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Microsoft Virtual Machine Manager Snapshots and FAQ

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This topic is about System Center Virtual Machine Manager Snapshots.

What are virtual machine snapshots?

Virtual machine snapshots capture the state, data, and hardware configuration of a running virtual machine.

What are snapshots used for?

Snapshots provide a fast and easy way to revert the virtual machine to a previous state. For this reason, virtual machine snapshots are intended mainly for use in development and test environments. Having an easy way to revert a virtual machine can be
very useful if you need to recreate a specific state or condition so that you can troubleshoot a problem. There are certain
circumstances in which it may make sense to use snapshots in a production environment. For example, you can use snapshots to provide a way to revert a potentially risky operation in a production environment, such as applying an update to the software running in the virtual machine.

How are snapshots stored?

Snapshot data files are stored as .avhd files. Taking multiple snapshots can quickly consume storage space. In the first release version of Hyper-V (KB950050) and Hyper-V in Windows Server Service Pack 2, snapshot, snapshot data files usually are
located in the same folder as the virtual machine by default. In Hyper-V in Windows Server 2008 R2, the files usually are located in the same folder as the virtual hard disk. The following exceptions affect the location of the snapshot data files: If the virtual machine was imported with snapshots, they are stored in their own folder. If the virtual machine has no snapshots and you configure the virtual machine snapshot setting, all snapshots you take afterwards will be stored in the folder you specify.

  Caution
Do not delete .avhd files directly from the storage location. Instead, use Hyper-V Manager to select   the virtual machine, and then delete the snapshots from the snapshot tree. Do not expand a virtual hard disk when it is used in a virtual machine that has snapshots. Doing so will make the snapshots unusable.

What other important considerations should I be aware of when using snapshots?

Keep the following considerations in mind, especially if you plan to use snapshots on a virtual machine in a production environment:

  • The presence of a virtual machine snapshot reduces the disk performance of the virtual machine.
  • When you delete a snapshot, the .avhd files that store the snapshot data remain in the storage location until the virtual machine is shut down, turned off, or put into a saved state. As a result, when you delete a snapshot, you will need to put the production virtual machine into one of those states at some point to be able to complete the safe removal of the snapshot.
  • We do not recommend using snapshots on virtual machines that provide time-sensitive services, or when performance or the availability of storage space is critical.

Should snapshots be used as a substitute for backups?

No, because virtual machine snapshots are not the same as backups created by a Volume Shadow Copy Service (VSS) writer. We do not recommend using virtual machine snapshots as a permanent data or system recovery solution. Even though virtual
machine snapshots provide a convenient way to store different points of system state, data, and configuration, there are some inherent risks of unintended data loss if they are not managed appropriately. A backup solution helps provide protection that is not provided by snapshots. One reason that are not an acceptable substitute for a backup is that they do not protect against problems that may occur on the server running Hyper-V, such as a hardware malfunction on the physical computer or a software-related issue in the management operating system. Another reason is that applications that run in a virtual machine are not aware of the snapshot, and will not be able to adjust appropriately. For example, if you used a virtual machine snapshot to restore an Exchange server, the server would expect the same set of client connections that were present when the snapshot was taken. For more
information about backing up Hyper-V and its virtual machines see :

Planning for Backup (http://go.microsoft.com/fwlink/?LinkId=143125).

Protecting Hyper-V with Microsoft DPM 2010

 

 Important
Microsoft does not support the use of snapshots on virtual machines hosting the Active Directory Domain Services role (also known as “domain controllers”) or virtual machines hosting the Active Directory Lightweight Directory Services role. For more
information, see Planning Considerations for Virtualized Domain Controllers
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Author: James van den Berg

I'm Microsoft Architect and ICT Specialist and Microsoft MVP System Center Cloud and Datacenter Management

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