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Backing up virtual machines: Guest-level backups still warranted

While host-level virtual machine backups are the norm, guest-level backups are still common in three situations.

Server virtualization and related backup technology have matured to the point that host-level backups have become the norm. Backing up virtualization hosts at the host level allows virtual machines to be backed up along with supporting components such as VM snapshots and VM configuration files.

Before virtualization-aware backup software went mainstream, users who wished to back up VMs largely resorted to performing guest-level backups by installing a backup agent on individual VMs. Given the state of virtualization-aware backup applications, one has to consider whether guest-level backups have become completely obsolete or if there is still a legitimate use for them.

Today, there are three main use cases in which guest-level backups are used: in lightly virtualized organizations, in organizations with tight IT budgets, and when VMs run operating systems that are not officially supported by the hypervisor.

Use case one: Lightly virtualized organizations

Although backing up virtual machines is usually performed at the host level, there are still a number of situations that may warrant a guest-level backup.

Imagine for a moment that an organization primarily uses physical servers equipped with a backup agent. If the organization decides to virtualize one of those physical boxes, the backup agent will still exist within the newly virtualized machine and the agent will still be functional. This means the organization can continue to back up that server in the same manner it always has.

For such an organization, it may not make sense to invest money in a new, virtualization-aware backup application. Even if the backup application the organization uses happens to be virtualization-aware, those tasked with backing up virtual machines may not wish to complicate the process by adopting a separate set of backup procedures for VMs. If the organization is very lightly virtualized, it may make sense to continue to back up all servers (physical and virtual) in a consistent manner rather than split the backups into physical backups and virtual backups.

Use case two: Tight IT budgets

Smaller shops and cash-strapped organizations make a conscious effort to minimize IT spending. Such an organization might be running an old copy of Backup Exec or CommVault Simpana and subscribe to the philosophy that if the backup application still gets the job done, there is no reason to replace it. In these situations, it is perfectly OK for an organization to continue to use a legacy backup product, as long as it meets data protection goals and storage administrators understand the inherent limits of guest backups.

Use case three: Unsupported operating systems

Hypervisor vendors such as Microsoft and VMware provide a set of drivers that allow VMs to better interact with the hypervisor. VMware refers to these drivers as VMware Tools, while Microsoft calls them Hyper-V Integration Services. These drivers allow a VM's operating system (OS) to fully recognize virtual hardware and to use hypervisor-level system services such as time synchronization. In some cases, these drivers also impact the way in which backups are made.

In Hyper-V, for example, the backup process leverages the Backup Integration Service to prepare the VM to be backed up. The problem is that Integration Services are not compatible with all guest OSes -- only specific versions of Windows and Linux are supported. Similarly, not all guest OSes are compatible with the VMware Tools.

Guest OSes can function without Hyper-V Integration Services or VMware Tools installed, but the absence of these components can impact the backup process. In Hyper-V, for instance, VMs that do not have the Backup Integration Service enabled must be placed into a saved state prior to being backed up at the host level.

Some administrators may prefer to back up these types of VMs at the guest level rather than dealing with the hassles that come from trying to perform a host-level backup without the aid of Integration Services.

Guest-level backups are anything but dead. Although backing up virtual machines is usually performed at the host level, there are still a number of situations that may warrant a guest-level backup.

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