Don’t get caught – VMware Snapshot is not a backup!

Monday, February 23, 2015

VMware Snapshot is a great tool if used properly. The problem is that some people think they can save money and use it as a quasi-backup only to find themselves in big trouble. If you understand what Snapshot is and isn’t you’ll be able to use it to your advantage.

If you’re not familiar with it, Snapshot is a tool that VMware provides as a way of capturing a virtual machine at a current point in time. Snapshot is a great way to capture the current state of a virtual machine to test changes, take a backup or perform risky changes. It can save VMware virtual servers from patches and upgrades gone wrong.

VMware SnapshotHowever, Snapshots should not be confused with virtual machine backups. Each has its own specific use case. A snapshot is not a full copy of a virtual machine. It is only a change log of the original virtual disk and can be used to restore a VM to a particular point in time when a failure or system error occurs during an upgrade or patch. It should not be relied upon as a direct backup process.

Snapshots are not normally automated events. They are manually created to make a moment in time that can be jumped back to. If the upgrade/patch/testing fails, then you have the ability to revert your VM back to the moment in time before the upgrade. Whilst this sounds like an undo feature, the only point that you can undo back to is to the one you created with the Snapshot.

As they are created for a specific event, most snapshots are deleted within an hour and VMware recommends deleting snapshots within 24 hours. They are for short term use only. In comparison, a backup is designed for long-term storage that can be reverted to in case of data loss or file corruption.

Snapshot Overview:

  • Represents the state of a virtual machine at the time it was taken
  • Includes the files and memory state of a virtual machine's guest operating system
  • Includes the settings and configuration of a virtual machine and its virtual hardware
  • Is stored as a set of files in the same directory as other files that comprise a virtual machine
  • Should be taken when testing something with unknown or potentially harmful effects
  • Is not meant to be a method of backup and recovery. If the files containing a virtual machine are lost, its snapshot files are also lost.
  • Negatively impacts the performance of a virtual machine. This is based on how long it has been in place and how much the virtual machine and its guest operating system have changed since the time it was taken. It is not recommended to run production virtual machines on snapshots on a permanent basis.
  • Can take up as much disk space as the virtual machine itself. If multiple snapshots are possible, the amount of disk space used increases with the number of snapshots in place.

When using VMware Snapshot, it makes sense to follow VMware best practice:

  • Use only 2-3 snapshots in a chain
  • Commit snapshots within 24-72 hours
  • Check snapshots created automatically by backup applications are committing correctly and regularly
  • Set vSphere alerts to notify you of virtual machines running on snapshots
  • Check for the existence of snapshots before modifying virtual machine hard disks or using vmotion
  • Ensure you have sufficient storage before creating or using snapshots
  • Avoid using snapshots on high transaction virtual machines such as database or email servers

VMware Snapshot is a wonderful tool that can be a lifesaver when you're performing upgrades or software maintenance. It gives you the ability to revert back to the moment in time before the upgrade so if anything goes wrong, you can always just roll back in a few minutes. It’s a tool that can  save you tons of time and heartache.

 


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If you’re still using Windows Server 2003 you will find our article 259 Days Till The End of Windows Server 2003 from October last year useful.