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Enabling Application Consistent or Crash Consistent Backups for Microsoft Hyper-V

You can configure a subclient to enable application consistent or crash consistent backups of virtual machines:

  • Application consistent backups quiesce virtual machines to capture pending transactions and stored data. Application consistent backups are enabled by default.
  • Crash consistent backups capture stored data in virtual machines quickly while the virtual machine remains online.

Application Consistent Backups

Application consistent backups might require taking a virtual machine offline temporarily, because it is difficult to capture pending transactions while a virtual machine is running. Application consistent backups are generally necessary for virtual machines that host high transaction applications, such as applications that rely on frequent database updates. Application consistent backups require Hyper-V Integration Services and VSS on the virtual machine.

Note: You must use the application consistent backup option to back up shared VHDX files.  Although you must specify the application consistent backup option, shared VHDX disk files can only be backed up in a crash consistent format (a Microsoft limitation).

Crash Consistent Backups

If a backup of a virtual machine needs to capture only stored data, a crash consistent backup can be performed quickly while the virtual machine remains online.  You can enable crash consistent backups in the following cases:

  • For non-Windows virtual machines, legacy Windows virtual machines, and Windows virtual machines that do not require application consistent backups.
  • When backups fail because of Volume Shadow Copy Service (VSS) errors. Running a crash consistent backup ensures that virtual machines are backed up, even when VSS cannot be engaged successfully.

The crash consistent backup method has the advantage of completing quickly without interrupting the virtual machine; however, it is not appropriate for virtual machines that need an application consistent backup. Crash consistent backups use Hyper-V snapshot and checkpoint capabilities to create the backup.

Note: Windows Server 2008 R2 does not support online merging of virtual machine snapshots. If crash consistent backups are used for backups performed by a Virtual Server Agent installed on a Windows Server 2008 R2 host, residual .avhd files remain after the backup completes. The virtual machine powers off and waits for consolidation, which is required to force .avhd files to be merged back into the parent .vhd file.

Procedure

  1. From the CommCell Browser, expand Client Computers > Client > Virtual Server > Hyper-V > Backup Set.
  2. Right-click the subclient and select Properties.
  3. On the Backup Options tab, select one of the following options:
    • Perform application consistent backup
    • Perform crash consistent backup
  4. Click OK.