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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: Microsoft VSS completes the requested VSS Writer freeze task and notifies VBR. (Freeze Timeout timer starts) VBR verifies the writers are frozen.1 VBR uses VIM API to send a snapshot creation request to vSphere.2 The ESXi host creates a snapshot on the VM. vSphere notifies VBR that the snapshot has been created.2 VBR sends the Thaw request to Microsoft VSS within the Guest OS.1 Microsoft VSS processes the Thaw request, resuming VSS writers’ I/O. 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.
Persistent VSS Snapshots 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.
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