FAQ

The Microsoft Hyper-V snapshot explained

Snapshots are an important part of a Hyper-V administrator's toolkit. A Hyper-V snapshot allows an admin to roll back to a previous state, undoing changes and potentially saving recovery time. However, if you don't know how to use them properly, you could be setting yourself up for disaster and inadvertently slowing VM performance. Before you start taking virtual pictures of your own, make sure you know when and how to use a Hyper-V snapshot.

What is a Hyper-V snapshot?

While it seems like a simple question, defining

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what a Hyper-V snapshot is can be tricky. The confusion stems from inconsistent terminology and naming conventions, which seem to be an all-too-common problem with IT lingo. The most important thing is not to confuse a Volume Shadow Copy Service (VSS) snapshot with a Hyper-V snapshot. While both are used to restore a virtual machine (VM) to a prior state, they work in fundamentally different ways. VSS operates on the block level of the file system and only backs up disk information, while a Hyper-V snapshot captures disk and memory information of a VM by creating a separate automatic Virtual Hard Disk (.AVHD) file to track changes. Unlike VSS snapshots, Hyper-V snapshots aren't the answer to backups or disaster recovery. Instead, you should use them as a troubleshooting option, should a configuration change or patch update cause problems.

How do Hyper-V snapshots work, and what are they used for?

When you create a Hyper-V snapshot, you're actually creating a differencing disk. A differencing disk stores changes, or differences, that would otherwise be written to the original virtual hard disk. As the VM continues to change, the differencing disk grows in size. VSS snapshots differ because they create a copy of the disk image at a specific time. Think of a Hyper-V snapshot like an "undo" option, rather than an independent backup.

Hyper-V snapshots are extremely useful for quickly reverting a VM back to a previous state, after an administrator realizes that an update failed or that a configuration change caused unexpected problems. The downside is that, if left unchecked, those expanding differencing disks can slow VM performance and fragment a hard drive.

How do I take a Hyper-V snapshot?

Many admins choose to create snapshots right before making any potentially risky change to a VM's state. The actual act of creating a Hyper-V snapshot from the Hyper-V Manager in the Microsoft Management Console is a simple process: Simply right-click on a VM and select Snapshot. The time it takes to create a snapshot will depend on many factors, including the size of the VM and the disk's overall I/O load.

It's important to delete and merge older snapshots that can continue to grow and hurt performance. In Windows Server 2008 R2, completely deleting the snapshot means shutting down the VM.

What changed with Hyper-V snapshots in Windows Server 2012?

Snapshots created in Windows Server 2012 can be merged and completely deleted without shutting down the VM. This live merging of snapshots can save administrators time and aggravation by allowing them to perform updates to VMs without the risk of secondary downtime (once to perform the update and again to delete the snapshot). This new feature brings Hyper-V snapshots more in line with VMware snapshots, which do not require the shutdown of a VM to merge snapshots.


This was first published in March 2013

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