Backing up virtual servers: Traditional apps and new tools


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Most IT shops are still relying on traditional backup apps with their server clients to back up virtual servers, but that approach has its limitations. Today, there are plenty of good alternatives for virtual machine backup.

In most IT shops, virtual servers are backed up just like physical servers at first, but as the number of virtual servers increases traditional backup methods start breaking down. The fact that a single physical machine can host many virtual machines (VMs) poses challenges that simply don't exist when backing up dedicated physical servers.

With multiple VMs competing for processing, storage and networking resources, contention for those resources is the No. 1 challenge of virtual server data protection. Concurrent backup jobs on multiple virtual machines can seriously impact the performance of applications hosted on those VMs. And when traditional backup methods are used to protect virtual servers, some key capabilities are sacrificed, such as application-consistent data protection and the ability to restore sub-VM-level objects such as files without having to restore the whole virtual machine. As virtual servers proliferate in the data center, there's a clear call to IT managers to rethink their backup strategy to efficiently protect VMs and the applications they host.

Virtual server data protection options

Contemporary virtual server environments can be protected using one of the following backup methods:

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  • Backup agents on VMs
  • Continuous data protection (CDP)
  • VM image backup on the hypervisor using a backup proxy server

Backup agents on VMs. Backing up virtual machines by placing backup agents on each VM extends the most popular backup method of physical servers into the virtual server realm. Instead of having one backup agent per physical server, each VM gets its own agent and backup jobs run independently for each virtual machine. This approach is workable as long as the number of VMs is small; as the number of VMs per hypervisor grows, resource contention will create performance problems. Besides being able to leverage an existing backup product and approach, a backup agent can be the most straightforward way of ensuring application consistency. For many apps, especially non-Windows applications and applications that aren't integrated with Microsoft Volume Shadow Copy Service (VSS), backup agents may be the only way of ensuring application consistency of backup data.

Continuous data protection. A CDP product that runs on each VM is one step up from backup agents running traditional full and incremental backups. A CDP product captures changes on an ongoing basis and puts a much smaller burden on the host machine than traditional backups do. CDP products work either at the file or block level, and usually provide integration with mainstream applications to enable restoring to consistent points in time. In addition to large backup application vendors that have added continuous data protection to their suites, CDP products are offered by a list of smaller vendors such as FalconStor Software Inc., InMage Systems Inc., Vision Solutions (acquired Double-Take Software) and others.

VM image backup on the hypervisor using a backup proxy server. Backing up VM images on the hypervisor, rather than backing up virtual machines via agents within each VM, is appealing for many reasons: it enables efficient backups with little processing overhead; eliminates the need to install and manage backup agents on each VM; and by centralizing the backup of all VMs at the host, the backup of multiple VMs can be orchestrated to minimize performance problems and resource contention. To offload the backup task from the hypervisor machine, VM snapshots are usually replicated or mounted to a backup host or proxy server, minimizing the performance impact while backups are performed.

Host-side VM image backup, however, is usually only acceptable if virtual machines can be backed up in a consistent state; all major hypervisor vendors have added the ability to quiesce a VM while a snapshot of the VM image is taken. Another challenge with VM image backup is restoring granularity. While some backup products are only able to restore virtual machines, others are able to restore sub-VM objects such as files. Consistent data protection of apps within each VM is more challenging with VM image backup than it is with backup agents within virtual machines. Application-consistent data protection is usually limited to applications that are integrated with VSS. For apps that aren't integrated with VSS, crash-consistent backups may be the only option; but if application consistency is required, backup agents on VMs are the way to go.

This was first published in July 2011

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