vMotion Fails to Start: Disk(s) Have Both Changed Block Tracking and Multi-Writer Sharing Enabled

Products

• VMware vSphere ESXi 8.0
• VMware vSphere ESXi

Issue/Introduction

• When performing a vMotion operation, the task fails to start, and the following error is displayed:
  Migration failed to start because there exist disk(s) enabled both Changed Block Tracking (CBT) and Multi-Writer sharing at the same time.
• This issue occurs when attempting to perform an online vMotion of a virtual machine that has one or more shared VMDKs configured with both Multi-Writer mode and Changed Block Tracking (CBT) enabled simultaneously.

Environment

• VMware vSphere ESXi

Cause

• The multi-writer option in vSphere allows the VMFS-backed disks to be shared by multiple VM’s simultaneously.
• Changed Block Tracking (CBT) is a feature that identifies blocks of data that have changed or in use. It enables incremental backups to identify changes from the last previous backup, writing only changed or in-use blocks.
• This is an unsupported configuration (CBT and Multi-writer): CBT and Multi-writer can never be configured simultaneously on VMs

Resolution

If the virtual machine’s configuration does not require the Multi-Writer option, or the VM is no longer part of an application cluster, or the cluster configuration is broken, you can resolve the vMotion failure by updating the VM configuration as outlined below.

There are two methods to allow successful vMotion of a VM that currently has both CBT and Multi-Writer enabled:

• Disable Changed Block Tracking (CBT) by using Changed Block Tracking (CBT) on virtual machines
• Disable Multi-Writer Mode: If you prefer to retain CBT (for example, to preserve backup operations), you can disable Multi-Writer from the VM configuration file (.vmx) through CLI
  • Edit the .vmx file and comment out all scsi#:#.sharing parameters by prefixing them with a hash (#). For example:
    [root@ESXi:/vmfs/volumes/datastore/VMName] cat VMName.vmx | grep -i multi
#scsi1:0.sharing = "multi-writer"
#scsi1:1.sharing = "multi-writer"
#scsi1:2.sharing = "multi-writer"
#scsi1:3.sharing = "multi-writer"
  • Reload the vmx configuration for the changes to recognized by vSphere. Please see Reloading a vmx file without removing the virtual machine from inventory.

Additional Information

CBT-enabled VM having multi-writer disk(s) is not supported