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A job utilizing Application-Aware Processing fails with either of the errors: Failed to call RPC function 'Vss.FinishSnapshot': Error code: 0x800401fd. Failed to invoke func [FinishSnapshot] Failed to call RPC function 'Vss.KeepSnapshot': Error code: 0x800401fd. Failed to invoke func [KeepSnapshot] A correlating entry can be found in the VeeamVssSupport log on the machine that was being backed up: 2:25:06 PM 224 INFO RPC: KeepSnapshotEx 2:25:06 PM 224 INFO Keep snapshot. JobId: {vss_job_guid} 2:25:06 PM 224 INFO Snapshot job id: {vss_job_guid}, ttl: 1200. 2:25:06 PM 224 KeepSnapshot. TTL: [1200] 2:25:06 PM 224 INFO Keep snapshot. JobId: {vss_job_guid}. Ok. 2:25:06 PM 224 INFO RPC: KeepSnapshotEx. Ok. 2:45:07 PM 4484 WARN Snapshot timeout occurred. Addtional log examples are available in the More Information section.
This error occurs when the guest OS fails to receive a keep-alive request (KeepSnapshotEx) within the configured snapshot timeout window. This may occur due to issues within the machine itself or external factors (e.g., network connection dropout, guest OS I/O performance, virtual environment disk performance).
Understanding KeepSnapshotEx Calls During Application-Aware Processing preparation, a temporary service (VeeamVssSupport) is deployed to manage VSS snapshot operations within the guest. This service is programmed to close the VSS snapshot if it does not receive keep-alive calls during a set timeout period. By default, the snapshot timeout window is 20 minutes (1200 seconds). The Veeam Backup Server sends KeepSnapshotEx calls every X seconds, where X is equal to the VSSGuestSnapshotTimeout divided by 10. This timeout window ensures that, should something occur external to the machine, the VSS snapshot will not remain active indefinitely. Adjust Snapshot Timeout Timing If this issue occurs persistently, the snapshot keep-alive timeout window can be changed by modifying the following registry value on the Veeam Backup Server. Note: This will affect all machines that are processed using Application-Aware Processing. Key Location: HKLM\SOFTWARE\Veeam\Veeam Backup and Replication\Value Name: VSSGuestSnapshotTimeoutValue Type: DWORD (32-Bit) ValueValue Data(Default-Decimal): 1200 [seconds] No reboot or service restart is required; the new timeout value will be used during the next job run.
The examples below demonstrate: A network connection drop that was long enough to prevent the VeeamVssSupport service from receiving a KeepSnapshotEx call within the timeout window. The VeeamVssSupport service closing the snapshot due to the snapshot timeout. The network connection resuming, which allows the VeeamVssSupport service to receive the KeepSnapshotEx or FinishSnapshotEx call; however, it fails because the timeout had already been triggered, resulting in the closure of the snapshot.
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