SIRIS, ALTO, and NAS: Critical Backup Failure: Failed to create VM snapshot at second attempt - aborting



When attempting an agentless backup of a Windows or Linux virtual machine, you receive the error message, "Critical Backup Failure: Failed to create VM snapshot at second attempt - aborting."


  • Datto SIRIS
  • Datto ALTO
  • Datto Agentless backups


The VMware hypervisor cannot take a snapshot. This condition commonly occurs when:

  • the required services for the snapshot are stopped or disabled on the virtual machine.
  • Outdated VMware tools or a VSS quiescing error on the virtual machine.


The troubleshooting steps presented in this article are offered to help diagnose the issue. VMWare is not a Datto product and so our ability to troubleshoot non-Datto related issues may be limited.

Troubleshooting steps

Verify that the vm snapshots that Datto uses in the backup process are working correctly.

Once you've got a good snapshot, you can reattempt pairing and backing up to the Datto Appliance. 

1. Open the vSphere connection to the ESX host.

2. Right-click the virtual machine name and navigate to Take snapshot.  Select the Quiesce guest file system check box.

3. Clear the Snapshot the virtual machine's memory check box and click OK.

Check the guest VM

1. On the guest VM, open services.msc. Verify the following services have a Running status and are set to startup type Automatic:

  • Virtual Disk
  • VMware Snapshot Provider 
  • VMware Tools
  • Volume Shadow Copy

2. If one of the services is not set to Automatic, right-click the service and go to Properties. Set the startup type to Automatic.

3. Restart the VMWare Tools service.

4. Attempt another quiesced snapshot in vSphere.

On Windows 2000 machines, you only need to restart the VMware Tools service.  You will not have Virtual Disk, Volume Shadow Copy, and VMware Snapshot Provider services.

continued failures to take a snapshot in vSphere, may indicate a VSS or VMware quiescing problem. Before troubleshooting further, try updating VMware tools and rebooting the VM; In most cases, this will resolve the issue.

Additional troubleshooting

Check Server Vss Health
On the guest VM, check the status of the VSS Writers  

1. Review the application logs for any recent VSS-related errors.

2. Open an administrative command prompt.

3. Run the command vssadmin list writers.

4. Review the output for any writers in a failed state.

5. Attempt to resolve the error with one of the following KBs:
Resolving VSS Writer Errors In Windows XP, Windows 7, And Windows Server 2003 (external link)

Resolving VSS Writer Errors In Windows 8, Server 2012, And Up (external link)

6. Attempt another quiesced snapshot in vSphere.

Explore Additional guest VM resources

Services that generate heavy I/O workload, like Exchange, Active Directory, LDAP, and MS-SQL, may succeed in taking VSS snapshots if you allocate more resources to them. Try giving the guest VM more processors and RAM.

Attempt a non-quiesced snapshot

Attempt a snapshot with quiescing disabled for troubleshooting purposes.

With quiescing disabled, the VM backs up in a crash-consistent state. This means that all files that were open during the backup will still exist, but are not guaranteed to be free of incomplete I/O operations or data corruption. 

1. Stop the VMware Snapshot Provider service.

2. Restart the VMware Tools service.

3. Attempt another snapshot in vSphere with Quiesce guest file system unchecked.

To disable quiescing permanently, right-click the  VMware Snapshot Provider service, then go to Properties and set the Startup Type to Disabled.

On a Linux guest VM

In the Linux VM, log in as root and run the following commands to restart VMware tools:

/etc/init.d/vmware-tools start
/etc/init.d/vmware-tools stop
/etc/init.d/vmware-tools restart
/etc/init.d/vmware-tools status

Additional Resources

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