VSS Freeze Wait Timeout

Adjust the VSS Preparation Timeout Value

The VSS Preparation Timeout can be adjusted by creating and setting the following registry value on the Veeam Backup Server.

Key Location: HKLM\SOFTWARE\Veeam\Veeam Backup and Replication
Value Name: VssPreparationTimeout
Value Type: DWORD (32-Bit) Value
Value Data(Default): 900000

Note:

• This change will affect the VSS Preparation Timeout for all jobs managed by the Veeam Backup Server.
• The value is in milliseconds (decimal)
• The default timeout is 900000ms (15 minutes).
• Try doubling the value initially by setting it to '1800000'.
• No reboot or service restart is required; the new timeout value will be used during the next job run.

Summary

This error is related to the timeout imposed by Microsoft VSS on how long VSS writers may remain frozen. This timeout is not configurable. The VSS writers are frozen during Application-Aware Processing to ensure the writers are not active when the snapshot is created. The Microsoft VSS subsystem will only allow VSS writers to be frozen for at most 60 seconds (20 for Exchange). Within that freeze timeout window, several tasks must be completed.

For example, in a vSphere environment, the following must occur within the freeze timeout window:

1. Verification of freeze state¹
2. Send snapshot creation request via VIM API²
3. Snapshot creation on the ESXi host
4. Return of snapshot information via VIM API²
5. Thaw request to Microsoft VSS¹
6. Thawing of VSS writers’ I/O

¹ In vSphere environments, if a network connection to the guest OS is not available, VIX API will be used, which introduces additional latency.
² These steps should usually be near-instantaneous, but if the vCenter is overloaded or has a high latency to the ESXi hosts, the delay may be significant.

Troubleshooting Suggestions

• If this error occurs when processing an Exchange server, review KB1680: VSS Timeout When Backing up Exchange VM.
• Consider adding a Guest Interaction Proxy in the same subnet as the machine being backed up to ensure Application-Aware Processing can be performed with minimal network latency.
• For vSphere environments, check if the VM has snapshots present before the job starts.
  While it is supported to back up a VM with snapshots present, their presence may increase the time it takes to create and remove subsequent snapshots.
• Use VSSTrace to gather extended VSS logging to help identify issues within the Microsoft VSS subsystem.^[Advanced] 
  See KB3164: How to Use VSSTrace to Collect VSS Diagnostic Logging
• Otherwise, this usually indicates excessive VMware snapshot creation time, an in-guest performance issue, delays related to VIX API, or a combination of factors. The list of possible solutions is identical to KB1680, but the timeout is 60 seconds instead of 20. This often occurs with VSS-related event log errors containing the word “abort”.
  
• This error message also sometimes appears when there are other problems with VSS that should generate additional logging. Collect the logs in KB1789 and provide them when opening a technical support case.