Why is UDT (Unified Data Transport) feature not being used on migrations of powered-on VMs in vSphere?

Products

• VMware vSphere ESXi

Issue/Introduction

• Looking to use UDT feature to speed up VM "hot" migrations
• This could be cross-vCenter or within same vCenter
• The Provisioning and vMotion networks are enabled according to the requirements of UDT:
  • TechDocs: Edit a VMkernel Adapter Configuration
  • KB 394773 
• UDT-related log messages are not found. These commands produce no results after performing migrations:
  • grep -i UDT /var/log/vmkernel.log
  • grep -i UDT /var/log/hostd.log

Environment

VMware vSphere 8.x

Cause

The following 

• VMware Blogs: Hot and Cold Migrations; Which Network is Used? 
  • vSphere vMotion does a really good job live-migrating workloads without any concessions on application availability or the risk of data corruption
  • Cold migrations: The biggest misconception for cold migration and cold data, is that the vMotion network is leveraged to perform the migration. However, cold data like powered off virtual machines, clones and snapshots are migrated over the Provisioning network if that is enabled.
  • Hot Migrations: (live migration) using the vSphere vMotion feature
  • Even though the live-migration itself uses the vMotion enabled network, the cold data during a vMotion is still transferred using the Provisioning network if configured
• TechDocs: Edit a VMkernel Adapter Configuration states that the Provisioning service handles cold migration, cloning, and snapshot migration.
• KB 394773 clarifies that UDT (Unified Data Transport) enhances cold migration by using NFC as a control channel but offloads the data transfer to vSphere vMotion protocol.

Resolution

To use UDT, the VMs need to be powered off, which will migrate data over the vMotion network

For powered-on VMs, the VMs will be migrated over the same vMotion network as standard