Hyper-V backups can fail for any number of reasons, but there are some things to look for when backups don’t work the way that they are supposed to. The techniques discussed in this article are not specific to any one particular backup application, but instead, focus on Hyper-V itself.

When backups fail, the first thing that you should do is to check the backup logs in an effort to learn more about the problem. Specifically, you need to determine if the problem is confined to a particular host, a particular virtual machine, or perhaps related to the backup target itself.

Troubleshooting VM Related Backup Problems

If you find that most of your virtual machines are backed up without issue, while others experience problems then you will need to determine what it is about those specific virtual machines that are interfering with the backup process.

  1. If you are backing the virtual machine up at the guest level, then it’s a good idea to make sure that the backup agent is installed and up to date, and that the Windows firewall is not preventing backup communications.
  2. Backups that are performed at the host level can be online or offline.
    • Online backups occur while the VM is running
    • Offline backups place the virtual machine into a saved state while the backup is being prepared.

    Virtual machines that are running newer versions of Windows or Linux can typically be backed up online. Hyper-V is designed to create an offline backup if the criteria for an online backup cannot be met, but on occasion, the backup will simply fail instead.

  3. All of the virtual machine’s volumes must be formatted as NTFS, and the virtual machine must contain only basic disks. Online backups cannot be used if the virtual machine is configured to use dynamic disks. You can use the Disk Management Console (diskmgmt.msc) inside of the virtual machine to verify disk usage.
  4. Another requirement is that the guest operating system must support the Volume Shadow Copy Services (VSS), and the VSS storage assignments should not be modified.
  5. Finally, the Backup integration service for the virtual machine should be enabled.
    • You can verify the Integration Services configuration in the Hyper-V Manager by right clicking on the virtual machine, and selecting the Settings command from the shortcut menu.
    • Doing so will cause Windows to display the settings window for the virtual machine. You can use this window to see which of the Integration Services are enabled.

    Keep in mind that older guest operating systems may require the Integration Services to be installed manually.

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  6. Finally, if the virtual machine is clustered, then the cluster resource group should be online.

Other Common Causes of Backup Failure

In addition to the items listed in the previous section, there are several other things that can cause Hyper-V backup failures.

  1. One of the first things that you should check is the system clock. Your virtual machines, host servers, and your backup server all need to have their clocks set to the same time. The Kerberos protocol, which is heavily used by Windows, is time sensitive and clock skew can cause Kerberos to stop working.
  2. Another commonly overlooked contributor to backup failures is incorrect DNS records. Hyper-V, like any other virtualization platform, is a highly dynamic environment. In most organizations, new virtual machines are being created all the time, and it can be just as common for old virtual machines that are no longer needed to be deleted. Depending on how a virtual machine is removed, its DNS record may or may not be deleted.
  3. A lingering DNS record from a virtual machine that has been deleted is not necessarily a problem by itself. What can be a problem though is if a new virtual machine is later created, and is given the same fully qualified domain name as an old virtual machine that has been deleted, but for which a DNS record still exists. Depending on what the virtual machine is designed to do, problems stemming from the DNS mismatch may not be readily apparent. Even so, a DNS mismatch will very often cause backup problems.
  4. One more issue that can cause problems with backing up Hyper-V virtual machines is low disk space. As previously noted, Hyper-V uses the Volume Shadow Copy Services (VSS) for online backups. In order for VSS to work properly, there must be an adequate amount of free space on the volume that is being backed up. The guidelines for shadow copy storage have changed considerably over the years, and current Microsoft documentation does not seem to list a specific amount of space that should be reserved for shadow copies. However, a good rule of thumb is to make sure that the volume that is being backed up has at least ten percent of its capacity or 1 GB (whichever is greater) free for shadow copy use.

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