As more servers are virtualized, backing up and protecting them becomes more of a problem. It's not enough for IT to back up each virtual server and its data. Protection is needed for the virtual server's image -- its OS, configuration and settings -- and the metadata on the physical server that identifies the virtual server's relationship to networked storage. The challenge is to choose the right virtual server backup option:

-- Traditional agent-based backup software, which installs a software agent on each virtual machine to back it up.

-- Serverless backup, which offloads backup processing from virtual machines (VMs) to a separate physical server.

-- Snapshotting VMs with 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 VM, back up its contents and restore the VM.

-- A combination of agent-based software and cloning.

Each virtual-machine backup approach has its advantages and disadvantages. Chief among the disadvantages is the effect on network performance and utilization. While virtualization can result in better utilization of server resources, backing up all the newly created VMs concurrently for a physical server can overwhelm the network and take resources from applications running in other VMs.

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

"When you have a shared I/O channel for all your PCI devices, traditional PCI devices can severely slow you down when you talk about six to 10 VMs 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."

Also 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 backed up VM, as well as one for the physical machine hosting the VMs, 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.

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