What are Hyper-V snapshots? It depends whom you ask

What's a Hyper-V snapshot? One thing's certain: A Hyper-V snapshot is not a primary backup method. But with numerous software vendors that abuse the term, it's easy to get confused.

Do you use Microsoft Hyper-V snapshots to back up your virtual machines (VMs)? Are you sure?

In many cases, the term snapshot refers to a step in the backup process. But that's not the case with Hyper-V. Hyper-V snapshots are not like traditional backup snapshots, and if you use them as a long-term recovery solution, you may be in for a rude awakening.

 What are Hyper-V snapshots?
Hyper-V's built-in snapshots provide a quick-and-dirty solution for reverting a VM to a previous state. So if a problem occurs with a VM, you can restore it to a working configuration.

More resources on Hyper-V snapshots
How Hyper-V snapshots work

Taking snapshots with Hyper-V

Managing virtual hard disks
in Hyper-V

Snapshots are great not only for troubleshooting but also for test and development environments. The problem is, Hyper-V snapshot are meant for rolling back a VM state and should not be used for backup purposes. The ability to revert VMs to a previous state is intended for short-term situations because using dated AVHD files can lead to data corruption, domain membership problems and application issues.

Hyper-V snapshots vs. backup snapshots
Much of the confusion about snapshots exists because of naming conventions. Many IT product companies use the term snapshot to refer to a step in the backup process. The Volume Shadow Copy Service (VSS), for example, uses a VSS snapshot to flush in-memory updates to a disk so that the backup can starts from a known point with good data. This process, however, has nothing to do with the snapshots that are initiated from a virtual console.

While VSS is a primary abuser of this term, third-party products are also culprits. Some backup applications -- particularly ones that deal with databases -- use the term snapshot to refer to database quiescence: a process that pauses a database or file system so that its memory contents can be flushed to a disk. Quiescence creates a known point in time from which backups can begin while the database continues to operate.

Hyper-V uses VSS snapshots for backups. Wait … what?
Hyper-V further complicates this terminology by using VSS snapshots to accomplish certain backups. VSS works in combination with a VSS-aware agent from a backup solution, and it also operates natively on all Windows computers (i.e., a Hyper-V host, VMs) -- so VSS is present in every VM and host.

VSS is a core component of Hyper-V backups when backup agents are installed to a Hyper-V host. In this situation, you can back up a VM as a single file instead of amassing all its individual files. A VSS snapshot is required for single-file backups because the VM's file system must be properly quiesced for consistency purposes.

Ultimately, VSS snapshots enable other backup solutions to function. VSS snapshots exist long enough so that a backup agent can begin from a known data point. As a result, VSS snapshots are quite different than the VM snapshots that are created in the Hyper-V Manager console, which must be manually removed or deleted.

Greg Shields
 

Greg Shields is an independent author, instructor, Microsoft MVP and IT consultant based in Denver. He is a co-founder of Concentrated Technology LLC and has nearly 15 years of experience in IT architecture and enterprise administration. Shields specializes in Microsoft administration, systems management and monitoring, and virtualization. He is the author of several books, including Windows Server 2008: What's New/What's Changed, available from Sapien Press.


This was first published in July 2010

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