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

Products

• VMware vSphere ESXi

Issue/Introduction

Powering ON a VM fails with the error. 

Operation failed!
Task name Power On virtual machine
Target        VMName
Status       Cannot enable changed block tracking and multi-writer sharing at the same time. 

Changing the sharing option from Multi-writer to Unspecified/No sharing fails. 

Operation failed!
Task name Reconfigure virtual machine 
Target        VM Name 
Status       The operation is not supported on the object. 

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 VM's configuration does not support multi-writer OR the VM is removed from an application cluster OR the cluster configuration is broken and you are unable to power ON the VM. 

There are 2 ways to power ON the VM using such configuration - 

1. Disable CBT using the Changed Block Tracking (CBT) on virtual machines KB 

2. If you don't want to do Step-1 because it will interfere with the next backup; disable multi-writer from the VMX file thru CLI 

3. Insert a # at the beginning of the scsi#:#.sharing parameter in the VMX file for all the disks. 

scsi0:0.deviceType = "scsi-hardDisk"
scsi0:0.fileName = "VMware.vmdk"
#scsi0:0.sharing = "multi-writer"
sched.scsi0:0.shares = "normal"

 

You can verify that all the multi-writer lines have been commented out by searching the file using the grep command

Example output of how it should look after the lines have been prefixed with a hash. 

 

[root@ESXi:/vmfs/volumes/5f4facab-########-####-############/VMName] cat VMName.vmx | grep -i multi
#scsi1:0.sharing = "multi-writer"
#scsi1:1.sharing = "multi-writer"
#scsi1:2.sharing = "multi-writer"
#scsi1:4.sharing = "multi-writer"
#scsi1:5.sharing = "multi-writer"

You should be able to power ON the VM.

In some cases you may need to reload the vmx configuration for the changes to recognized by vSphere please see Reloading a vmx file without removing the virtual machine from inventory for more details on the reload process.

Additional Information

Enabling or disabling simultaneous write protection provided by VMFS using the multi-writer flag

Reloading a vmx file without removing the virtual machine from inventory