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This article was initially created for Veeam Backup & Replication 6.1.
While the article remains technically accurate regarding the behavior of Microsoft VSS and Veeam's interactions with VSS, the issue discussed rarely occurs thanks to the improvements made in Veeam Backup & Replication version 8.
Veeam Backup & Replication 8 introduced Persistent VSS Snapshots, which significantly diminished the occurrences of VSS Timeouts during VSS preparation of Exchange servers.
However, some customer may still be affected by these timeouts if their Exchange server does not meet the requirements for Persistent VSS Snapshots to be utilized. Review Limitations for Persistent VSS Snapshot Technology for more information.
The backup of an Exchange server VM fails with:
Unfreeze error:[Backup job failed] Cannot create shadow copy of the volumes containing writer’s data A VSS critical writer has failed. Writer name: [Microsoft Exchange Writer]. Class ID: [{76fe1ac4-15f7-4bcd-987e-8e1acb462fb7}]. Instance ID: [{0db23250-4d1e-42c1-8d14-2be32f448184}]. Writer's state: [VSS_WS_FAILED_AT_FREEZE]. Error code: [0x800423f2].]
If you run the command ‘vssadmin list writers’ on the Exchange server after the job fails, typically you will see an Exchange Writer has failed because of a timeout error (error code 9).
VSSControl: Failed to freeze guest, wait timeout
This "wait timeout" refers to the limit imposed by Microsoft VSS on the maximum duration VSS writers may be frozen. This timeout is not configurable. Veeam Backup & Replication (VBR) uses VSS to freeze the VSS writers immediately before creating the VMware snapshot and then sends the thaw command as soon as snapshot creation is complete. Microsoft VSS will only hold a freeze on the writers for up to 60 seconds (20 for Exchange) and will automatically unfreeze (thaw) the writers once that timeout is met. This means that the following steps must all be able to complete within that timeframe:
1 If a network connection to the guest OS is unavailable, VIX API will be used, introducing additional latency.
2 These steps should usually be near-instantaneous, but the delay may be significant if the vCenter is heavily loaded or has a high latency to the ESXi hosts.
When using Veeam Backup & Replication version 8 or higher, the software will automatically switch to using Persistent VSS Snapshots with Exchange servers, which mitigate issues associated with the 20-second freeze limitation.
Review the Limitations for Persistent VSS Snapshot Technology to ensure the Persistent VSS Snapshots feature can function.
If Persistent VSS Snapshots cannot be used, you will need to determine why the tasks that must be performed within the Microsoft VSS freeze timeout window could not occur in a timely manner. This issue is an infrastructure issue that can be difficult to narrow down. The following is a comprehensive list of solutions that customers have used to resolve the issue:
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