Backup hotadd fails and performance is slow in vSphere 8.0 Update 2 and later

Products

• VMware vSphere ESXi

Issue/Introduction

• In vSphere 8.0 Update 2, you may find that third-party backup solutions (such as Veeam or backup_exec) fail to use the hotadd transport mode. Affected virtual machines often show "DirectPath I/O" as active for VMXNET3 adapters in the vSphere Client UI, even if passthrough was not manually enabled. This results in significantly slower backup windows as the system falls back to less efficient transport methods.

Cause

This is caused by a behavioral change introduced with Uniform Passthrough v2. In vSphere 8.0 Update 2, VMXNET3 adapters are "DirectPath-ready" by default. The uptCompatibility setting is enabled in the VM configuration, which prevents some backup proxies from successfully performing hotadd operations on the virtual machine.

Resolution

If the ESXi host does not have a SmartNIC, the UptCompatibility setting can be safely disabled. To restore hotadd functionality in these cases, the uptCompatibility feature can be disabled within the virtual machine configuration (VMX) file.

1. Power off the affected Virtual Machine.
2. Access the ESXi host where the VM resides via SSH.
3. Locate the VMX file path:
   
   esxcli vm process list | grep -i "<VM Name>"

4. Edit the VMX file using a text editor:
   
   vi /vmfs/volumes/<datastore>/<VM-Name>.vmx
   
   
5. Locate the line ethernet0.uptCompatibility = "TRUE" and change the value to "FALSE".
   
   
6. Save with wq! and exit the editor.
   
   
7. Reload the VM configuration:
   
   - Identify the Vmid: vim-cmd vmsvc/getallvms | grep "<VM-Name>"
   - Reload the VM: vim-cmd vmsvc/reload <Vmid>
   
   
8. Power on the Virtual Machine.

Additional Information

DirectPath I/O option not available on UI