Creating snapshots of servers that have database operations mid-transaction can result in the storage of inconsistent data. VSS or Volume Shadow Copy Service is a functionality that provides a consistent point-in-time snapshot of a volume.
VSS is a Windows built-in application that allows the backup of VSS-compatible applications, such as SQL Server, Exchange Server, SharePoint, and Active Directory. Powershell scripts are executed on the Windows Instance VSS agent using an AWS Run Command, to generate a consistent snapshot.
The overall process flow:
- Volume Freeze:
- The volume is frozen with the write operation suspended.
- Writes during the frozen period are buffered, and any new data is written after the volume is unfrozen.
- The snapshot is generated and stored.
- Volume Unfreeze: The volume is then unfrozen, and the write operation is resumed.
When generating a consistent snapshot on a Windows instance the VSS agent needs to be installed on the server. Most Windows servers should have VSS already installed.
Generating VSS consistent snapshots
You can choose to generate VSS consistent snapshots when you are creating a new backup policy.
To generate VSS consistent snapshots for a pre-existing policy:
- On the top navigation bar, click Policies and then select the policy you wish to edit.
- Navigate to the Additional options tab, and select the Execute VSS Consistent Scripts (Windows Only) checkbox.
The backup policy will now generate consistent snapshots for any Windows server with VSS installed.
Note: If the selected backup policy has servers defined that do not have VSS installed, then a standard AWS EBS snapshot is generated.
If you were an existing CloudRanger customer prior to October 27, 2017, you may have to update your existing AWS account credentials to use this functionality.