wolf says. "PCI-Express should be the bus of choice for all new virtualization deployments, as it offers a transfer rate up to 16Gbps in full duplex, compared to PCI Extended, which has a maximum throughput of 4Gbs."

Another factor to consider is the cost associated with agent-based backup software used in a virtual environment. Since most vendors of backup software require a separate license for each virtual machine that is being backed up, as well as one for the physical machine hosting the virtual machines, licensing costs can increase quickly.

The advantage of agent-based backup software is that IT administrators are familiar with it, having deployed it for many years to back up the physical machines in their environment.